User's Guide
SNVU285 – December 2013
LP8557EVM User Guide
1
Introduction
The LP8557EVM evaluation kit is ready to be used with a power source and a PWM input. The boost
switching frequency and the maximum led current are set by resistors R3 and R4.
2
LP8557EVM Startup
The LP8557EVM requires a power source and Pulse Width Modulated (PWM) input. See
for the
connector descriptions and the location of the PWM input on the left side of the board.
1. Connect the LED board to the LP8557 evaluation board. Set jumpers for 6 LEDs per string as shown
in
2. Connect external power and ground to the board. Suggest 3.6V to VIN. VDD is not required if P2 is in
place.
3. Connect Ground to GND jack.
4. Connect a pulse generator or PWM source to the PWM input. R2 is a 10 K
Ω
resistor to ground on this
input.
5. Set pulse stream to 30 kHz frequency, 50% duty cycle, 3V input high level.
6. Turn on power supply.
7. Enable pulse stream output.
The LEDs will light at 50% brightness if a 50% duty cycle is used. The brightness will vary with the
positive duty cycle of the PWM input signal.
3
Changing Boost Frequency and PWM Frequency on the LP8557EVM
The LP8557EVM is configured to operate at 500 kHz boost switching frequency and 9.8 kHz PWM
frequency by the resistor R3 equal to zero ohms. This resistor can be changed to a value from Table 4 of
the LP8557 datasheet which is presented here.
Table 1. Setting Boost Switching and PWM Dimming Frequencies with an External Resistor
RFSET [
Ω
] (Tolerance)
ƒ
SW
(kHz)
ƒ
PWM
(kHz)
470k - 1M (±5%)
500
4.9
300k, 330k (±5%)
500
9.8
200k (±5%)
500
24.6
147k, 150k, 154k, 158k (±1%)
500
19.5
121k (±1%)
500
24.4
100k (±1%)
500
29.3
86.6k (±1%)
500
34.2
75.0k (±1%)
500
39.1
63.4k (±1%)
1000
4.9
52.3k, 53.6k (±1%)
1000
9.8
44.2k, 45.3k (±1%)
1000
24.6
39.2k (±1%)
1000
19.5
34.0k (±1%)
1000
24.4
30.1k (±1%)
1000
29.3
1
SNVU285 – December 2013
LP8557EVM User Guide
Copyright © 2013, Texas Instruments Incorporated