L6732 datasheet(13/37 Pages) STMICROELECTRONICS

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L6732

Device description

13/37

5.5

Error amplifier

5.6

Soft-start

When both VCC and VIN are above their turn-ON thresholds (VIN is monitored by the OCH

pin) the start-up phase takes place. Otherwise the SS pin is internally shorted to GND. At

start-up, a ramp is generated charging the external capacitor CSS with an internal current

generator. The initial value for this current is 35 µA and charges the capacitor up to 0.5 V.

After that it becomes 10 µA until the final charge value of approximately 4 V (see Figure 6).

The output of the error amplifier is clamped with this voltage (VSS) until it reaches the

programmed value. No switching activity is observable if VSS is lower than 0.5 V and both

MOSFETs are OFF. When VSS is between 0.5 V and 1.1 V the low-side MOSFET is turned

on because the comp signal is lower than the valley of the triangular wave and so the duty-

cycle is 0 %. As VSS reaches 1.1 V (i.e. the oscillator triangular wave inferior limit) even the

high-side MOSFET begins to switch and the output voltage starts to increase. The L6732

can only source current during the soft-start phase in order to manage the pre-bias start-up

applications. This means that when the start-up occurs with output voltage greater than 0V

(pre-bias startup), even when Vss is between 0.5 V and 1.1 V the low-side MOSFET is kept

OFF (see Figure 7 and Figure 8).

Figure 5.

Error amplifier reference

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