LP8557 datasheet(13/49 Pages) TI2

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PWM

Detector

Curve

Shaper

Hybrid

PWM&I

Dimming

PWM

Gen

PWM

Input

CURRENT

PWM

MAXCURR

THRESHOLD

PWM/INT

Change in Duty

on this Edge

On-time

tON1

On-time

tON2

tPWM

Cycle Time

tPWM

Cycle Time

PWM

Duty =

LP8557, LP85571

www.ti.com

SNVSA15B – DECEMBER 2013 – REVISED DECEMBER 2015

Feature Description (continued)

8.3.2.1 PWM Input Duty Measurement

When using PWM input for brightness control the input PWM duty cycle is measured as described in following

diagram and the brightness is controlled based on the result. When changing the brightness it must be noted that

the measurement cycle is from rising edge to next rising edge and brightness change must be done accordingly

(time from rising to rising edge is constant (=cycle time) and falling edge defines the brightness).

Figure 22. PWM Input Duty Cycle Measurement

8.3.2.2 BRTMODE = 00b

With BRTMODE = 00b, the LED output current is controlled by the PWM input duty cycle. The PWM detector

block measures the duty cycle at the PWM pin and uses it to generate a PWM-based brightness code. Before

the output is generated, the code goes through the curve shaper block. Then the code goes into the hybrid PWM

& I Dimming block which determines the range of the PWM and Current control. The outcome of the hybrid PWM

& I dimming block is current and/or up to 6 PWM output signals.

Figure 23. Brightness Data Path for BRTMODE = 00b

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