LP8555 Datasheet, PDF (50/64 Pages) Texas Instruments – LP8555 High-Efficiency LED Backlight Driver for Tablet PCs

LP8555
SNVS857 – FEBRUARY 2014
www.ti.com
9.2.1.4.5 Boost Input Capacitor
A ceramic capacitor with at least 10-V voltage rating is recommended for the boost input capacitor. If input
voltage is higher, then the rating should be selected accordingly. The DC-bias effect can reduce the effective
capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically, a 10-μF
capacitor per boost is sufficient. X5R/X7R are recommended types.
9.2.1.4.6 Diode
A Schottky diode should be used for the output diode. Peak repetitive current should be greater than inductor
peak current (3.1 A) to ensure reliable operation. Average current rating should be greater than the maximum
output current. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing
efficiency in portable applications. Choose a reverse breakdown voltage of the Schottky diode significantly larger
(~40 V) than the output voltage. Do not use ordinary rectifier diodes, since slow switching speeds and long
recovery times cause the efficiency and the load regulation to suffer.
9.2.1.5 Application Performance Plots
Typical performance plots with default EPROM configuration. The LP8555EVM was used for taking the
oscilloscope plots.
VDD = 3.7V
I_LED = 25 mA
12 x 7 LEDs
Figure 45. Start-up Waveform with I²C Write
Full Brightness Slope Function Disabled.
VDD = 3.7V
V_BOOST = 28V
Load = 150 mA
Figure 46. Typical Boost Waveform, Boost A
VDD = 3.7V
f_SW = 1 MHz
Figure 47. 180° Phase Difference Between Boost A and B
ƒ_PWM = 19.5 kHz
6 Strings
Figure 48. Typical LED Current and Voltage Waveforms.
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