L6732TR datasheet(13/36 Pages) STMICROELECTRONICS

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L6732

Device description

13/36

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 35µA and charges the capacitor up to 0.5V. After that it becomes

10µA until the final charge value of approximately 4V (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 0.5V and both MOSFETs are OFF. When VSS is

between 0.5V and 1.1V 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 1.1V (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 prebias 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 0.5V and

1.1V the low-side MOSFET is kept OFF (see Figure 7 and Figure 8).

Figure 5.

Error amplifier reference

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