PM6641TR datasheet(28/47 Pages) STMICROELECTRONICS

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Device description

PM6641

28/47

7.6

Outputs Soft-Start

The soft start function of each switching regulator is achieved by ramping up the SS pin

voltage with a constant slew rate dV/dt.

When the switching section is enabled (EN high), the SS pin constant current charges the

capacitor connected between SS and ground pins.

The SS voltage is used as reference of the switching regulator and the output voltage of the

converter follows the ramp of the SS voltage. When the SS pin voltage is higher than 0.8 V,

the error amplifier will use the internal 0.8 V ±1 % reference to regulate the output voltage.

Figure 31. SW regulator programmable Soft-Start

During the soft start period the current limit is set to the nominal value.

The dV/dt slope is set by charging the external capacitor with a 10 µA current. The

capacitance values will be of the order of magnitude of 10 nF for a 1 msec soft-start

duration, as pointed out by the following formula:

Equation 6

During the soft-start the output under voltage management is not enabled, whereas the

output over voltage, the current limit and the thermal overheat are always monitored.

When the first switching regulator is turned on the output soft-start begins after an additional

delay of about 180 µs, due to PM6641 initializing and fuses reading.

SS_1Sx

SS DO

VREF

Iss

Regulator

⋅

∆

⋅

PM6641TR Datasheet(PDF) 28 Page - STMicroelectronics