Datasheet

LED7707 Operation description
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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 13) 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.
DIMmin,DIM
fs10D
⋅
μ
=

10µs minimum on-time10µs minimum on-time