LED7708TR datasheet(20/53 Pages) STMICROELECTRONICS

20 / 53 page

Recommended operating conditions

LED7708

20/53

Doc ID 022698 Rev 1

8.1

Device power-up and soft-start

The VSI pin (supply input for the serial interface) has a high level threshold lower than the

upper UVLO threshold at the LDO3 pin, ensuring that the serial interface is enabled prior to

the device being turned on. Anyway, after the POR, the DEN (device enable) bit of the

internal DEVCFG0 control register is low (default value) and the device is off. The soft-start

sequence takes place as soon as this bit is asserted.

If the LED7708 is set to operate in standalone, the soft-start procedure takes place in two

steps (Figure 4):

The switching frequency is reduced to one fourth of the nominal value and the boost

current limit is set to 20%. This phase finishes approximately 1 ms after the output

voltage reaches the maximum value (not the OVP level). During this phase the

channels are disabled.

Both the switching frequency and the boost current limit are released to their respective

nominal values and then the channels are enabled.

Figure 4.

Soft-start waveforms (standalone operation)

A slightly different soft-start procedure takes place if the LED7708 is set to operate as

master in conjunction with other slave devices:

The switching frequency is reduced to one eighth of the nominal value and the boost

current limit is set to 20%. This phase finishes approximately 1 ms after the output

voltage reaches the maximum value (not the OVP level). During this phase the

channels are active.

The switching frequency is increased to half the nominal value. This phase is held for

approximately 2 ms.

The switching frequency is further increased to the nominal value while the current limit

changes to 40% (~5 ms duration).

The current limit is released to its nominal values.

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