PM6600 datasheet(35/43 Pages) STMICROELECTRONICS

35 / 43 page

PM6600

Backlight driver section

35/43

Equation 18

Due to the spread of the LEDs' forward voltage, the total drop across the LED's strings will

be different. The device will manage the unconnected ROWs according to the MODE pin

setting (see Table 3).

8.2

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 500 ns.

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

following formula:

Equation 19

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

managed.

The device can manage the condition fDIM = 0 Hz. However, in order to avoid any flickering

issue due to the human eye cutoff frequency, we recommend to use fDIM > 100 Hz (condition

verified with discrete smd leds without any ligth guide).

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.

ROWi

mean

ROW

mean

ROW

mean

ROW

min

ROW

min

ROWx

mean

ROW

mean

ROW

max

ROW

max

ROWx

∑

−

≥

−

∆

≤

−

∆

DIM

min

DIM

500

⋅

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