LP8555 datasheet(14/64 Pages) TI1

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PWM Detector

Curve Shaper

Hybrid PWM

and Current

Dimming

MAXCURR

THRESHOLD

PWM

Generator

PWM

CURRENT

PWM Input

PWM/INT

Change in Duty

on this Edge

On-time

tON1

On-time

tON2

tPWM

Cycle Time

tPWM

Cycle Time

PWM

Duty =

LP8555

SNVS857 – FEBRUARY 2014

www.ti.com

8.3.2 Brightness Control

The brightness can be controlled using an external PWM signal or the Brightness registers accessible via an I2C

interface, or both. Which of these two input sources are selected is set by the BRTMODE EPROM bits. How the

brightness is controlled in each of the four possible modes is described in the following sections.

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 23. PWM Input Duty Cycle Measurement

8.3.2.2 BRTMODE = 00

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

block measures the duty cycle at the PWM/INT terminal 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 and Current Dimming block which determines the range of the PWM and Current control. The

outcome of the Hybrid PWM and Current Dimming block is Current and/or up to 6 PWM output signals.

Figure 24. BRTMODE = 00 Brightness Control

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