LT1241 Series High Speed Current Mode Pulse Width Modulators FEATURES DESCRIPTION Low Start-Up Current: < 250A nn 50ns Current Sense Delay nn Current Mode Operation: To 500kHz nn Pin Compatible with UC1842 Series nn Undervoltage Lockout with Hysteresis nn No Cross-Conduction Current nn Trimmed Bandgap Reference nn 1A Totem Pole Output nn Trimmed Oscillator Frequency and Sink Current nn Active Pull-Down on Reference and Output During Undervoltage Lockout nn High Level Output Clamp: 18V nn Current Sense Leading Edge Blanking The LT (R)1241 series devices are 8-pin, fixed frequency, current mode, pulse width modulators. They are improved plug compatible versions of the industry standard UC1842 series. These devices have both improved speed and lower quiescent current. The LT1241 series is optimized for off-line and DC/DC converter applications. They contain a temperature-compensated reference, high gain error amplifier, current sensing comparator and a high current totem pole output stage ideally suited to driving power MOSFETs. Start-up current has been reduced to less than 250A. Cross-conduction current spikes in the output stage have been eliminated, making 500kHz operation practical. Several new features have been incorporated. Leading edge blanking has been added to the current sense comparator. Trims have been added to the oscillator circuit for both frequency and sink current, and both of these parameters are tightly specified. The output stage is clamped to a maximum VOUT of 18V in the on state. The output and the reference output are actively pulled low during undervoltage lockout. nn APPLICATIONS nn nn Off-Line Converters DC/DC Converters All registered trademarks and trademarks are the property of their respective owners. BLOCK DIAGRAM REFERENCE ENABLE 5V REF MAIN BIAS RT/CT 4 COMP 1 FB 2 OSCILLATOR 5.6V UV LOCKOUT REFERENCE PULL-DOWN OUTPUT PULL-DOWN 8 VREF 7 VCC 6 OUTPUT 5 GND T 1V S R - BLANKING 1mA - 2.5V ISENSE + 2R R + + 1.5V 3 18V - 1241 BD01 Rev B Document Feedback For more information www.analog.com 1 LT1241 Series ABSOLUTE MAXIMUM RATINGS Supply Voltage...........................................................25V Output Current......................................................... 1A* Output Energy (Capacitive Load per Cycle)................ 5J Analog Inputs (Pins 2, 3)................................ -0.3 to 6V Error Amplifier Output Sink Current........................10mA Power Dissipation at TA 25C...................................1W Operating Junction Temperature Range LT124XC.................................................. 0C to 100C LT124XI ............................................. - 40C to 100C LT124XM (Obsolete)........................... - 55C to 125C Storage Temperature Range.................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................... 300C *The 1A rating for output current is based on transient switching requirements. PIN CONFIGURATION TOP VIEW COMP 1 8 VREF FB 2 7 VCC ISENSE 3 6 OUTPUT RT/CT 4 5 GND TOP VIEW J8 PACKAGE 8-LEAD CERDIP TJMAX = 125C, JA = 100C/W COMP 1 8 VREF FB 2 7 VCC ISENSE 3 6 OUTPUT RT/CT 4 5 GND N8 PACKAGE 8-LEAD PDIP TJMAX = 100C, JA = 130C/W OBSOLETE PACKAGE TOP VIEW COMP 1 8 VREF FB 2 7 VCC ISENSE 3 6 OUTPUT RT/CT 4 5 GND S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 100C, JA = 150C/W ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL http://www.linear.com/product/LT1241#orderinfo PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE OBSOLETE PACKAGE LT124XCJ8#PBF LT124XCJ8#TRPBF 124X 8-Lead CERDIP 0C to 100C LT124XCN8#PBF LT124XCN8#TRPBF 124X 8-Lead PDIP 0C to 100C LT124XCS8#PBF LT124XCS8#TRPBF 124X 8-Lead Plastic SO 0C to 100C LT124XIN8#PBF LT124XIN8#TRPBF 124XI 8-Lead PDIP -40C to 125C LT124XIS8#PBF LT124XIS8#TRPBF 124XI 8-Lead Plastic SO -40C to 125C OBSOLETE PACKAGE LT124XMJ8#PBF LT124XMJ8#TRPBF 8-Lead CERDIP -55C to 125C Consult ADI Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. 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/. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix. Rev B 2 For more information www.analog.com LT1241 Series ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Notes 2 and 3) PARAMETER CONDITIONS MIN TYP MAX UNITS IO = 1mA, TJ = 25C 4.925 5.000 5.075 V Reference Section Output Voltage Line Regulation 12V < VCC < 25V l 3 20 mV Load Regulation 1mA < IVREF < 20mA l -6 -25 mV l Temperature Stability 0.1 Total Output Variation Line, Load, Temp Output Noise Voltage 10Hz < F < 10kHz, TJ = 25C Long Term Stability TA = 125C, 1000 Hrs Output Short-Circuit Current 4.87 mV/C 5.13 50 l -30 V V 5 25 mV -90 -180 mA Oscillator Section Initial Accuracy RT = 10k, C T = 3.3nF, TJ = 25C 47.5 50 52.5 kHz RT = 13.0k, C T = 500pF, TJ = 25C 228 248 268 kHz 1 % Voltage Stability 12V < VCC < 25V, TJ = 25C Temperature Stability TMIN < TJ < TMAX - 0.05 Amplitude TJ = 25C (Pin 4) 1.7 Clock Ramp Reset Current VOSC (Pin 4) = 2V, TJ = 25C %/C V 7.9 8.2 8.5 mA 2.42 2.50 2.58 V -2 A 65 90 0.7 1.3 Error Amplifier Section Feedback Pin Input Voltage VPIN1 = 2.5V l Input Bias Current VFB = 2.5V l Open-Loop Voltage Gain 2 < VO < 4V l Unity-Gain Bandwidth TJ = 25C 60 dB 2 MHz Power Supply Rejection Ratio 12V < VCC < 25V l Output Sink Current VPIN2 = 2.7V, VPIN1 = 1.1V l 2 6 mA dB Output Source Current VPIN2 = 2.3V, VPIN1 = 5V l -0.5 -0.75 mA Output Voltage High Level VPIN2 = 2.3V, RL = 15k to GND l 5 5.6 Output Voltage Low Level VPIN2 = 2.7V, RL = 15k to Pin 8 l Error Amplifier Section V 0.2 1.1 V V/V Current Sense Section Gain Maximum Current Sense Input Threshold VPIN3 < 1.1V l 2.85 3.00 3.15 l 0.90 1.00 1.10 V Power Supply Rejection Ratio l 70 dB Input Bias Current l -1 10 A Delay to Output l 50 100 ns Blanking Time 100 ns Blanking Override Voltage 1.5 V Rev B For more information www.analog.com 3 LT1241 Series ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Notes 2 and 3) PARAMETER CONDITIONS MIN TYP MAX 0.25 0.75 0.4 2.2 UNITS Output Section Output Low Level IOUT = 20mA IOUT = 200mA l l Output High Level IOUT = 20mA IOUT = 200mA l l Rise Time CL = 1nF, TJ = 25C Fall Time CL = 1.0nF, TJ = 25C Output Clamp Voltage IO = 1mA V V 12.0 11.75 V V 50 l 80 ns 30 60 ns 18 19.5 V Undervoltage Lockout Start-Up Threshold LT1241 LT1242/LT1244 LT1243/LT1245 l l l 9.0 15 7.8 9.6 16 8.4 10.2 17 9.0 V V V Minimum Operating Voltage LT1241/LT1243/LT1245 LT1242/LT1244 l l 7.0 9.0 7.6 10 8.2 11 V V 1.6 5.5 0.4 2.0 6.0 0.8 V V V 46 94 48 96 % % l 0 % Start-Up Current l 170 250 A Operating Current l 7 10 mA Hysteresis LT1241 LT1242/LT1244 LT1243/LT1245 PWM Maximum Duty Cycle LT1241/LT1244/LT1245 LT1242/LT1243 TJ = 25C TJ = 25C Minimum Duty Cycle Total Device 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: Unless otherwise specified, VCC = 15V, RT = 10k, CT = 3.3nF. Note 3: Low duty cycle pulse techniques are used during test to maintain junction temperature close to ambient. Rev B 4 For more information www.analog.com LT1241 Series TYPICAL PERFORMANCE CHARACTERISTICS Undervoltage Lockout - LT1242, LT1244 Undervoltage Lockout - LT1241 11 MINIMUM OPERATING VOLTAGE 7 15 11 10 6 -50 -25 25 75 0 50 TEMPERATURE (C) 100 MINIMUM OPERATING VOLTAGE 9 -50 -25 125 0 50 25 75 TEMPERATURE (C) LT1241 * TPC01 Start-Up Current LT1242/4 50 TJ = 25C 4 6 8 10 VCC (V) 12 14 16 160 9 120 100 80 40 6 0 -50 -25 18 25 75 0 50 TEMPERATURE (C) 100 5 -50 -25 125 58 LT1242, LT1243 56 FREQUENCY (kHz) 6 5 4 3 VCC = 15V RT = 10k CL = 15pF Oscillator Sink Current VCC = 5V RT = 10k CT = 3300pF 54 52 50 48 46 44 1M LT1241 * TPC18 40 -50 -25 125 8.7 42 100k OSCILLATOR FREQUENCY (Hz) 100 LT1241 * TPC06 Oscillator Frequency LT1241, LT1244, LT1245 25 75 0 50 TEMPERATURE (C) LT1241 * TPC05 9 0 10k 7 20 60 1 8 60 10 2 VCC = 15V RT = 10k CT = 3300pF 140 Supply Current vs Oscillator Frequency 7 125 Supply Current LT1241 * TPC04 8 100 10 OSCILLATOR SINK CURRENT (mA) 2 25 75 0 50 TEMPERATURE (C) LT1241 * TPC03 ICC (mA) START-UP CURRENT (A) LT1241 MINIMUM OPERATING VOLTAGE 6 -50 -25 125 180 100 0 8 Start-Up Current 150 LT1243/5 START-UP THRESHOLD 7 200 START-UP THRESHOLD START-UP CURRENT (A) 100 9 LT1241 * TPC02 200 SUPPLY CURRENT (mA) 10 VCC (V) 9 8 START-UP THRESHOLD 16 START-UP THRESHOLD VCC (V) VCC (V) 11 17 10 0 Undervoltage Lockout - LT1243, LT1245 25 75 0 50 TEMPERATURE (C) 100 125 LT1241 * TPC07 8.6 VPIN4 = 2V 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 -50 -25 0 50 25 75 TEMPERATURE (C) 100 125 LT1241 * TPC08 Rev B For more information www.analog.com 5 LT1241 Series TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage Reference Short-Circuit Current 5.03 5.02 5.01 5.00 4.99 4.98 4.97 4.96 4.95 -50 -25 0 50 25 75 TEMPERATURE (C) 100 125 2.55 120 100 80 60 40 20 -50 -25 0 50 25 75 TEMPERATURE (C) LT1241 * TPC10 90 20 45 0 0 10 100 1k 100k 10k FREQUENCY (Hz) 1M 2.47 2.46 2.45 -50 -25 125 1.02 1.01 2.5 TJ = 25C TJ = 125C 1.0 0.5 100 OUTPUT SOURCE CURRENT (mA) 200 1.00 0.99 0.98 0.97 0.96 0 50 25 75 TEMPERATURE (C) 100 LT1241 * TPC13 125 TJ = - 55C 0.6 TJ =125C 0.4 TJ = 25C 0.2 0 125 0 1 2 4 5 3 ERROR AMP OUTPUT VOLTAGE (V) 6 LT1241 * TPC17 Low Level Output Saturation Voltage During Undervoltage Lockout 4.0 TJ = 125C 0.5 TJ = 25C 0 100 0.8 LT1241 * TPC12 OUTPUT SATURATION VOLTAGE (V) TJ = -55C 0 75 1.0 Low Level Output Saturation Voltage 3.5 1.5 50 LT1241 * TPC11 1.0 2.0 25 Current Sense Input Threshold 1.03 0.95 -50 -25 -45 10M 4.0 3.0 0 TEMPERATURE (C) 1.04 High Level Output Saturation Voltage OUTPUT SATURATION VOLTAGE (V) 2.48 1.2 LT1241 * TPC16 0 2.49 OUTPUT SATURATION VOLTAGE (V) -20 2.50 CURRENT SENSE INPUT THRESHOLD (V) 135 CURRENT SENSE CLAMP VOLTAGE (V) 180 PHASE (DEG) AVOL OPEN-LOOP VOLTAGE GAIN (dB) VCC = 15V VO = 2.0V TO 4.0V RL = 100k TA = 25C PHASE 40 2.52 2.51 1.05 225 60 2.53 Current Sense Clamp Voltage 100 GAIN 100 2.54 LT1241 * TPC09 Error Amplifier Open-Loop Gain and Phase 80 Feedback Pin Input Voltage 140 FEEDBACK PIN INPUT VOLTAGE (V) IO = 1mA 5.04 REFERENCE VOLTAGE (V) REFERENCE SHORT-CIRCUIT CURRENT (mA) 5.05 0 TJ = -55C 100 OUTPUT SINK CURRENT (mA) 200 LT1241 * TPC14 3.5 3.0 2.5 TJ = -55C 2.0 1.5 TJ = 125C 1.0 TJ = 25C 0.5 0 0 5 OUTPUT SINK CURRENT (mA) 10 LT1241 * TPC15 Rev B 6 For more information www.analog.com LT1241 Series TYPICAL PERFORMANCE CHARACTERISTICS Output Deadtime vs Oscillator Frequency - LT1241, LT1243, LT1245 Output Deadtime vs Oscillator Frequency - LT1242, LT1244 60 2nF 1nF % OF DEADTIME 40 30 500pF 20 100pF 70 10nF 65 5nF 2nF 1nF 100pF 0 100 OSCILLATOR FREQUENCY (kHz) 1000 500pF 60 5nF CT =10nF 100pF 50 0 100 OSCILLATOR FREQUENCY (kHz) LT1241 * TPC19 1000 1M OUTPUT VOLTAGE 5V/DIV Current Sense Delay CURRENT SENSE INPUT 1V/DIV OUTPUT VOLTAGE 5V/DIV LT1241 * TPC22 OUTPUT CROSSCONDUCTION CURRENT 20mA/DIV TIME 50ns/DIV 100k OSCILLATOR FREQUENCY (Hz) LT1241 * TPC21 Output Cross-Conduction Current OUTPUT VOLTAGE VCC = 15V CL = 1nF 1 10k VCC = 15V TJ = 25C LT1241 * TPC20 Output Rise and Fall Time 500pF 2nF 10 55 10 200pF 1nF RT (k) 5nF % OF DEADTIME 100 75 50 0 Timing Resistor vs Oscillator Frequency VCC = 15V CL = 1nF TIME 50ns/DIV LT1241 * TPC23 VCC = 15V CL = 1nF TIME 50ns/DIV LT1241 * TPC24 Rev B For more information www.analog.com 7 LT1241 Series PIN FUNCTIONS COMP (Pin 1): Compensation Pin. This pin is the output of the Error Amplifier and is made available for loop compensation. It can also be used to adjust the maximum value of the current sense clamp voltage to less than 1V. This pin can source a minimum of 0.5mA (0.8mA typ) and sink a minimum of 2mA (4mA typ) FB (Pin 2): Voltage Feedback Pin. This pin is the inverting input of the error amplifier. The output voltage is normally fed back to this pin through a resistive divider. The noninverting input of the error amplifier is internally committed to a 2.5V reference point. ISENSE (Pin 3): Current Sense Pin. This is the input to the current sense comparator. The trip point of the comparator is set by, and is proportional to, the output voltage of the Error Amplifier. RT/CT (Pin 4): The oscillator frequency and the deadtime are set by connecting a resistor (RT) from VREF to RT/CT and a capacitor (CT) from RT/CT to GND. The rise time of the oscillator waveform is set by the RC time constant of RT and CT. The fall time, which is equal to the output deadtime, is set by a combination of the RC time constant and the oscillator sink current (8.2mA typ). GND (Pin 5): Ground. OUTPUT (Pin 6): This pin is the output of a high current totem pole output stage. It is capable of driving up to 1A of current into a capacitive load such as the gate of a MOSFET. VCC (Pin 7): This pin is the positive supply of the control IC. VREF (Pin 8): Reference. This is the reference output of the IC. The reference output is used to supply charging current to the external timing resistor RT. The reference provides biasing to a large portion of the internal circuitry, and is used to generate several internal reference levels including the VFB level and the current sense clamp voltage. Rev B 8 For more information www.analog.com LT1241 Series APPLICATIONS INFORMATION START-UP DEVICE MINIMUM OPERATING THRESHOLD MAXIMUM VOLTAGE DUTY CYCLE REPLACES LT1241 9.6V 7.6V 50% NONE LT1242 16V 10V 100% UC1842 LT1243 8.4V 7.6V 100% UC1843 LT1244 16V 10V 50% UC1844 LT1245 8.4V 7.6V 50% UC1845 Oscillator The LT1241 series devices are fixed frequency current mode pulse width modulators. The oscillator frequency and the oscillator discharge current are both trimmed and tightly specified to minimize the variations in frequency and deadtime. The oscillator frequency is set by choosing a resistor and capacitor combination, RT and CT. This RC combination will determine both the frequency and the maximum duty cycle. The resistor RT is connected from VREF (Pin 8) to the RT/CT pin (Pin 4). The capacitor CT is connected from the RT/CT pin to ground. The charging current for CT is determined by the value of RT. The discharge current for CT is set by the difference between the current supplied by RT and the discharge current of the LT124X. The discharge current of the device is trimmed to 8.2mA. For large values of RT discharge time will be determined by the discharge current of the device and the value of CT. As the value of RT is reduced it will have more effect on the discharge time of CT. During an oscillator cycle capacitor CT is charged to approximately 2.8V and discharged to approximately 1.1V. The output is enabled during the charge time of CT and disabled, in an off state, during the discharge time of CT. The deadtime of the circuit is equal to the discharge time of CT. The maximum duty cycle is limited by controlling the deadtime of the oscillator. There are many combinations of RT and CT that will yield a given oscillator frequency, however there is only one combination that will yield a specific deadtime at that frequency. Curves of oscillator frequency and deadtime for various values of RT and CT appear in the Typical Performance Characteristics section. Frequency and deadtime can also be calculated using the following formulas: Oscillator Rise Time: t r = 0.583 * RC Oscillator Discharge Time: t d = 3.46 * RC (0.0164)R - 11.73 Oscillator Period: TOSC = tr + td Oscillator Frequency: fOSC = 1 T OSC Maximum Duty Cycle: LT1241, LT1244, LT1245 DMAX = -t T = OSC d 2T OSC 2T OSC tr LT1242, LT1243 DMAX = -t T = OSC d T OSC T OSC tr The above formulas will give values that will be accurate to approximately 5%, at the oscillator, over the full operating frequency range. This is due to the fact that the oscillator trip levels are constant versus frequency and the discharge current and initial oscillator frequency are trimmed. Some fine adjustment may be required to achieve more accurate results. Once the final RT/CT combination is selected the oscillator characteristics will be repeatable from device to device. Note that there will be some slight differences between maximum duty cycle at the oscillator and maximum duty cycle at the output due to the finite rise and fall times of the output. The output switching frequency will be equal to the oscillator frequency for LT1242 and LT1243. The output switching frequency will be equal to one-half the oscillator frequency for LT1241, LT1244 and LT1245. The oscillator of LT1241 series devices will run at frequencies up to 1MHz, allowing 500kHz output switching frequencies for all devices. Rev B For more information www.analog.com 9 LT1241 Series APPLICATIONS INFORMATION Error Amplifier The LT1241 series of devices contain a fully compensated error amplifier with a DC gain of 90dB and a unity-gain frequency of 1MHz. Phase margin at unity-gain is 80. The noninverting input is internally committed to a 2.5V reference point derived from the 5V reference of Pin 8. The inverting input (Pin 2) and the output (Pin 1) are made available to the user. The output voltage in a regulator circuit is normally fed back to the inverting input of the error amplifier through a resistive divider. The output of the error amplifier is made available for external loop compensation. The output current of the error amplifier is limited to approximately 0.8mA sourcing and approximately 6mA sinking. In a current mode PWM the peak switch current is a function of the output voltage of the error amplifier. In the LT1241 series devices the output of the error amplifier is offset by two diodes (1.4V at 25C), divided by a factor of three, and fed to the inverting input of the current sense comparator. For error amplifier output voltages less than 1.4V the duty cycle of the output stage will be zero. The maximum offset that can appear at the current sense input is limited by a 1V clamp. This occurs when the error amplifier output reaches 4.4V at 25C. The output of the error amplifier can be clamped below 4.4V in order to reduce the maximum voltage allowed across the current sensing resistor to less than 1V. The supply current will increase by the value of the output source current when the output voltage of the error amplifier is clamped. Current Sense Comparator and PWM Latch LT1241 series devices are current mode controllers. Under normal operating conditions the output (Pin 6) is turned on at the start of every oscillator cycle, coincident with the rising edge of the oscillator waveform. The output is then turned off when the current reaches a threshold level proportional to the error voltage at the output of the error amplifier. Once the output is turned off it is latched off until the start of the next cycle. The peak current is thus proportional to the error voltage and is controlled on a cycle by cycle basis. The peak switch current is normally sensed by placing a sense resistor in the source lead of the output MOSFET. This resistor converts the switch current to a voltage that can be fed into the current sense input. For normal operating conditions the peak inductor current, which is equal to the peak switch current, will be equal to: IPK = (VPIN1 - 1.4V) (3RS ) During fault conditions the maximum threshold voltage at the input of the current sense comparator is limited by the internal 1V clamp at the inverting input. The peak switch current will be equal to: I PK(MAX) = 1V RS In certain applications, such as high power regulators, it may be desirable to limit the maximum threshold voltage to less than 1V in order to limit the power dissipated in the sense resistor or to limit the short-circuit current of the regulator circuit. This can be accomplished by clamping the output of the error amplifier. A voltage level of approximately 1.4V at the output of the error amplifier will give a threshold voltage of 0V. A voltage level of approximately 4.4V at the output of the error amplifier will give a threshold level of 1V. Between 1.4V and 4.4V the threshold voltage will change by a factor of one-third of the change in the error amplifier output voltage. The threshold voltage will be 0.333V for an error amplifier voltage of 2.4V. To reduce the maximum current sense threshold to less than 1V the error amplifier output should be clamped to less than 4.4V. Rev B 10 For more information www.analog.com LT1241 Series APPLICATIONS INFORMATION Blanking A unique feature of the LT1241 series devices is the built-in blanking circuit at the output of the current sense comparator. A common problem with current mode PWM circuits is erratic operation due to noise at the current sense input. The primary cause of noise problems is the leading edge current spike due to transformer interwinding capacitance and diode reverse recovery time. This current spike can prematurely trip the current sense comparator causing an instability in the regulator circuit. A filter at the current sense input is normally required to eliminate this instability. This filter will in turn slow down the current sense loop. A slow current sense loop will increase the minimum pulse width which will increase the short-circuit current in an overload condition. The LT1241 series devices blank (lock out) the signal at the output of the current sense comparator for a fixed amount of time after the switch is turned on. This effectively prevents the PWM latch from tripping due to the leading edge current spike. The blanking time will be a function of the voltage at the feedback pin (Pin 2). The blanking time will be 100ns for normal operating conditions (VFB = 2.5V). The blanking time goes to zero as the feedback pin is pulled to 0V. This means that the blanking time will be minimized during start-up and also during an output short-circuit fault. This blanking circuit eliminates the need for an input filter at the current sense input except in extreme cases. Eliminating the filter allows the current sense loop to operate with minimum delays, reducing peak currents during fault conditions. Undervoltage Lockout The LT1241 series devices incorporate an undervoltage lockout comparator which prevents the internal reference circuitry and the output from starting up until the supply voltage reaches the start-up threshold voltage. The quiescent current, below the start-up threshold, has been reduced to less than 250A (170A typ.) to minimize the power loss due to the bleed resistor used for start-up in off-line converters. In undervoltage lockout both VREF (Pin 8) and the output (Pin 6) are actively pulled low by Darlington connected PNP transistors. They are designed to sink a few milliamps of current and will pull down to about 1V. The pull-down transistor at the reference pin can be used to reset the external soft start capacitor. The pull-down transistor at the output eliminates the external pull-down resistor required, with earlier devices, to hold the external MOSFET gate low during undervoltage lockout. Output The LT1241 series devices incorporate a single high current totem pole output stage. This output stage is capable of driving up to 1A of output current. Crossconduction current spikes in the output totem pole have been eliminated. This device is primarily intended for driving MOSFET switches. Rise time is typically 40ns and fall time is typically 30ns when driving a 1.0nF load. A clamp is built into the device to prevent the output from rising above 18V in order to protect the gate of the MOSFET switch. The output is actively pulled low during undervoltage lockout by a Darlington PNP. This PNP is designed to sink several milliamps and will pull the output down to approximately 1V. This active pull-down eliminates the need for an external resistor which was required in older designs. The output pin of the device connects directly to the emitter of the upper NPN drive transistor and the collector of the lower NPN drive transistor in the totem pole. The collector of the lower transistor, which is n-type silicon, forms a p-n junction with the substrate of the device. This junction is reverse biased during normal operation. In some applications the parasitic LC of the external MOSFET gate can ring and pull the OUTPUT pin below ground. If the OUTPUT pin is pulled negative by more than a diode drop the parasitic diode formed by the collector Rev B For more information www.analog.com 11 LT1241 Series APPLICATIONS INFORMATION of the output NPN and the substrate will turn on. This can cause erratic operation of the device. In these cases a Schottky clamp diode is recommended from the output to ground. Reference The internal reference of the LT1241 series devices is a 5V bandgap reference, trimmed to within 1% initial tolerance. The reference is used to power the majority of internal logic and the oscillator circuitry. The oscillator charging current is supplied from the reference. The feedback pin voltage and the clamp level for the current sense comparator are derived from the reference voltage. The reference can supply up to 20mA of current to power external circuitry. Note that using the reference in this manner, as a voltage regulator, will significantly increase power dissipation in the device which will reduce the useful operating ambient temperature range. Design/Layout Considerations LT1241 series devices are high speed circuits capable of generating pulsed output drive currents of up to 1A peak. The rise and fall time for the output drive current is in the range of 10ns to 20ns. High speed circuit techniques must be used to insure proper operation of the device. Do not attempt to use Proto-boards or wire-wrap techniques to breadboard high speed switching regulator circuits. They will not work properly. Printed circuit layouts should include separate ground paths for the voltage feedback network, oscillator capacitor, and switch drive current. These ground paths should be connected together directly at the ground pin (Pin 5) of the LT124X. This will minimize noise problems due to pulsed ground pin currents. VCC should be bypassed, with a minimum of 0.1F, as close to the device as possible. High current paths should be kept short and they should be separated from the feedback voltage network with shield traces if possible. Rev B 12 For more information www.analog.com LT1241 Series TYPICAL APPLICATIONS External Clock Synchronization Soft-Start VREF 8 VREF 5V REF 8 RT R RT/CT EXTERNAL SYNC INPUT 4 0.01F + OSCILLATOR 5.6V C 1V 1mA FB 2 CT 47 5V REF COMP 1 - 2R - + D1 R + + 2.5V 1.5V ISENSE D1 IS REQUIRED IF THE SYNC AMPLITUDE IS LARGE ENOUGH TO PULL THE BOTTOM OF CT MORE THAN LT1241 * TA01 300mV BELOW GROUND. 3 - LT1241 * TA02 Adjustable Clamp Level with Soft-Start 5V REF MAIN BIAS VREF 8 REFERENCE ENABLE 4 100k COMP 1 FB 2 C R1 OSCILLATOR 5.6V T 1mA - 7 OUTPUT 6 S R 1V 18V 2R + - R 2.5V VIN VCC OUTPUT PULL-DOWN RT/CT R2 UV LOCKOUT REFERENCE PULL-DOWN GND 5 BLANKING + + 1.5V - RS ISENSE 3 VCLAMP ( 1.67 R2 +1 R1 ( IPK (MAX) VCLAMP RS WHERE: 0V VCLAMP 1.0V tSOFT-START = -ln 1 - VC 3 * VCLAMP C R1 R2 R1 + R2 LT1241 * TA03 Rev B For more information www.analog.com 13 LT1241 Series TYPICAL APPLICATIONS 300kHz Off-Line Power Supply HOT 1 R5 1M 1/2W 90VAC TO 240VAC 3 C2 0.1F 250V MP3-X2 T1 BALEN 2 NEU C3 0.1F 250V MP3-X2 D5 C4 4700pF 250V Y-CAP - + 2KBPO8M 4 1212-R6103 COILTRONICS C6 4700pF 250V Y-CAP C5 4700pF 250V Y-CAP AC GND RT1 MCID404 2KBPOO5M + R3 200k 1/2W R2 660k 1/10W C7 0.22F MKS-2 2 R13 12k C10 0.1F MKS-2 4 12T 5 7 30T 1 CTX210433-1 3 13T 6 VCC COMP 4 RT/CT OUTPUT ISENSE GND 5 C15 3.3F 50V C16 3.3F 50V R15 750 1W RTN C13 4700pF 1kV Y-CAP LT1241 * TA06 C12 22F 25V LT1241 1 20V 1.5A D2 BAV21 R14 39 FB 8 V REF R10 20k D1 MUR160 8 30T 2 LP = 100H 7 R9 200k C1 470pF R4 660k 1/10W C8 100pF C9 0.01F, 100V MKS-2 R5 27k 2W R7 510 1/10W D7 BAV21 L1 5 1/2 TURN AIRCORE T2 R1 200k 1/2W C14 100F 400V D6 1N5245B 15V R8 152k D3 MUR420 R11 12 6 3 Q1 MPT2N60 D4 BAT 85 R12 1k 1/10W R18 2 1/4W C11 220pF R16 2 1/4W R17 2 1/4W NOTES: UNLESS OTHERWISE SPECIFIED 1. ALL RESISTANCES ARE IN OHMS, 1/4W, 5%. 2. ALL CAPACITANCES ARE IN MICROFARADS, 50V, 10%. Rev B 14 For more information www.analog.com LT1241 Series TYPICAL APPLICATIONS Slope Compensation at ISENSE Pin 5V REF MAIN BIAS VREF 8 RT REFERENCE ENABLE OUTPUT PULL-DOWN RT/CT 4 OSCILLATOR CT COMP 1 FB 2 UV LOCKOUT REFERENCE PULL-DOWN 5.6V - + 2.5V T 1V 18V - R VIN 7 OUTPUT 6 S R 1mA 2R VCC GND 5 BLANKING + + 1.5V - ISENSE RS 3 LT1241 * TA04 Rev B For more information www.analog.com 15 LT1241 Series PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT1241#packaging for the most recent package drawings. J8 Package 8-Lead CERDIP (Narrow .300 Inch, Hermetic) (Reference LTC DWG # 05-08-1110) CORNER LEADS OPTION (4 PLCS) .023 - .045 (0.584 - 1.143) HALF LEAD OPTION .045 - .068 (1.143 - 1.650) FULL LEAD OPTION .005 (0.127) MIN .405 (10.287) MAX 8 7 6 5 .025 (0.635) RAD TYP .220 - .310 (5.588 - 7.874) 1 .300 BSC (7.62 BSC) 2 3 4 .200 (5.080) MAX .015 - .060 (0.381 - 1.524) .008 - .018 (0.203 - 0.457) 0 - 15 NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP/PLATE OR TIN PLATE LEADS .045 - .065 (1.143 - 1.651) .014 - .026 (0.360 - 0.660) .100 (2.54) BSC .125 3.175 MIN J8 0801 OBSOLETE PACKAGE Rev B 16 For more information www.analog.com LT1241 Series PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT1241#packaging for the most recent package drawings. N Package 8-Lead PDIP (Narrow .300 Inch) (Reference LTC DWG # 05-08-1510 Rev I) .400* (10.160) MAX 8 7 6 5 1 2 3 4 .255 .015* (6.477 0.381) .300 - .325 (7.620 - 8.255) .008 - .015 (0.203 - 0.381) ( +.035 .325 -.015 8.255 +0.889 -0.381 ) .045 - .065 (1.143 - 1.651) .065 (1.651) TYP .100 (2.54) BSC .130 .005 (3.302 0.127) .120 (3.048) .020 MIN (0.508) MIN .018 .003 (0.457 0.076) N8 REV I 0711 NOTE: 1. DIMENSIONS ARE INCHES MILLIMETERS *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm) Rev B For more information www.analog.com 17 LT1241 Series PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT1241#packaging for the most recent package drawings. S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610 Rev G) .050 BSC .189 - .197 (4.801 - 5.004) NOTE 3 .045 .005 8 .245 MIN .160 .005 .010 - .020 x 45 (0.254 - 0.508) NOTE: 1. DIMENSIONS IN 5 .150 - .157 (3.810 - 3.988) NOTE 3 1 RECOMMENDED SOLDER PAD LAYOUT .053 - .069 (1.346 - 1.752) 0- 8 TYP .016 - .050 (0.406 - 1.270) 6 .228 - .244 (5.791 - 6.197) .030 .005 TYP .008 - .010 (0.203 - 0.254) 7 .014 - .019 (0.355 - 0.483) TYP INCHES (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm) 4. PIN 1 CAN BE BEVEL EDGE OR A DIMPLE 2 3 4 .004 - .010 (0.101 - 0.254) .050 (1.270) BSC SO8 REV G 0212 Rev B 18 For more information www.analog.com LT1241 Series REVISION HISTORY (Revision history begins at Rev B) REV DATE DESCRIPTION B 04/18 Deleted CERDIP J8 package PAGE NUMBER 2 Rev B Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license For is granted implication or otherwise under any patent or patent rights of Analog Devices. moreby information www.analog.com 19 LT1241 Series TYPICAL APPLICATION Slope Compensation at Error Amp 5V REF MAIN BIAS VREF 8 RT CT RSLOPE REFERENCE PULL-DOWN RT/CT OSCILLATOR COMP 1 Rf 5.6V 1V - S R FB + 2.5V 18V 2R - R 7 OUTPUT 6 T 1mA 2 VCC OUTPUT PULL-DOWN 4 TO VOUT UV LOCKOUT REFERENCE ENABLE GND 5 BLANKING + + 1.5V - ISENSE 3 LT1241 * TA05 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3895 150V Low IQ, Synchronous Step-Down DC/DC Controller 4V VIN 140V, 150V ABS Max, 0.8V VOUT 24V, IQ = 50A PLL Fixed Frequency 50kHz to 900kHz LTC3810 100V Synchronous Step-Down DC/DC Controller Constant On-Time Valley Current Mode 6.2V VIN 100V, 0.8V VOUT 0.93VIN, SSOP-28 LTC3864 60V, Low IQ, High Voltage DC/DC Controller with 100% Duty Cycle Fixed Frequency 50kHz to 850kHz, 3.5V VIN 60V, 0.8V VOUT VIN , IQ = 40A, MSOP-12E, 3mm x 4mm DFN-12 LTC3891 60V, Low IQ, Synchronous Step-Down DC/DC Controller with 99% Duty Cycle 4V VIN 60V, 0.8V VOUT 24V, IQ = 50A PLL Fixed Frequency 50kHz to 900kHz LT3840 60V, Low IQ, Synchronous Step-Down Controller 2.5V VIN 60V, 1.23V VOUT 60V, IQ = 75A Synchronizable Fixed with Integrated Buck-Boost Bias Voltage Regulator Frequency 100kHz to 600kHz LTC3892/LTC3892-1 60V Low IQ, Dual, 2-Phase Synchronous StepDown DC/DC Controller with 99% Duty Cycle LTC3890/LTC3890-1 60V, Low IQ, Dual 2-Phase Synchronous StepLTC3890-2/LTC3890-3 Down DC/DC Controller with 99% Duty Cycle 4V VIN 60V, 0.8V VOUT 0.99VIN, PLL Fixed Frequency 50kHz to 900kHz, Adjustable 5V to 10V Gate Drive, IQ = 29A PLL Fixed Frequency 50kHz to 900kHz, 4V VIN 60V, 0.8V VOUT 24V, IQ = 50A LTC7813 60V Low IQ, Synchronous Boost + Buck DC/DC Controller 4.5V(Down to 2.2V After Start-Up) VIN 60V, 0.8V VOUT 60V, Adjustable 5V to 10V Gate Drive, IQ = 33A LTC7801 150V Low IQ, Synchronous Step-Down DC/DC Controller 4V VIN 140V, 150V ABS Max, 0.8V VOUT 60V, IQ = 40A PLL Fixed Frequency 320kHz to 2.25MHz LTC7103 105V, 2.3A Low EMI Synchronous Step-Down Regulator 4.4V VIN 105V, 1V VOUT VIN, IQ = 2A Fixed Frequency 200kHz to 2MHz, 5mm x 6mm QFN Rev B 20 D16858-0-4/18(B) www.analog.com ANALOG DEVICES, INC. 1992-2018