Recommended External
Components
OUTPUT CAPACITOR, COUT:
The output capacitor COUT directly affects the magnitude of
the output ripple voltage. In general, the higher the value of
COUT, the lower the output ripple magnitude. Multilayer ce-
ramic capacitors with low ESR are the best choice. At the
lighter loads, the low ESR ceramics offer a much lower
VOUT ripple than the higher ESR tantalums of the same value.
At the higher loads, the ceramics offer a slightly lower VOUT
ripple magnitude than the tantalums of the same value. How-
ever, the dv/dt of the VOUT ripple with the ceramics is much
lower than the tantalums under all load conditions. Capacitor
voltage rating must be sufficient, 10V or greater is recom-
mended.
Some ceramic capacitors, espesically those in small pack-
ages, exhibit a strong capacitance reduction with the in-
creased applied voltage. The capacitance value can fall to
below half of the nominal capacitance. Too low output capac-
itance will increase the noise and it can make the boost
converter unstable.
INPUT CAPACITOR, CIN:
The input capacitor CIN directly affects the magnitude of the
input ripple voltage and to a lesser degree the VOUT ripple. A
higher value CIN will give a lower VIN ripple. Capacitor voltage
rating must be sufficient, 10V or greater is recommended.
OUTPUT DIODE, D1:
A Schottky diode should be used for the output diode. To
maintain high efficiency the average current rating of the
schottky diode shoulde be larger than the peak inductor cur-
rent (1A). Schottky diodes with a low forward drop and fast
switching speeds are ideal for increasing efficiency in portable
applications. Choose a reverse breakdown of the schottky
diode larger than the output voltage. Do not use ordinary rec-
tifier diodes, since slow switching speeds and long recovery
times cause the efficiency and the load regulation to suffer.
INDUCTOR, L:
The LP55281's high switching frequency enables the use of
the small surface mount inductor. A 4.7 µH shielded inductor
is suggested for 2 MHz operation, 10 µH should be used at 1
MHz. The inductor should have a saturation current rating
higher than the peak current it will experience during circuit
operation (~1A). Less than 300 mΩ ESR is suggested for high
efficiency. Open core inductors cause flux linkage with circuit
components and interfere with the normal operation of the
circuit. This should be avoided. For high efficiency, choose an
inductor with a high frequency core material such as ferrite to
reduce the core losses. To minimize radiated noise, use a
toroid, pot core or shielded core inductor. The inductor should
be connected to the SW pin as close to the IC as possible.
Recommended inductors are LPS3015 and LPS4012 from
Coilcraft and VLF4012 from TDK.
LIST OF RECOMMENDED EXTERNAL COMPONENTS
Symbol Symbol explanation Value Unit Type
CVDD1 C between VDD1 and GND 100 nF Ceramic, X7R/X5R
CVDD2 C between VDD2 and GND 100 nF Ceramic, X7R/X5R
CVDDIO C between VDDIO and GND 100 nF Ceramic, X7R/X5R
CVDDA C between VDDA and GND 1 µF Ceramic, X7R/X5R
COUT C between FB and GND 10 µF Ceramic, X7R/X5R
CIN C between battery voltage and GND 10 µF Ceramic, X7R/X5R
LBOOST L between SW and VBAT at 2 MHz 4.7 µH Shielded, low ESR, ISAT 1A
CVREF C between VREF and GND 100 nF Ceramic, X7R
CVDDIO C between VDDIO and GND 100 nF Ceramic, X7R
RRGB R between IRGB and GND 8.2 kΩ±1%
RRT R between IRT and GND 82 kΩ±1%
D1Rectifying Diode (Vf @ maxload) 0.3 V Schottky diode
CASE C between Audio input and ASEx 100 nF Ceramic, X7R/X5R
LEDs User defined
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LP55281