LED7707 datasheet(14/48 Pages) STMICROELECTRONICS

14 / 48 page

Operation description

LED7707

14/48

In applications where the dimming signal is used to turn on and off the device, the EN pin

can be connected to the DIM pin as shown in Figure 7.

Figure 7.

External sync waveforms

5.1.3

Soft-start

The soft-start function is required to perform a correct start-up of the system, controlling the

inrush current required to charge the output capacitor and to avoid output voltage overshoot.

The soft-start duration is set connecting an external capacitor between the SS pin and

ground. This capacitor is charged with a 5

μA (typ.) constant current, forcing the voltage on

the SS pin to ramp up. When this voltage increases from zero to nearly 1.2 V, the current

limit of the power MOSFET is proportionally released from zero to its final value. However,

because of the limited minimum on-time of the switching section, the inductor might saturate

due to current runaway. To solve this problem the switching frequency is reduced to one half

of the nominal value at the beginning of the soft-start phase. The nominal switching

frequency is restored after the SS pin voltage has crossed 0.8 V.

Figure 8.

Soft-start sequence waveforms in case of floating rows

DIM

SGND

LED7707

220k

100n

BAS69

AM00584v1

AVCC

tss

SS pin voltage

Protections turn active

1.2V

0.8V

Current limit

EN pin voltage

100%

Output voltage

OVP

95% of

OVP

Floating ROWs detection

2.4V

Nominal switching

frequency release

AM00585v1

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