Backlight driver section

LED7706

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7.2

PWM dimming

The brightness control of the LEDs is performed by a Pulse-Width Modulation of the rows

current. When a PWM signal is applied to the DIM pin, the current generators are turned on

and off mirroring the DIM pin behavior. Actually, the minimum dimming duty-cycle depends

on the dimming frequency.

Figure 15.

PWM dimming waveforms

The real limit to the PWM dimming is the minimum on-time that can be managed for the

current generators; this minimum on-time is approximately 500 ns.

Thus, the minimum dimming duty-cycle depends on the dimming frequency according to the

following formula:

Equation 17

For example, at a dimming frequency of 20 kHz, 1% of dimming duty-cycle can be

managed.

During the off-phase of the PWM signal the boost converter is paused, the current

generators are turned off and the output voltage is frozen across the output capacitor.

During the start-up sequence the dimming duty-cycle is forced to 100 % to detect floating

rows regardless of the applied dimming signal.

DIM

min

,

DIM

f

ns

500

D

⋅

=