LP8556 datasheet(24/52 Pages) TI

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LED1

LED2

LED3

LED4

LED5

72 degrees

Phase Delay

1/(fPWM)

Cycle Time

VBOOST

LED1

LED2

LED3

LED4

LED5

LED6

60 degrees

Phase Delay

1/(fPWM)

Cycle Time

VBOOST

LP8556

SNVS871B – JULY 2012 – REVISED SEPTEMBER 2012

www.ti.com

PHASE SHIFT PWM SCHEME

Phase shift PWM scheme allows delaying the time when each LED driver is active. When the LED drivers are

not activated simultaneously, the peak load current from the boost output is greatly decreased. This reduces the

ripple seen on the boost output and allows smaller output capacitors. Reduced ripple also reduces the output

ceramic capacitor audible ringing. PSPWM scheme also increases the load frequency seen on the boost output

six times and therefore transfers the possible audible noise to the frequencies outside of the audible range.

Description of the PSPWM mode is seen in the following diagrams. PSPWM mode is set with <PS_MODE[2:0]>

bits.

PS_MODE[2:0]

Waveforms

Connection

000

6 LED strings with 60 degree phase shift. One driver for each LED string.

001

5 LED strings with 72 degree phase shift. One driver for each LED string.

(Driver #6 not used).

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LP8556 Datasheet(PDF) 24 Page - Texas Instruments

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