LED7707 datasheet(22/48 Pages) STMICROELECTRONICS

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Operation description

LED7707

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5.2.2

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.

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 10

μs.

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

following formula:

Equation 10

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

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

generators are turned off. The output voltage can be considered almost constant because of

the relatively slow discharge of the output capacitor. During the start-up sequence (see

Section 5.1.3 on page 14

) the dimming duty-cycle is forced to 100% to detect floating rows

regardless of the applied dimming signal.

Figure 15.

PWM dimming waveforms

5.3

Fault management

The main loop keeps the row having the lowest voltage drop regulated to about 700 mV.

This value slightly depends on the voltage across the remaining active rows. After the soft-

start sequence, all protections turn active and the voltage across the active current

generators is monitored to detect shorted LEDs.

DIM

min

DIM

⋅

10µs minimum on-time

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