LTC6945 Ultralow Noise and Spurious 0.35GHz to 6GHz Integer-N Synthesizer Description Features Low Noise Integer-N PLL nn 350MHz to 6GHz VCO Input Range nn -226dBc/Hz Normalized In-Band Phase Noise Floor nn -274dBc/Hz Normalized In-Band 1/f Noise nn -157dBc/Hz Wideband Output Phase Noise Floor nn Excellent Spurious Performance nn Output Divider (1 to 6, 50% Duty Cycle) nn Low Noise Reference Buffer nn Output Buffer Muting nn Charge Pump Supply from 3.15V to 5.25V nn Charge Pump Current from 250A to 11.2mA nn Configurable Status Output nn SPI Compatible Serial Port Control nn PLLWizardTM Software Design Tool Support nn Applications Wireless Base Stations (LTE, WiMAX, W-CDMA, PCS) Broadband Wireless Access nn Microwave Data Links nn Military and Secure Radio nn Test and Measurement nn nn The LTC(R)6945 is a high performance, low noise, 6GHz phaselocked loop (PLL), including a reference divider, phasefrequency detector (PFD) with phase-lock indicator, charge pump, integer feedback divider and VCO output divider. The part features a buffered, programmable VCO output divider with a range of 1 through 6. The differential, low noise output buffer has user-programmable output power ranging from -6dBm to 3dBm, and may be muted through either a digital input pin or software. The low noise reference buffer outputs a typical 0dBm square wave directly into a 50 impedance from 10MHz to 250MHz, or may be disabled through software. The ultralow noise charge pump contains selectable high and low voltage clamps useful for VCO monitoring, and also may be set to provide a V+/2 bias. All device settings are controlled through a SPI-compatible serial port. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and PLLWizard is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Typical Application LTC6945 Data Converter Sample Clock LOOP 3.3V BANDWIDTH 0.1F ~2.3kHz 51.1 100MHz REF 1F 0.1F 0.1F 47nF 432 LTC6945 CS GND SCLK SDI 3.3V 0.01F GND GND 100pF GND GND GND VCO+ fPFD = 25MHz VD+ VCO- MUTE GND RF- RF+ VRF+ BB 16.5 SDO 0.1F DSB INTEGRATION (100Hz TO 1GHz) RMS NOISE = 0.014 RMS JITTER = 39fs fPFD = 25MHz BW = 2.3kHz -110 1F REF- REF+ VREF+ CP VCP+ GND VREFO+ VVCO+ STAT 3.3V 1GHz Sample Clock Phase Noise -100 570nF REFO SPI BUS CRYSTEK CVCSO-914-1000 16.5 16.5 100pF 1.0F PHASE NOISE (dBc/Hz) 3.3V 5V 5V VTUNE -120 -130 -140 -150 -160 -170 -180 100 100pF 1k 1M 10k 100k OFFSET FREQUENCY (Hz) 10M 40M 6945 TA01b 3.3V 68nH 100pF 50 3.3V 0.01F 68nH 3.3V 100pF ALT SAMPLE CLOCK 500MHz OR 1GHz SAMPLE CLOCK 1GHz, 7dBm 6945 TA01b 6945fa For more information www.linear.com/LTC6945 1 LTC6945 Pin Configuration Supply Voltages V+ (VREF+, VREFO+, VRF+, V VCO+, VD+) to GND.......3.6V VCP+ to GND..........................................................5.5V Voltage on CP Pin..................GND - 0.3V to VCP+ + 0.3V Voltage on All Other Pins...........GND - 0.3V to V+ + 0.3V Operating Junction Temperature Range, TJ (Note 2) LTC6945I................................................ -40C to 105C Junction Temperature, TJMAX................................. 125C Storage Temperature Range................... -65C to 150C GND VCP+ CP REF- TOP VIEW VREF+ (Note 1) REF+ Absolute Maximum Ratings 28 27 26 25 24 23 22 VVCO+ VREFO+ 1 REFO 2 21 GND STAT 3 20 GND CS 4 19 GND 29 GND SCLK 5 18 GND SDI 6 17 GND SDO 7 16 VCO+ VD+ 8 15 VCO- BB VRF+ RF+ RF - GND MUTE 9 10 11 12 13 14 UFD PACKAGE 28-LEAD (4mm x 5mm) PLASTIC QFN TJMAX = 125C, JCbottom = 3C/W, JCtop = 26C/W EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION JUNCTION TEMPERATURE RANGE LTC6945IUFD#PBF LTC6945IUFD#TRPBF 6945 28-Lead (4mm x 5mm) Plastic QFN -40C to 105C Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VREF+ = VREF0+ = VD+ = VRF+ = VVCO+ = 3.3V, VCP+ = 5V unless otherwise specified. All voltages are with respect to GND. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 250 MHz Reference Inputs (REF+, REF-) fREF Input Frequency VREF Input Signal Level Single-Ended, 1F AC-Coupling Capacitors Input Slew Rate l 10 l 0.5 l 20 l 1.65 1.85 2.25 V l 6.2 8.4 11.6 k Input Duty Cycle 2 2.7 50 Self-Bias Voltage Input Resistance Differential Input Capacitance Differential VP-P V/s % 3 pF Reference Output (REFO) fREFO Output Frequency PREFO Output Power fREFO = 10MHz, RLOAD = 50 Output Impedance, Disabled 2 l 10 250 MHz l -0.2 3.2 dBm 800 6945fa For more information www.linear.com/LTC6945 LTC6945 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VREF+ = VREF0+ = VD+ = VRF+ = VVCO+ = 3.3V, VCP+ = 5V unless otherwise specified. All voltages are with respect to GND. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VCO Input (VCO+, VCO-) fVCO Input Frequency l 350 6000 MHz PVCOI Input Power Level RZ = 50, Single-Ended l -8 0 6 dBm Input Resistance Single-Ended, Each Input l 97 121 145 l 350 6000 MHz l 1 RF Output (RF+, RF-) fRF Output Frequency O Output Divider Range All Integers Included Output Duty Cycle Output Resistance + Single-Ended, Each Output to VRF Output Common Mode Voltage PRF(SE) 6 50 l l 111 136 % 159 2.4 VRF + V -9.7 -6.8 -3.9 -1.2 -6.0 -3.6 -0.4 2.3 dBm dBm dBm dBm Output Power, Single-Ended, fRF = 900MHz RFO[1:0] = 0, RZ = 50, LC Match RFO[1:0] = 1, RZ = 50, LC Match RFO[1:0] = 2, RZ = 50, LC Match RFO[1:0] = 3, RZ = 50, LC Match l l l l Output Power, Muted RZ = 50, Single-Ended, fRF = 900MHz, O = 2 to 6 l -60 dBm Mute Enable Time l 110 ns Mute Disable Time l 170 ns l 100 MHz Phase/Frequency Detector fPFD Input Frequency Lock Indicator, Available on the STAT Pin and via the SPI-Accessible Status Register tLWW Lock Window Width LKWIN[1:0] = 0 LKWIN[1:0] = 1 LKWIN[1:0] = 2 LKWIN[1:0] = 3 3.0 10.0 30.0 90.0 ns ns ns ns tLWHYS Lock Window Hysteresis Increase in tLWW Moving from Locked State to Unlocked State 22 % Output Current Range 12 Settings (See Table 5) Output Current Source/Sink Accuracy VCP = VCP+/2, All Settings Output Current Source/Sink Matching ICP = 250A to 1.4mA, VCP = VCP+/2 ICP = 2mA to 11.2mA, VCP = VCP+/2 Output Current vs Output Voltage Sensitivity (Note 3) l 0.1 Output Current vs Temperature VCP = VCP+/2 l 170 ppm/C Output Hi-Z Leakage Current ICP = 700A, CPCLO = CPCHI = 0 (Note 3) ICP = 11.2mA, CPCLO = CPCHI = 0 (Note 3) 0.5 5 nA nA Low Clamp Voltage CPCLO = 1 0.84 V Charge Pump ICP VCLMP(LO) 0.25 + 11.2 mA 6 % 3.5 2 % % 1.0 %/V VCLMP(HI) High Clamp Voltage CPCHI = 1, Referred to VCP -0.96 V VMID Mid-Supply Output Bias Ratio Referred to (VCP+ - GND) 0.48 V/V Reference (R) Divider R Divide Range All Integers Included l 1 1023 Counts 6945fa For more information www.linear.com/LTC6945 3 LTC6945 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VREF+ = VREF0+ = VD+ = VRF+ = VVCO+ = 3.3V, VCP+ = 5V unless otherwise specified. All voltages are with respect to GND. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 65535 Counts VCO (N) Divider N Divide Range All Integers Included l 32 1.55 Digital Pin Specifications VIH High Level Input Voltage MUTE, CS, SDI, SCLK l VIL Low Level Input Voltage MUTE, CS, SDI, SCLK l VIHYS Input Voltage Hysteresis MUTE, CS, SDI, SCLK 250 Input Current MUTE, CS, SDI, SCLK l IOH High Level Output Current SDO and STAT, VOH = VD+ - 400mV l IOL Low Level Output Current SDO and STAT, VOL = 400mV l SDO Hi-Z Current V 0.8 -2.3 1.8 mV 1 A -1.4 mA 3.4 mA 1 l V A Digital Timing Specifications (See Figures 8 and 9) tCKH SCLK High Time l 25 ns tCKL SCLK Low Time l 25 ns tCSS CS Setup Time l 10 ns tCSH CS High Time l 10 ns tCS SDI to SCLK Setup Time l 6 ns tCH SDI to SCLK Hold Time l 6 tDO SCLK to SDO Time To VIH/VIL/Hi-Z with 30pF Load ns l 16 ns Power Supply Voltages VREF+ Supply Range VREFO+ Supply Range VD+ Supply Range VRF+ Supply Range VVCO+ Supply Range VCP+ Supply Range l 3.15 3.3 3.45 V l 3.15 3.3 3.45 V l 3.15 3.3 3.45 V l 3.15 3.3 3.45 V l 3.15 3.3 3.45 V l 3.15 5.25 V 500 A Power Supply Currents IDD VD+ Supply Current + Supply Current Digital Inputs at Supply Levels l ICP = 11.2mA ICP = 1.0mA PDALL = 1 l l l 34 12 235 39 14.5 385 mA mA A ICC(CP) VCP ICC(REFO) VREFO+ Supply Currents REFO Enabled, RZ = l 7.8 9.0 mA ICC Sum VREF+, VRF+, VVCO+ Supply RF Muted, OD[2:0] = 1 RF Enabled, RFO[1:0] =0, OD[2:0] = 1 RF Enabled, RFO[1:0] = 3, OD[2:0] = 1 RF Enabled, RFO[1:0] =3, OD[2:0] = 2 RF Enabled, RFO[1:0] =3, OD[2:0] = 3 RF Enabled, RFO[1:0] =3, OD[2:0] = 4 to 6 PDALL = 1 l l l l l l l 70 79 88 105 111 116 202 78 88 98 117 124 128 396 mA mA mA mA mA mA A 4 Currents 6945fa For more information www.linear.com/LTC6945 LTC6945 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VREF+ = VREF0+ = VD+ = VRF+ = VVCO+ = 3.3V, VCP+ = 5V unless otherwise specified. All voltages are with respect to GND. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Phase Noise and Spurious LM(MIN) Output Phase Noise Floor (Note 5) RFO[1:0] = 3, OD[2:0] = 1, fRF = 6GHz RFO[1:0] = 3, OD[2:0] = 2, fRF = 3GHz RFO[1:0] = 3, OD[2:0] = 3, fRF = 2GHz RFO[1:0] = 3, OD[2:0] = 4, fRF = 1.5GHz RFO[1:0] = 3, OD[2:0] = 5, fRF = 1.2GHz RFO[1:0] = 3, OD[2:0] = 6, fRF = 1.0GHz -155 -155 -156 -156 -157 -158 dBc/Hz dBc/Hz dBc/Hz dBc/Hz dBc/Hz dBc/Hz LM(NORM) Normalized In-Band Phase Noise Floor ICP = 11.2mA (Notes 6, 7, 8) -226 dBc/Hz LM(NORM -1/f) Normalized In-Band 1/f Phase Noise ICP = 11.2mA (Notes 6, 9) -274 dBc/Hz LM(IB) In-Band Phase Noise Floor (Notes 6, 7, 8, 10) -99 dBc/Hz Integrated Phase Noise from 100Hz to 40MHz (Notes 4, 7, 10) 0.13 RMS Spurious Reference Spur, PLL locked (Notes 4, 7, 10, 11) -102 dBc Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC6945I is guaranteed to meet specified performance limits over the full operating junction temperature range of -40C to 105C. Under maximum operating conditions, air flow or heat sinking may be required to maintain a junction temperature of 105C or lower. It is strongly recommended that the exposed pad (Pin 29) be soldered directly to the ground plane with an array of thermal vias as described in the Applications Information section. Note 3: For 0.9V VCP (VCP+ - 0.9V). Note 4: VCO is Crystek CVCO55CL-0902-0928. Note 5: fVCO = 6GHz, fOFFSET = 40MHz. Note 6: Measured inside the loop bandwidth with the loop locked. Note 7: Reference frequency supplied by Wenzel 501-04608A, fREF = 10MHz, PREF = 13dBm. Note 8: Output phase noise floor is calculated from normalized phase noise floor by LM(OUT) = -226 + 10log10(fPFD) + 20log10(fRF/fPFD). Note 9: Output 1/f phase noise is calculated from normalized 1/f phase noise by LM(OUT -1/f) = -274 + 20log10 (fRF) - 10log10 (fOFFSET). Note 10: ICP = 11.2mA, fPFD = 250kHz, fRF = 914MHz, FILT[1:0] = 3, Loop BW = 7kHz. Note 11: Measured using DC1649. Typical Performance Characteristics REF Input Sensitivity vs Frequency BST = 1 FILT = 0 TJ = 105C TJ = 25C TJ = -40C -20 -25 -30 -35 -40 2 1 0 -1 -45 -2 -50 -3 -55 0 25 50 75 100 125 150 175 200 225 250 FREQUENCY (MHz) 6945 G01 -4 REFO Phase Noise -140 TJ = 105C TJ = 25C TJ = -40C 3 POUT (dBm) SENSITIVITY (dBm) REFO Output Power vs Frequency 4 PHASE NOISE (dBc/Hz) -15 0 25 50 75 100 125 150 175 200 225 250 FREQUENCY (MHz) 6945 G02 -145 POUT = 1.45dBm fREF = 10MHz BST = 1 FILT = 3 NOTE 7 -150 -155 -160 100 10k 100k 1M 1k OFFSET FREQUENCY (Hz) 5M 6945 G03 6945fa For more information www.linear.com/LTC6945 5 LTC6945 Typical Performance Characteristics Charge Pump Sink Current Error vs Voltage, Temperature 5 4 4 4 3 3 3 2 2 2 1 1 1 -1 0 -1 -2 -2 -3 0 0.5 1 1.5 2 2.5 3 3.5 4 OUTPUT VOLTAGE (V) 4.5 -4 -5 5 0 0.5 1 1.5 2 2.5 3 3.5 4 OUTPUT VOLTAGE (V) Charge Pump Source Current Error vs Voltage, Temperature 5 1.0 4 0.5 3 0 ICP = 11.2mA TJ = 105C TJ = 25C -4 TJ = -40C -5 0 0.5 1 1.5 2 2.5 3 3.5 4 OUTPUT VOLTAGE (V) -2.5 -3 -3.0 4.5 5 -20 -25 -30 -3.5 HD2 (dBc) PVCO = 0dBm LC = 180nH CS = 270pF TJ = 105C TJ = 25C TJ = -40C HD3 (dBc) POUT AT fVCO/O (dBm) fRF = fVCO/O PVCO = 0dBm LC = 180nH CS = 270pF -60 -70 -80 -110 6945 G10 O=1 O=6 O=4 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 fVCO (GHz) 6945 G09 Frequency Step Transient 2.10 O=1 fRF = fVCO/O PVCO = 0dBm LC = 180nH CS = 270pF O=3 -90 -100 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 fVCO (GHz) O=5 -40 -55 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 FREQUENCY (GHz) -50 -20 5 -40 O=6 O=1 O=2 -50 -30 O=3 4.5 O=3 6945 G08 -5 O=2 fRF = fVCO/O PVCO = 0dBm LC = 180nH CS = 270pF -35 MUTE Output Power vs fVCO and Output Divide (Single-Ended On RF-) RF Output HD3 vs Output Divide (Single-Ended On RF-) -15 1.5 2 2.5 3 3.5 4 OUTPUT VOLTAGE (V) -45 6945 G07 -10 1 RF Output HD2 vs Output Divide (Single-Ended On RF-) -120 O=2 2.05 FREQUENCY (GHz) ERROR (%) POUT (dBm) -2.0 -2 0.5 0 6945 G06 RF Output Power vs Frequency (Single-Ended On RF-) -1.5 -1 -30 -5 5 -1.0 0 250A 1mA 11.2mA -4 -0.5 1 -25 -3 6945 G05 6945 G04 2 4.5 0 -1 -2 ICP = 11.2mA TJ = 105C TJ = 25C TJ = -40C -3 250A 1mA 11.2mA -4 -5 ERROR (%) 5 0 6 Charge Pump Source Current Error vs Voltage, Output Current 5 ERROR (%) ERROR (%) Charge Pump Sink Current Error vs Voltage, Output Current O=4 O=5 2.00 1.95 1.90 O=6 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 fVCO (GHz) 6945 G11 1.85 fPFD = 1MHz BW = 40kHz 100MHz STEP 0 5 10 15 20 25 30 TIME (s) 35 40 45 6945 G12 6945fa For more information www.linear.com/LTC6945 LTC6945 Typical Performance Characteristics VCO Input Sensitivity vs Frequency, Temperature Closed-Loop Phase Noise, fRF = 914MHz PHASE, NOISE (dBc/Hz) -20 -25 -30 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 FREQUENCY (GHz) -90 -100 -100 -110 -110 -120 -130 -140 RMS NOISE = 0.13 fPFD = 250kHz -150 BW = 7kHz NOTES 7, 10 -160 VCO = CRYSTEK CVCO55CL-0902-0928 -170 1M 1k 100 10k 100k OFFSET FREQUENCY (Hz) 10M 40M Spurious Response fRF = 914MHz, fREF = 10MHz, fPFD = 250kHz, Loop BW = 7kHz -130 -140 RMS NOISE = 0.33 fPFD = 1MHz -150 BW = 40kHz NOTE 7 -160 VCO = RFMD UMX-586-D16-G -170 1M 1k 100 10k 100k OFFSET FREQUENCY (Hz) 0 RBW = 1Hz VBW = 1Hz -20 NOTES 7, 10, 11 6945 G15 RBW = 1Hz VBW = 1Hz NOTES 7, 11 -20 -40 POUT (dBm) -40 -60 -80 -102dBc -113dBc -102dBc -111dBc -60 -80 -102dBc -100 -113dBc -120 -120 -140 -140 -10 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 10 FREQUENCY OFFSET (MHz, IN 10kHz SEGMENTS) -102dBc -112dBc -10 -3 -2 -1 0 1 2 3 10 FREQUENCY OFFSET (MHz, IN 10kHz SEGMENTS) 6945 G17 6945 G16 Spurious Response fRF = 5725MHz, fREF = 10MHz, fPFD = 5MHz, Loop BW = 21kHz Supply Current vs Temperature 0 88 RBW = 1Hz VBW = 1Hz -20 NOTES 7, 11 3.3V CURRENT (mA) -60 -80 -100dBc -112dBc -101dBc -112dBc -120 -140 -20 -15 -10 -5 0 5 10 15 20 FREQUENCY OFFSET (MHz, IN 10kHz SEGMENTS) 35.5 PDREFO = 1 87 O = 1 RFO = 3 86 MUTE = 0 ICP = 11.2mA 35.0 34.5 85 34.0 84 33.5 83 33.0 82 32.5 81 32.0 80 -40 -20 6945 G18 0 20 80 40 60 TEMPERATURE, TJ (C) 100 5V CURRENT (mA) -40 -100 10M 40M Spurious Response fRF = 2100MHz, fREF = 10MHz, fPFD = 1MHz, Loop BW = 40kHz 0 -100 -120 6945 G14 6945 G13 POUT (dBm) -35 TJ = 105C TJ = 25C TJ = -40C POUT (dBm) SENSITIVITY (dBm) -15 -90 PHASE, NOISE (dBc/Hz) -10 Closed-Loop Phase Noise, fRF = 2100MHz 31.5 6945 G19 6945fa For more information www.linear.com/LTC6945 7 LTC6945 Pin Functions VREFO+ (Pin 1): 3.15V to 3.45V Positive Supply Pin for REFO Circuitry. This pin should be bypassed directly to the ground plane using a 0.1F ceramic capacitor as close to the pin as possible. REFO (Pin 2): Reference Frequency Output. This produces a low noise square wave, buffered from the REF differential inputs. The output is self-biased and must be AC-coupled with a 22nF capacitor. STAT (Pin 3): Status Output. This signal is a configurable logical OR combination of the UNLOCK, LOCK, THI and TLO status bits, programmable via the STATUS register. See the Operations section for more details. CS (Pin 4): Serial Port Chip Select. This CMOS input initiates a serial port communication burst when driven low, ending the burst when driven back high. See the Operations section for more details. SCLK (Pin 5): Serial Port Clock. This CMOS input clocks serial port input data on its rising edge. See the Operations section for more details. SDI (Pin 6): Serial Port Data Input. The serial port uses this CMOS input for data. See the Operations section for more details. SDO (Pin 7): Serial Port Data Output. This CMOS threestate output presents data from the serial port during a read communication burst. Optionally attach a resistor of >200k to GND to prevent a floating output. See the Operations section for more details. VD+ (Pin 8): 3.15V to 3.45V Positive Supply Pin for Serial Port Circuitry. This pin should be bypassed directly to the ground plane using a 0.1F ceramic capacitor as close to the pin as possible. MUTE (Pin 9): RF Mute. The CMOS active-low input mutes the RF differential outputs while maintaining internal bias levels for quick response to de-assertion. GND (Pins 10, 17, 18, 19, 20, 21): Negative Power Supply (Ground). These pins should be tied directly to the ground plane with multiple vias for each pin. 8 RF-, RF+ (Pins 11, 12): RF Output Signals. The VCO output divider is buffered and presented differentially on these pins. The outputs are open collector, with 136 (typical) pull-up resistors tied to VRF+ to aid impedance matching. If used single-ended, the unused output should be terminated to 50. See the Applications Information section for more details on impedance matching. VRF+ (Pin 13): 3.15V to 3.45V Positive Supply Pin for RF Circuitry. This pin should be bypassed directly to the ground plane using a 0.01F ceramic capacitor as close to the pin as possible. BB (Pin 14): RF Reference Bypass. This output must be bypassed with a 1.0F ceramic capacitor to GND. Do not couple this pin to any other signal. VCO-, VCO+ (Pins 15, 16): VCO Input Signals. The differential signal placed on these pins is buffered with a low noise amplifier and fed to the internal output and feedback dividers. These self-biased inputs must be AC-coupled and present a single-ended 121 (typical) resistance to aid impedance matching. They may be used singleended by bypassing VCO- to GND with a capacitor. See the Applications Information section for more details on impedance matching. VVCO+ (Pin 22): 3.15V to 3.45V Positive Supply Pin for VCO Circuitry. This pin should be bypassed directly to the ground plane using a 0.01F ceramic capacitor as close to the pin as possible. GND (23): Negative Power Supply (Ground). This pin is attached directly to the die attach paddle (DAP) and should be tied directly to the ground plane. VCP+ (Pin 24): 3.15V to 5.25V Positive Supply Pin for Charge Pump Circuitry. This pin should be bypassed directly to the ground plane using a 0.1F ceramic capacitor as close to the pin as possible. CP (Pin 25): Charge Pump Output. This bi-directional current output is normally connected to the external loop filter. See the Applications Information section for more details. 6945fa For more information www.linear.com/LTC6945 LTC6945 Pin Functions VREF+ (Pin 26): 3.15V to 3.45V Positive Supply Pin for Reference Input Circuitry. This pin should be bypassed directly to the ground plane using a 0.1F ceramic capacitor as close to the pin as possible. capacitors. If used single-ended, bypass REF- to GND with a 1F capacitor. If the single-ended signal is greater than 2.7VP-P, bypass REF- to GND with a 47pF capacitor. GND (Exposed Pad Pin 29): Negative Power Supply (Ground). The package exposed pad must be soldered directly to the PCB land. The PCB land pattern should have multiple thermal vias to the ground plane for both low ground inductance and also low thermal resistance. REF+, REF- (Pins 27, 28): Reference Input Signals. This differential input is buffered with a low noise amplifier, which feeds the reference divider and reference buffer. They are self-biased and must be AC-coupled with 1F Block Diagram 28 27 REF- 1 2 26 REF+ 24 VREF+ 23 VCP+ GND VREFO+ REFO 250MHz 100MHz R_DIV 250A TO 11.2mA PFD /1 TO 1023 CP LOCK 3 4 5 6 7 GND 20 CS GND 19 /32 TO 65535 SERIAL PORT GND 18 N_DIV GND 17 SDI 350MHz TO 6GHz SDO VCO+ 16 O_DIV /1 TO 6, 50% + 8 GND 21 STAT SCLK 25 VVCO+ 22 VD MUTE MUTE 9 GND 10 RF- RF+ VRF+ 11 12 13 VCO- 15 350MHz TO 6GHz BB 14 6945 BD 6945fa For more information www.linear.com/LTC6945 9 LTC6945 Operation The LTC6945 is a high performance PLL, and, combined with an external high performance VCO, can produce low noise LO signals up to 6GHz. It is able to achieve superior integrated phase noise performance due to its extremely low in-band phase noise performance. REFERENCE INPUT BUFFER The PLL's reference frequency is applied differentially on pins REF+ and REF-. These high impedance inputs are self-biased and must be AC-coupled with 1F capacitors (see Figure 1 for a simplified schematic). Alternatively, the inputs may be used single-ended by applying the reference frequency at REF+ and bypassing REF- to GND with a 1F capacitor. If the single-ended signal is greater than 2.7VP-P, then use a 47pF capacitor for the GND bypass. VREF+ VREF+ BIAS LOWPASS 1.9V 27 28 REF+ 4.2k 4.2k FILT[1:0] REF - Table 1. FILT[1:0] Programming FILT[1:0] fREF 3 <20MHz 2 NA 1 20MHz to 50MHz 0 >50MHz Table 2. BST Programming BST VREF 1 <2.0VP-P 0 2.0VP-P REFERENCE OUTPUT BUFFER The reference output buffer produces a low noise square wave with a noise floor of -155dBc/Hz (typical) at 10MHz. Its output is low impedance, and produces 2dBm typical output power into a 50 load at 10MHz. Larger output swings will result if driving larger impedances. The output is self-biased, and must be AC-coupled with a 22nF capacitor (see Figure 2 for a simplified schematic). The buffer may be powered down by using bit PDREFO found in the serial port Power register h02. 6945 F01 VREFO+ BST Figure 1. Simplified REF Interface Schematic A high quality signal must be applied to the REF inputs as they provide the frequency reference to the entire PLL. To achieve the part's in-band phase noise performance, apply a CW signal of at least 6dBm into 50, or a square wave of at least 0.5VP-P with slew rate of at least 40V/s. Additional options are available through serial port register h08 to further refine the application. Bits FILT[1:0] control the reference input buffer's lowpass filter, and should be set based upon fREF to limit the reference's wideband noise. The FILT[1:0] bits must be set correctly to reach the LM(NORM) normalized in-band phase noise floor. See Table 1 for recommended settings. The BST bit should be set based upon the input signal level to prevent the reference input buffer from saturating. See Table 2 for recommended settings and the Applications Information section for programming examples. 10 REFO 2 800 6945 F02 Figure 2. Simplified REFO Interface Schematic REFERENCE (R) DIVIDER A 10-bit divider, R_DIV, is used to reduce the frequency seen at the PFD. Its divide ratio R may be set to any integer from 1 to 1023, inclusive. Use the RD[9:0] bits found in registers h03 and h04 to directly program the R divide ratio. See the Applications Information section for the relationship between R and the fREF , fPFD, fVCO and fRF frequencies. 6945fa For more information www.linear.com/LTC6945 LTC6945 Operation PHASE/FREQUENCY DETECTOR (PFD) The phase/frequency detector (PFD), in conjunction with the charge pump, produces source and sink current pulses proportional to the phase difference between the outputs of the R and N dividers. This action provides the necessary feedback to phase-lock the loop, forcing a phase alignment at the PFD's inputs. The PFD may be disabled with the CPRST bit which prevents UP and DOWN pulses from being produced. See Figure 3 for a simplified schematic of the PFD. D Q RST CPRST DELAY Q Table 3. LKWIN[1:0] Programming LKWIN[1:0] tLWW fPFD 0 3ns >5MHz 1 10ns 5MHz 2 30ns 1.7MHz 3 90ns 550kHz The PFD phase difference must be less than tLWW for the COUNTS number of successive counts before the lock indicator asserts the LOCK flag. The LKCT[1:0] bits found in register h09 are used to set COUNTS depending upon the application. See Table 4 for LKCT[1:0] programming and the Applications Information section for examples. UP R DIV D The user sets the phase difference lock window time, tLWW , for a valid LOCK condition with the LKWIN[1:0] bits. See Table 3 for recommended settings for different fPFD frequencies and the Applications Information section for examples. Table 4. LKCT[1:0] Programming DOWN 6945 F03 N DIV LKCT[1:0] COUNTS 0 32 1 128 RST Figure 3. Simplified PFD Schematic LOCK INDICATOR The lock indicator uses internal signals from the PFD to measure phase coincidence between the R and N divider output signals. It is enabled by setting the LKEN bit in the serial port register h07, and produces both LOCK and UNLOCK status flags, available through both the STAT output and serial port register h00. 2 512 3 2048 When the PFD phase difference is greater than tLWW , the lock indicator immediately asserts the UNLOCK status flag and clears the LOCK flag, indicating an out-of-lock condition. The UNLOCK flag is immediately de-asserted when the phase difference is less than tLWW . See Figure 4 for more details. +tLWW PHASE DIFFERENCE AT PFD 0 -tLWW UNLOCK FLAG LOCK FLAG t = COUNTS/fPFD 6945 F04 Figure 4. UNLOCK and LOCK Timing For more information www.linear.com/LTC6945 6945fa 11 LTC6945 Operation CHARGE PUMP The charge pump, controlled by the PFD, forces sink (DOWN) or source (UP) current pulses onto the CP pin, which should be connected to an appropriate loop filter. See Figure 5 for a simplified schematic of the charge pump. VCP+ CHARGE PUMP FUNCTIONS VCP+ + - UP CPUP CPMID 0.9V - + THI CP VCP+/2 - + DOWN CPDN + - 25 TLO 0.9V 6945 F05 Figure 5. Simplified Charge Pump Schematic The output current magnitude ICP may be set from 250A to 11.2mA using the CP[3:0] bits found in serial port register h09. A larger ICP can result in lower in-band noise due to the lower impedance of the loop filter components. See Table 5 for programming specifics and the Applications Information section for loop filter examples. Table 5. CP[3:0] Programming CP[3:0] ICP 0 250A 1 350A 2 500A 3 700A 4 1.0mA 5 1.4mA 6 2.0mA 7 2.8mA 8 4.0mA 9 5.6mA 10 8.0mA 11 11.2mA 12 to 15 Invalid The CPINV bit found in register h0A should be set for applications requiring signal inversion from the PFD, such 12 as for loops using negative-slope tuning oscillators, or inverting op amps in conjunction with positive-slope tuning oscillators. A passive loop filter as shown in Figure 15, used in conjunction with a positive-slope VCO, requires CPINV = 0. The charge pump contains additional features to aid in system start-up and monitoring. See Table 6 for a summary. Table 6. CP Function Bit Descriptions BIT DESCRIPTION CPCHI Enable High Voltage Output Clamp CPCLO Enable Low Voltage Output Clamp CPDN Force Sink Current CPINV Invert PFD Phase CPMID Enable Mid-Voltage Bias CPRST Reset PFD CPUP Force Source Current CPWIDE Extend Current Pulse Width THI High Voltage Clamp Flag TLO Low Voltage Clamp Flag The CPCHI and CPCLO bits found in register h0A enable the high and low voltage clamps, respectively. When CPCHI is enabled and the CP pin voltage exceeds approximately VCP+ - 0.9V, the THI status flag is set, and the charge pump sourcing current is disabled. Alternately, when CPCLO is enabled and the CP pin voltage is less than approximately 0.9V, the TLO status flag is set, and the charge pump sinking current is disabled. See Figure 5 for a simplified schematic. The CPMID bit also found in register h0A enables a resistive VCP+/2 output bias which may be used to prebias troublesome loop filters into a valid voltage range before attempting to lock the loop. When using CPMID, it is recommended to also assert the CPRST bit, forcing a PFD reset. Both CPMID and CPRST must be set to "0" for normal operation. The CPUP and CPDN bits force a constant ICP source or sink current, respectively, on the CP pin. The CPRST bit may also be used in conjunction with the CPUP and CPDN 6945fa For more information www.linear.com/LTC6945 LTC6945 Operation bits, allowing a pre-charge of the loop to a known state, if required. CPUP, CPDN, and CPRST must be set to "0" to allow the loop to lock. The CPWIDE bit extends the charge pump output current pulse width by increasing the PFD reset path's delay value (see Figure 3). CPWIDE is normally set to 0. VCO INPUT BUFFER The VCO frequency is applied differentially on pins VCO+ and VCO-. The inputs are self-biased and must be AC-coupled. Alternatively, the inputs may be used single-ended by applying the VCO frequency at VCO+ and bypassing VCO- to GND with a capacitor. Each input provides a single-ended VVCO+ + - 16 15 VCO+ 121 0.9V VVCO+ VVCO+ OUTPUT (O) DIVIDER The 3-bit O divider can reduce the frequency from the VCO input buffer to the RF output buffer to extend the output frequency range. Its divide ratio O may be set to any integer from 1 to 6, inclusive, outputting a 50% duty cycle even with odd divide values. Use the OD[2:0] bits found in register h08 to directly program the 0 divide ratio. See the Applications Information section for the relationship between O and the fREF , fPFD, fVCO and fRF frequencies. RF OUTPUT BUFFER The low noise, differential output buffer produces a differential output power of -6dBm to 3dBm, settable with bits RFO[1:0] according to Table 7. The outputs may be combined externally, or used individually. Terminate any unused output with a 50 resistor to VRF+. Table 7. RFO[1:0] Programming 121 VC0- 6945 F06 Figure 6. Simplified VCO Interface Schematic 121 resistance to aid in impedance matching at high frequencies. See the Applications Information section for matching guidelines. RFO[1:0} PRF (Differential) PRF (Single-Ended) 0 -6dBm -9dBm 1 -3dBm -6dBm 2 0dBm -3dBm 3 3dBm 0dBm Each output is open collector with 136 pull-up resistors to VRF+, easing impedance matching at high frequencies. See Figure 7 for circuit details and the Applications Information section for matching guidelines. The buffer may be muted with either the OMUTE bit, found in register h02, or by forcing the MUTE input low. VCO (N) DIVIDER The 16-bit N divider provides the feedback from the VCO input buffer to the PFD. Its divide ratio N may be set to any integer from 32 to 65535, inclusive. Use the ND[15:0] bits found in registers h05 and h06 to directly program the N divide ratio. See the Applications Information section for the relationship between N and the fREF , fPFD, fVCO and fRF frequencies. VRF+ VRF+ 136 136 RF+ RF- 9 MUTE OMUTE 12 11 MUTE RFO[1:0] 6945 F07 Figure 7. Simplified RF Interface Schematic 6945fa For more information www.linear.com/LTC6945 13 LTC6945 Operation SERIAL PORT Single Byte Transfers The SPI-compatible serial port provides control and monitoring functionality. A configurable status output, STAT, gives additional instant monitoring. The serial port is arranged as a simple memory map, with status and control available in 12, byte-wide registers. All data bursts are comprised of at least two bytes. The 7 most significant bits of the first byte are the register address, with an LSB of 1 indicating a read from the part, and LSB of 0 indicating a write to the part. The subsequent byte, or bytes, is data from/to the specified register address. See Figure 10 for an example of a detailed write sequence, and Figure 11 for a read sequence. Communication Sequence The serial bus is comprised of CS, SCLK, SDI and SDO. Data transfers to the part are accomplished by the serial bus master device first taking CS low to enable the LTC6945's port. Input data applied on SDI is clocked on the rising edge of SCLK, with all transfers MSB first. The communication burst is terminated by the serial bus master returning CS high. See Figure 8 for details. Figure 12 shows an example of two write communication bursts. The first byte of the first burst sent from the serial bus master on SDI contains the destination register address (Addr0) and an LSB of "0" indicating a write. The next byte is the data intended for the register at address Addr0. CS is then taken high to terminate the transfer. The first byte of the second burst contains the destination register address (Addr1) and an LSB indicating a write. The next byte on SDI is the data intended for the register at address Addr1. CS is then taken high to terminate the transfer. Data is read from the part during a communication burst using SDO. Readback may be multidrop (more than one LTC6945 connected in parallel on the serial bus), as SDO is three-stated (Hi-Z) when CS = 1, or when data is not being read from the part. If the LTC6945 is not used in a multidrop configuration, or if the serial port master is not capable of setting the SDO line level between read sequences, it is recommended to attach a high value resistor of greater than 200k between SDO and GND to ensure the line returns to a known level during Hi-Z states. See Figure 9 for details. MASTER-CS tCSS tCKL tCKH tCSS tCSH MASTER-SCLK tCS MASTER-SDI tCH DATA DATA 6945 F07 Figure 8. Serial Port Write Timing Diagram MASTER-CS 8TH CLOCK MASTER-SCLK tDO LTC6945-SDO Hi-Z tDO DATA Figure 9. Serial Port Read Timing Diagram 14 tDO tDO For more information www.linear.com/LTC6945 DATA Hi-Z 6945 F09 6945fa LTC6945 Operation MASTER-CS 16 CLOCKS MASTER-SCLK 7-BIT REGISTER ADDRESS MASTER-SDI LTC6945-SD0 8 BITS OF DATA A6 A5 A4 A3 A2 A1 A0 0 D7 D6 D5 D4 D3 D2 D1 D0 0 = WRITE Hi-Z 6945 F10 Figure 10. Serial Port Write Sequence MASTER-CS 16 CLOCKS MASTER-SCLK 7-BIT REGISTER ADDRESS MASTER-SDI 1 = READ A6 A5 A4 A3 A2 A1 A0 1 8 BITS OF DATA LTC6945-SDO Hi-Z X D7 D6 D5 D4 D3 D2 D1 D0 DX Hi-Z 6945 F11 Figure 11. Serial Port Read Sequence MASTER-CS Addr0 + Wr MASTER-SDI LTC6945-SDO Byte 0 Addr1 + Wr Byte 1 Hi-Z 6945 F12 Figure 12. Serial Port Single Byte Write Multiple Byte Transfers More efficient data transfer of multiple bytes is accomplished by using the LTC6945's register address autoincrement feature as shown in Figure 13. The serial port master sends the destination register address in the first byte and its data in the second byte as before, but continues sending bytes destined for subsequent registers. Byte 1's address is Addr0+1, Byte 2's address is Addr0+2, and so on. If the resister address pointer attempts to increment past 11 (h0B), it is automatically reset to 0. An example of an auto-increment read from the part is shown in Figure 14. The first byte of the burst sent from the serial bus master on SDI contains the destination register address (Addr0) and an LSB of "1" indicating a read. Once the LTC6945 detects a read burst, it takes SDO out of the Hi-Z condition and sends data bytes sequentially, beginning with data from register Addr0. The part ignores all other data on SDI until the end of the burst. Multidrop Configuration Several LTC6945s may share the serial bus. In this multidrop configuration, SCLK, SDI and SDO are common between all parts. The serial bus master must use a separate CS for each LTC6945 and ensure that only one device has CS asserted at any time. It is recommended to attach a high value resistor to SDO to ensure the line returns to a known level during Hi-Z states. 6945fa For more information www.linear.com/LTC6945 15 LTC6945 Operation MASTER-CS Addr0 + Wr MASTER-SDI LTC6945-SDO Byte 0 Byte 1 Byte 2 Hi-Z 6945 F12 Figure 13. Serial Port Auto-Increment Write MASTER-CS Addr0 + Rd MASTER-SDI LTC6945-SDO Hi-Z DON'T CARE Byte 0 Byte 1 Hi-Z Byte 2 6945 F13 Figure 14. Serial Port Auto-Increment Read Serial Port Registers or expanded: The memory map of the LTC6945 may be found in Table 8, with detailed bit descriptions found in Table 9. The register address shown in hexadecimal format under the ADDR column is used to specify each register. Each register is denoted as either read-only (R) or read-write (R/W). The register's default value on device power-up or after a reset is shown at the right. The read-only register at address h00 is used to determine different status flags. These flags may be instantly output on the STAT pin by configuring register h01. See the STAT Output section for more information. The read-only register at address h0B is a ROM byte for device identification. STAT Output The STAT output pin is configured with the x[5:0] bits of register h01. These bits are used to bit-wise mask, or enable, the corresponding status flags of status register h00, according to Equation 1. The result of this bit-wise Boolean operation is then output on the STAT pin: STAT = OR (Reg00[5,2:0] AND Reg01[5,2:0]) 16 STAT = (UNLOCK AND x[5]) OR (LOCK AND x[2]) OR (THI AND x[1]) OR (TLO AND x[0]) For example, if the application requires STAT to go high whenever the LOCK or THI flags are set, then x[2] and x[1] should be set to "1", giving a register value of h6. Block Power-Down Control The LTC6945's power-down control bits are located in register h02, described in Table 9. Different portions of the device may be powered down independently. Care must be taken with the LSB of the register, the POR (power-on reset) bit. When written to a "1", this bit forces a full reset of the part's digital circuitry to its power-up default state. (1) 6945fa For more information www.linear.com/LTC6945 LTC6945 Operation Table 8. Serial Port Register Contents ADDR MSB [6] [5] h00 h01 h02 [4] [3] [2] * * UNLOCK * * x[5] PDALL PDPLL * PDOUT [1] LSB R/W DEFAULT * * LOCK THI TLO R * * x[2] x[1] x[0] R/W h04 PDREFO * OMUTE POR R/W h0E h03 * * * * * * RD[9] RD[8] R/W h00 h04 RD[7] RD[6] RD[5] RD[4] RD[3] RD[2] RD[1] RD[0] R/W h01 h05 ND[15] ND[14] ND[13] ND[12] ND[11] ND[10] ND[9] ND[8] R/W h00 h06 ND[7] ND[6] ND[5] ND[4] ND[3] ND[2] ND[1] ND[0] R/W hFA h07 * * * * * * * LKEN R/W h01 h08 BST FILT[1] FILT[0] RFO[1] RFO[0] OD[2] OD[1] OD[0] R/W hF9 h09 LKWIN[1] LKWIN[0] LKCT[1] LKCT[0] CP[3] CP[2] CP[1] CP[0] R/W h9B h0A CPCHI CPCLO CPMID CPINV CPWIDE CPRST CPUP CPDN R/W hE4 h0B REV[2] REV[1] REV[0] PART[4] PART[3] PART[2] PART[1] PART[0] R h40 *unused Table 9. Serial Port Register Bit Field Summary BITS DESCRIPTION DEFAULT BITS BST REF Buffer Boost Current 1 OD[2:0] DESCRIPTION Output Divider Value (0 < OD[2:0] < 7) h1 CP[3:0] CP Output Current hB OMUTE Mutes RF Output 1 CPCHI CP Enable Hi Voltage Output Clamp 1 PART[4:0] CPCLO CP Enable Low Voltage Output Clamp 1 PDALL Full Chip Power-Down 0 CPDN CP Pump Down Only 0 PDOUT Powers Down O_DIV, RF Output Buffer 0 PDPLL Powers Down REF, REFO, R_DIV, PFD, CPUMP, N_DIV 0 CPINV CP Invert Phase 0 CPMID CP Bias to Mid-Rail 1 CPRST CP Three-State 1 CPUP CP Pump Up Only 0 CPWIDE CP Extend Pulse Width 0 FILT[1:0] REF Input Buffer Filter h3 LKCT[1:0] PLL Lock Cycle Count h1 PLL Lock Indicator Enable 1 LKEN LKWIN[1:0] PLL Lock Indicator Window LOCK ND[15:0] h2 PLL Lock Indicator Flag N Divider Value (ND[15:0] > 31) h00FA PDREFO POR Part Code DEFAULT h00 Powers Down REFO 1 Force Power-On Reset Register Initialization 0 RD[9:0] R Divider Value (RD[9:0] > 0) REV[2:0] Rev Code RFO[1:0] RF Output Power THI CP Clamp High Flag TLO CP Clamp Low Flag UNLOCK PLL Unlock Flag x[5,2:0] STAT Output OR Mask h001 h3 h04 6945fa For more information www.linear.com/LTC6945 17 LTC6945 Applications Information INTRODUCTION A PLL is a complex feedback system that may conceptually be considered a frequency multiplier. The system multiplies the frequency input at REF and outputs a higher frequency at RF. The PFD, charge pump, N divider, and external VCO and loop filter form a feedback loop to accurately control the output frequency (see Figure 15). The R and O dividers are used to set the output frequency resolution. LTC6945 REF (fREF) R_DIV ICP fPFD /R LOOP FILTER CP RZ KPFD CP CI N_DIV LF(s) /N RF O_DIV (fRF) /O VCO (fVCO) KVCO 6945 F15 Figure 15. PLL Loop Diagram OUTPUT FREQUENCY When the loop is locked, the frequency fVCO (in Hz) produced at the output of the VCO is determined by the reference frequency fREF , and the R and N divider values, given by Equation 2: f *N fVCO = REF R (2) Here, the PFD frequency fPFD produced is given by the following equation: fPFD = fREF R (3) and fVCO may be alternatively expressed as: fVCO = fPFD * N The output frequency fRF produced at the output of the O divider is given by Equation 4: fRF = 18 fVCO O Using the above equations, the output frequency resolution fSTEP produced by a unit change in N is given by Equation 5: fSTEP = fREF R*O (5) LOOP FILTER DESIGN A stable PLL system requires care in selecting the external loop filter values. The Linear Technology PLLWizard application, available from www.linear.com, aids in design and simulation of the complete system. The loop design should use the following algorithm: 1. Determine the output frequency, fRF , and frequency step size, fSTEP , based on application constraints. Using Equations 2, 3, 4 and 5, change fREF , N, R and O until the application frequency constraints are met. Use the minimum R value that still satisfies the constraints. 2. Select the loop bandwidth BW constrained by fPFD. A stable loop requires that BW is less than fPFD by at least a factor of 10. 3. Select loop filter component RZ and charge pump current ICP based on BW and the VCO gain factor KVCO. BW (in Hz) is approximated by the following equation: BW or : RZ = ICP * RZ * K VCO 2 * *N (6) 2 * * BW * N ICP * K VCO where KVCO is in Hz/V, ICP is in Amps, and RZ is in Ohms. KVCO is the VCO's frequency tuning sensitivity, and may be determined from the VCO specifications. Use ICP = 11.2mA to lower in-band noise unless component values force a lower setting. (4) 6945fa For more information www.linear.com/LTC6945 LTC6945 Applications Information 4. Select loop filter components CI and CP based on BW and RZ . A reliable loop can be achieved by using the following equations for the loop capacitors (in Farads): CI = 3.5 2 * * BW * RZ (7) 1 7 * * BW * RZ (8) CP = LOOP FILTERS USING AN OPAMP Some VCO tune voltage ranges are greater than the LTC6945's charge pump voltage range. An active loop filter using an op amp can increase the tuning voltage range. To maintain the LTC6945's high performance, care must be given to picking an appropriate op amp. The op amp input common mode voltage should be biased within the LTC6945 charge pump's voltage range, while its output voltage should achieve the VCO tuning range. See Figure 16 for an example op amp loop filter. The op amp's input bias current is supplied by the charge pump; minimizing this current keeps spurs related to fPFD low. The input bias current should be less than the charge pump leakage (found in the Electrical Characteristics section) to avoid increasing spurious products. CP ICP RZ LTC6945 5k 5k VCO + This programming example uses the DC1649. Assume the following parameters of interest : fREF = 100MHz at 7dBm into 50 fSTEP = 250kHz fVCO = 902MHz to 928MHz Determining Divider Values RP2 VCP+ DESIGN AND PROGRAMMING EXAMPLE fRF = 914MHz LF(s) - CP An additional R-C lowpass filter (formed by RP2 and CP2 in Figure 16) connected at the input of the VCO will limit the op amp noise sources. The bandwidth of this filter should be placed approximately 15 to 20 times the PLL loop bandwidth to limit loop phase margin degradation. RP2 should be small (preferably much less than RZ) to minimize its noise impact on the loop. However, picking too small of a value can make the op amp unstable as it has to drive the capacitor in this filter. KVCO = 15MHz/V to 21.6MHz/V LOOP FILTER CI Op amp noise sources are highpass filtered by the PLL loop filter and should be kept at a minimum, as their effect raises the total system phase noise beginning near the loop bandwidth. Choose a low noise op amp whose input-referred voltage noise is less than the thermal noise of RZ. Additionally, the gain bandwidth of the op amp should be at least 15 times the loop bandwidth to limit phase margin degradation. The LT1678 is an op amp that works very well in most applications. CP2 Following the Loop Filter Design algorithm, first determine all the divider values. Using Equations 2, 3, 4 and 5, calculate the following values: O=1 VCP+/2 R = 100MHz/250kHz = 400 47F fPFD = 250kHz KVCO (fVCO) 6945 F16 N = 914MHz/250kHz = 3656 Figure 16. Op Amp Loop Filter 6945fa For more information www.linear.com/LTC6945 19 LTC6945 Applications Information The next step in the algorithm is to determine the openloop bandwidth. BW should be at least 10x smaller than fPFD. Wider loop bandwidths could have lower integrated phase noise, depending on the VCO phase noise signature, while narrower bandwidths will likely have lower spurious power. Use a factor of 25 for this design: BW = 250kHz = 10kHz 25 Divider Programming Program registers Reg03 to Reg06 with the previously determined R and N divider values: Reg03 = h01 Reg04 = h90 Reg05 = h0E Reg06 = h48 Loop Filter Component Selection Now set loop filter resistor RZ and charge pump current ICP . Because the KVCO varies over the VCO's frequency range, using the KVCO geometric mean gives good results. Using an ICP of 11.2mA, RZ is determined: K VCO = 106 * 15 * 21.6 = 18MHz / V Reference Input Settings and Output Divider Programming From Table 1, FILT = 0 for a 100MHz reference frequency. Next, convert 7dBm into VP-P . For a CW tone, use the following equation with R = 50: VP-P R * 10(dBm - 21)/20 2 * * 10k * 3656 RZ = 11.2m * 18M RZ = 1.14k (9) This gives VP-P = 1.41V, and, according to Table 2, set BST = 1. Now calculate CI and CP from Equations 7 and 8: 3.5 = 48.9nF 2 * * 10k * 1.14k 1 CP = = 3.99nF 7 * * 10k * 1.14k CI = Now program Reg08, assuming maximum RF output power (RFO[1:0] = 3 according to Table 7) and OD[2:0] = 1: Reg08 = h99 Lock Detect and Charge Pump Current Programming Status Output Programming This example will use the STAT pin to monitor a phase lock condition. Program x[2] = 1 to force the STAT pin high whenever the LOCK bit asserts: Reg01 = h04 Power Register Programming For correct PLL operation all internal blocks should be enabled, but PDREFO should be set if the REFO pin is not being used. OMUTE may remain asserted (or the MUTE pin held low) until programming is complete. For PDREFO = 1 and OMUTE = 1: Next determine the lock indicator window from fPFD. From Table 3, LKWIN[1:0] = 3 for a tLWW of 90ns. The LTC6945 will consider the loop "locked" as long as the phase coincidence at the PFD is within 8, as calculated below: phase = 360 * tLWW * fPFD = 360 * 90n * 250k 8 LKWIN[1:0] may be set to a smaller value to be more conservative. However, the inherent phase noise of the loop could cause false "unlocks" for too small a value. Choosing the correct COUNTS depends upon the ratio of the bandwidth of the loop to the PFD frequency (BW/fPFD). Smaller ratios dictate larger COUNTS values. A COUNTS value of 128 will work for the ratio of 1/25. From Table 4, LKCT[1:0] = 1 for 128 counts. Reg02 = h0A 20 6945fa For more information www.linear.com/LTC6945 LTC6945 Applications Information Using Table 5 with the previously selected ICP of 11.2mA, gives CP[3:0] = 11. This is enough information to program Reg09: The LTC6945 achieves an in-band normalized phase noise floor of -226dBc/Hz (typical). To calculate its equivalent input phase noise floor LM(IN), use Equation 10: LM(IN) = -226 + 10 * log10(fREF) Reg09 = hDB To enable the lock indicator, write Reg07: Reg07 = h01 Charge Pump Function Programming The DC1649 includes an LT1678I op amp in the loop filter. This allows the circuit to reach the voltage range specified for the VCO's tuning input. However, it also adds an inversion in the loop transfer function. Compensate for this inversion by setting CPINV = 1. This example does not use the additional voltage clamp features to allow fault condition monitoring. The loop feedback provided by the op amp will force the charge pump output to be equal to the op amp positive input pin's voltage. Disable the charge pump voltage clamps by setting CPCHI = 0 and CPCLO = 0. Disable all the other charge pump functions (CPMID, CPRST, CPUP and CPDN) to allow the loop to lock: Reg0A = h10 The loop should now lock. Now unmute the output by setting OMUTE = 0 (assumes the MUTE pin is high): Reg02 = h08 REFERENCE SOURCE CONSIDERATIONS A high quality signal must be applied to the REF inputs as they provide the frequency reference to the entire PLL. As mentioned previously, to achieve the part's in-band phase noise performance, apply a CW signal of at least 6dBm into 50, or a square wave of at least 0.5VP-P with slew rate of at least 40V/s. The LTC6945 may be driven single-ended to CMOS levels (greater than 2.7VP-P). Apply the reference signal directly without a DC-blocking capacitor at REF+, and bypass REF- to GND with a 47pF capacitor. The BST bit must also be set to "0", according to guidelines given in Table 2. (10) For example, using a 10MHz reference frequency gives an input phase noise floor of -156dBc/Hz. The reference frequency source's phase noise must be approximately 3dB better than this to prevent limiting the overall system performance. IN-BAND OUTPUT PHASE NOISE The in-band phase noise produced at fRF may be calculated by using Equation 11. LM(OUT) = -226 + 10 * log10 ( fPFD ) (11) f +20 * log10 RF fPFD or LM(OUT) = -226 + 10 * log10 ( fPFD ) N +20 * log10 O As seen for a given PFD frequency fPFD, the output in-band phase noise increases at a 20dB-per-decade rate with the N divider count. So, for a given output frequency fRF, fPFD should be as large as possible (or N should be as small as possible) while still satisfying the application's frequency step size requirements. OUTPUT PHASE NOISE DUE TO 1/f NOise In-band phase noise at very low offset frequencies may be influenced by the LTC6945's 1/f noise, depending upon fPFD. Use the normalized in-band 1/f noise of -274dBc/Hz with Equation 12 to approximate the output 1/f phase noise at a given frequency offset fOFFSET: LM(OUT -1/f) (fOFFSET) = -274 + 20 * log10(fRF) - 10 * log10(fOFFSET) (12) 6945fa For more information www.linear.com/LTC6945 21 LTC6945 Applications Information Unlike the in-band noise floor LM(OUT), the 1/f noise LM(OUT -1/f) does not change with fPFD and is not constant over offset frequency. See Figure 17 for an example of in-band phase noise for fPFD equal to 3MHz and 100MHz. The total phase noise will be the summation of LM(OUT) and LM(OUT -1/f). -90 PHASE NOISE (dBc/Hz) TOTAL NOISE fPFD = 3MHz -100 TOTAL NOISE fPFD = 100MHz -110 -120 1/f NOISE CONTRIBUTION -130 10 1k 100 10k OFFSET FREQUENCY (Hz) 100k 6945 F17 Figure 17. Theoretical In-Band Phase Noise, fRF = 2500MHz Table 10. Single-Ended VCO+ Input Impedance FREQUENCY (MHz) IMPEDANCE () S11 (dB) 250 118 - j78 -5.06 500 83.6 - j68.3 -5.90 1000 52.8 - j56.1 -6.38 1500 35.2 - j41.7 -6.63 2000 25.7 - j30.2 -6.35 2500 19.7 - j20.6 -5.94 3000 17.6 - j11.2 -6.00 3500 17.8 - j3.92 -6.41 4000 19.8 + j4.74 -7.20 4500 21.5 + j15.0 -7.12 5000 21.1 + j19.4 -6.52 5500 27.1 + j22.9 -7.91 6000 38.3 + j33.7 -8.47 6500 36.7 + j42.2 -6.76 7000 46.2 + j40.9 -8.11 7500 76.5 + j36.8 -9.25 8000 84.1+ j52.2 -7.27 Table 11. Single-Ended RF Output Impedance VCO INPUT MATCHING The VCO inputs may be used differentially or single-ended. Each input provides a single-ended 121 resistance to aid in impedance matching at high frequencies. The inputs are self-biased and must be AC-coupled using a 100pF capacitors (or 270pF for VCO frequencies less than 500MHz). The inputs may be used single-ended by applying the ACcoupled VCO frequency at VCO+ and bypassing VCO- to GND with a 100pF capacitor (270pF for frequencies less than 500MHz). Measured VCO+ s-parameters (with VCO- bypassed with 100pF to GND) are shown in Table 10 to aid in the design of external impedance matching networks. FREQUENCY (MHz) IMPEDANCE () S11 (dB) 500 102.8 - j49.7 -6.90 1000 70.2 - j60.1 -6.53 1500 52.4 - j56.2 -6.35 2000 43.6 - j49.2 -6.58 2500 37.9 - j39.6 -7.34 3000 32.7 - j28.2 -8.44 3500 27.9 - j17.8 -8.99 4000 24.3 - j9.4 -8.72 4500 22.2 - j3.3 -8.26 5000 21.6 + j1.9 -8.02 5500 21.8 + j6.6 -7.91 6000 23.1 + j11.4 -8.09 6500 25.7 + j16.9 -8.38 RF OUTPUT MATCHING 7000 29.3 + j23.0 -8.53 The RF outputs may be used in either single-ended or differential configurations. Using both RF outputs differentially will result in approximately 3dB more output power than single-ended. Impedance matching to an external load in both cases requires external chokes tied to VRF+. Measured RF s-parameters are shown below in Table 11 to aid in the design of impedance matching networks. 7500 33.5 + j28.4 -8.56 8000 37.9 + j32.6 -8.64 22 6945fa For more information www.linear.com/LTC6945 LTC6945 Applications Information VRF+ 0 68nH, 100pF 180nH, 270pF -2 RF+(-) -4 CS S11 (dB) LC 50 RF-(+) -8 -10 VRF+ LC -6 -12 -14 CS -16 TO 50 LOAD 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 FREQUENCY (GHz) 6945 F18 6945 F19 Figure 18. Single-Ended Output Matching Schematic Figure 19. Single-Ended Return Loss Single-ended impedance matching is accomplished using the circuit of Figure 18, with component values found in Table 12. Using smaller inductances than recommended can cause phase noise degradation, especially at lower center frequencies. For lower frequencies, transmission line (TL) baluns such as the M/A-COM MABACT0065 and the TOKO #617DB-1673 provide good results. At higher frequencies, surface mount (SMT) baluns such as those produced by TDK, Anaren, and Johanson Technology, can be attractive alternatives. See Table 13 for recommended balun part numbers versus frequency range. Table 12. Suggested Single-Ended Matching Component Values fRF (MHz) LC (nH) CS (pF) 350 to 1500 180nH 270pF 1000 to 5800 68nH 100pF Return loss measured on the DC1649 using the above component values is shown in Figure 19. A broadband match is achieved using an (LC, CS) of either (68nH, 100pF) or (180nH, 270pF). However, for maximum output power and best phase noise performance, use the recommended component values of Table 12. LC should be a wirewound inductor selected for maximum Q factor and SRF, such as the Coilcraft HP series of chip inductors. The LTC6945's differential RF outputs may be combined using an external balun to drive a single-ended load. The advantages are approximately 3dB more output power than each output individually and better 2nd-order harmonic performance. The listed SMT baluns contain internal chokes to bias RF and also provide input-to-output DC isolation. The pin denoted as GND or DC FEED should be connected to the VRF+ voltage. Figure 20 shows a surface mount balun's connections with a DC FEED pin. VRF+ RF+ 12 3 LTC6945 RF- 2 1 TO 50 LOAD BALUN 11 4 5 6 6945 F20 BALUN PIN CONFIGURATION 1 UNBALANCED PORT 2 GND OR DC FEED 3 BALANCED PORT 4 BALANCED PORT 5 GND 6 NC Figure 20. Example of a SMT Balun Connection 6945fa For more information www.linear.com/LTC6945 23 LTC6945 Applications Information REFERENCE SIGNAL ROUTING AND SPURIOUS Table 13. Suggested Baluns fRF (MHz) PART NUMBER MANUFACTURER TYPE 350 to 900 #617DB-1673 TOKO TL 400 to 600 HHM1589B1 TDK SMT 600 to 1400 BD0810J50200 Anaren SMT 600 to 3000 MABACT0065 M/A-COM TL 1000 to 2000 HHM1518A3 TDK SMT 1400 to 2000 HHM1541E1 TDK SMT 1900 to 2300 2450BL15B100E Johanson SMT 2000 to 2700 HHM1526 TDK SMT 3700 to 5100 HHM1583B1 TDK SMT 4000 to 6000 HHM1570B1 TDK SMT The listed TL baluns do not provide input-to-output DC isolation and must be AC coupled at the output. Figure 21 displays RF connections using these baluns. VRF+ RF+ TO 50 LOAD 12 LTC6945 RF- The charge pump operates at the PFD's update frequency fPFD. The resultant output spurious energy is small and is further reduced by the loop filter before it modulates the VCO frequency. However, improper PCB layout can degrade the LTC6945's inherent spurious performance. Care must be taken to prevent the reference signal fREF from coupling onto the VCO's tune line, or into other loop filter signals. Example suggestions are the following. 1. Do not share power supply decoupling capacitors between same voltage power supply pins. 2. Use separate ground vias for each power supply decoupling capacitor, especially those connected to VREF+, VCP+, and VVCO+. 3. Physically separate the reference frequency signal from the loop filter and VCO. PRI 11 SEC 6945 F21 Figure 21. Example of a TL Balun Connection SUPPLY BYPASSING AND PCB LAYOUT GUIDELINES Care must be taken when creating a PCB layout to minimize power supply decoupling and ground inductances. All power supply V+ pins should be bypassed directly to the ground plane using a 0.1F ceramic capacitor as close to the pin as possible. Multiple vias to the ground plane should be used for all ground connections, including to the power supply decoupling capacitors. 6945 F22 Figure 22. Example Exposed Pad Land Pattern The package's exposed pad is a ground connection, and must be soldered directly to the PCB land. The PCB land pattern should have multiple thermal vias to the ground plane for both low ground inductance and also low thermal resistance (see Figure 22 for an example). See QFN Package Users Guide, page 8, on Linear Technology website's Packaging Information page for specific recommendations concerning land patterns and land via solder masks. Links are provided below. http://www.linear.com/designtools/packaging 24 6945fa For more information www.linear.com/LTC6945 LTC6945 Typical Applications LTC6945 Wideband Frequency Hopping Local Oscillator 5V 5V 4.99k 4.99k 4.99k 4.99k 0.1F 0.1F 14V 0.1F 14V 0.1F 47F + 3.3V LT1678IS8 1F 22nF 100MHz REF 5V 0.1F 1F 0.1F 3.3V 274 3.3V 0.01F REF- REF+ VREF+ CP VCP+ GND VREFO+ VVCO+ REFO LTC6945 STAT CS GND SCLK SPI BUS O_DIV = 2 0.1F 51.1 12V RFMD UMS-1400-A16-G 100 VTUNE MUTE1 180nH VD+ VCO- MUTE GND RF- RF+ VRF+ BB 0.1F MUTE1 3.3V MUTE2 100pF 100pF 1.0F 3.3V 3.3V 0.01F 0V 3.3V 3.3V 0V 180nH 270pF 180nH 3.3V 270pF 100ns 50 270pF LO2 = 350MHz TO 700MHz LO1 = 350MHz TO 700MHz 650 550 450 MUTE VOLTAGES (V) MUTE1 1 -0.2 0.2 0 TIME (s) -20 -30 -5.0 -40 MUTE1 2 -0.4 -10 -4.5 MUTE2 3 0 -0.6 0 -3.5 -4.0 0.4 0.6 6945 TA02b POWER (dBm) LOOUT POWER (MHz) 750 MUTE2 3 1 2 S LO OUT 6945 TA02a fPFD = 1MHz OD = 2 -50 -60 -70 -80 2 -90 1 0 -0.6 POWER COMBINER Frequency Hopping LOOUT Spectrum, LO1 = 450MHz Muted, LO2 = 700MHz Frequency Hopping LOOUT Power, LO1 = 450MHz, LO2 = 700MHz MUTE VOLTAGES (V) LOOUT FREQUENCY (MHz) Frequency Hopping LOOUT Frequency, LO1 = 450MHz, LO2 = 700MHz 700MHz TO 1400MHz VCO+ O_DIV = 2 MUTE2 270pF 50 GND GND SDO 100pF 13.3nF GND SDI 700MHz TO 1400MHz VTUNE 12V RFMD UMS-1400-A16-G GND GND SCLK SPI BUS LOOP BANDWIDTH = ~7.6kHz 100 GND LTC6945 + - 267nF 3.3V 0.01F REF- REF+ VREF+ CP VCP+ GND VREFO+ VVCO+ 3.3V 3.3V 0.01F 180nH 0.1F 3.3V 274 REFO 1.0F 3.3V 22nF 1F CS VCO+ LT1678IS8 5V 0.1F 267nF STAT 100pF 3.3V 1F GND VD+ VCO- MUTE GND RF- RF+ VRF+ BB 3.3V 0.1F GND GND SDO LOOP BANDWIDTH = ~7.6kHz 13.3nF GND GND SDI + - + 0.1F 47F -100 -0.4 -0.2 0.2 0 TIME (s) 0.4 0.6 6945 TA02c -110 400 -95dBc 450 500 550 600 650 FREQUENCY (MHz) 700 750 6945 TA02d 6945fa For more information www.linear.com/LTC6945 25 LTC6945 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UFD Package 28-Lead Plastic QFN (4mm x 5mm) (Reference LTC DWG # 05-08-1712 Rev B) 0.70 0.05 4.50 0.05 3.10 0.05 2.50 REF 2.65 0.05 3.65 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 3.50 REF 4.10 0.05 5.50 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 0.10 (2 SIDES) 0.75 0.05 R = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.35 x 45 CHAMFER 2.50 REF R = 0.115 TYP 27 28 0.40 0.10 PIN 1 TOP MARK (NOTE 6) 1 2 5.00 0.10 (2 SIDES) 3.50 REF 3.65 0.10 2.65 0.10 (UFD28) QFN 0506 REV B 0.200 REF 0.00 - 0.05 0.25 0.05 0.50 BSC BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X). 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 26 6945fa For more information www.linear.com/LTC6945 LTC6945 Revision History REV DATE DESCRIPTION A 3/15 Changed operating core temperature to operating junction temperature. PAGE NUMBER 2 Updated power supply currents. 4 6945fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representaFor more information www.linear.com/LTC6945 tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. 27 LTC6945 Typical Application LTC6945 Wideband Point-to-Point Radio Local Oscillator 5V 4.99k 4.99k 0.1F 11V 0.1F 47F + 51.1 100MHz REF 18.3nF 5V 0.1F 1F 0.1F 3.3V 1F 113 CS SCLK 3.3V 0.1F -80 100 VTUNE -90 5V RFMD UMZ-T2-227-O16-G 4.7nF GND GND GND GND SDI GND SDO VCO+ Radio Local Oscillator Phase Noise, fRF = 5725MHz LOOP BANDWIDTH = ~21.4kHz 230nF GND LTC6945 STAT + - 3.3V 0.01F REF- REF+ VREF+ CP VCP+ GND VREFO+ VVCO+ REFO SPI BUS LT1678IS8 100pF PHASE, NOISE (dBc/Hz) 0.1F 3.3V -100 -110 -120 -130 -140 -150 O_DIV = 2 VD+ VCO- MUTE GND RF- RF+ VRF+ BB 6945 TA03a RMS NOISE = 0.47 RMS JITTER = 230fs fPFD = 5MHz BW = 21kHz -160 100 100pF 1.0F 1k 1M 10k 100k OFFSET FREQUENCY (Hz) 10M 40M 6945 TA03b 3.3V 0.01F 3.3V 68nH 68nH 3.3V 100pF 50 100pF LO OUT 4900MHz TO 5900MHz IN STEPS OF 5MHz Related Parts PART NUMBER DESCRIPTION COMMENTS LTC6946 Ultralow Noise and Spurious Integer-N Synthesizer with VCO 370MHz to 6.4GHz, -226dBc/Hz Normalized In-Band Phase Noise Floor LTC6947 Ultralow Noise and Spurious Fractional-N Synthesizer 350MHz to 6GHz, -226dBc/Hz Normalized In-Band Phase Noise Floor LTC6948 Ultralow Noise and Spurious Frac-N Synthesizer with VCO 370MHz to 6.4GHz, -226dBc/Hz Normalized In-Band Phase Noise Floor LTC6950 Low Phase Noise and Spurious Integer-N PLL Core with Five Output Clock Distribution and EZSync 1.4GHz Max VCO Frequency, Additive Jitter <20fsRMS, -226dBc/Hz Normalized In-Band Phase Noise Floor LTC6957 Low Phase Noise, Dual Output Buffer/Driver/Logic Converter Optimized Conversion of Sine Waves to Logic Levels, LVPECL/LVDS/ CMOS LTC2000 16-/14-/11-Bit 2.5Gsps DAC Superior 80dBc SFDR at 70MHz Output, 40mA Nominal Drive and High Linearity LTC5569 Broadband Dual Mixer 300MHz to 4GHz, 26.8dBm IIP3, 2dB Gain, 11.7dB NF, 600mW Power LTC5588-1 Ultrahigh OIP3 I/Q Modulator 200MHz to 6GHz, 31dBm OIP3, -160.6dBm/Hz Noise Floor LT 5575 Direct Conversion I/Q Demodulator 800MHz to 2.7GHz, 22.6dBm IIP3, 60dBm IIP2, 12.7dB NF (R) 28 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LTC6945 (408) 432-1900 FAX: (408) 434-0507 www.linear.com/LTC6945 6945fa LT 0315 REV A * PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2011