L3751 datasheet(13/41 Pages) STMICROELECTRONICS

13 / 41 page

Figure 6. Internal sawtooth ramp feedforward - voltage feed forward

Vcomp

Ton

a*Vs

Ton/a

VIN

a*VIN

Ramp slope = Vs/T

Ramp slope = a*Vs/T

Figure 7. Internal sawtooth ramp feedforward - Fsw adjusted by external resistor

Vcomp

Ton

Ton/a

Fsw

a*Fsw

Ramp slope = Vs/T

Ramp slope = a*Vs/T

T/a

The device generates a synchronization signal available at the SYNCOUT pin that has 180° phase shift referred

to the rising edge of the internal oscillator. Synchronized operation is easily accomplished connecting the

SYNCOUT of the master to the SINCIN of the slave device to reduce the RMS current of the input filter and

increase the EMI performance.

6.3

Soft-start

The soft-start (SS) feature minimizes the inrush current surge, decreases the stress of the power components

during the power-up phase and makes the output voltage increase monotonically.

The device is inhibited as long as the EN/UVLO pin voltage is lower than the 0.4 V typ.

As soon as the EN/UVLO pin voltage exceeds the 1.2 V typ. rising threshold the soft-start phase takes place;

an internal 10 µA typ. current source charges the external soft-start capacitor to ramp the SS/TRK pin voltage,

clamping the internal error amplifier reference voltage. When the SS/TRK voltage exceeds the 0.8 V threshold the

internal EA reference drives the non-inverting input so the SS is ended.

L3751

Soft-start

DS13841 - Rev 1

page 13/41

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