L6727_10 Datasheet, PDF (11/34 Pages) STMicroelectronics – Single phase PWM controller

L6727
6
Soft-start and disable
Soft-start and disable
L6727 implements a soft-start to smoothly charge the output filter avoiding high in-rush
currents to be required from the input power supply. The device progressively increases the
internal reference from 0 V to 0.8 V in about 5.1 ms, in closed loop regulation, gradually
charging the output capacitors to the final regulation voltage.
In the event of an overcurrent triggering during soft start, the over current logic will override
the soft start sequence and will shut down both the high side and low side gates for the
internal soft start residual time (up to 2048 clock cycles) plus 2048 clock cycles, then it will
begin a new soft start.
The device begins soft start phase only when VCC power supply is above UVLO threshold
and overcurrent threshold setting phase has been completed.
6.1
Low-side-less start up (LSLess)
In order to manage start up over pre-biased output, L6727 performs a special sequence in
enabling LS driver to switch: during the soft-start phase, LS driver results disabled
(LS = OFF) until HS starts to switch. This avoids the dangerous negative spike on the output
voltage that can happen if starting over a pre-biased output.
If the output voltage is pre-biased to a voltage lower than the programmed one, neither HS
nor LS will turn on until the soft start ramp exceeds the output pre-bias voltage; then V_OUT
will ramp up from there, without any drop or current return.
If the output voltage is pre-biased to a voltage higher than the programmed one, HS would
never start to switch. In this case, at the end of soft start time, LS is enabled and discharges
the output to the final regulation value.
This particular feature of the device masks the LS turn-on only from the control loop point of
view: protections by-pass LSLESS, turning ON the LS MOSFET in case of need.
Figure 4. LSless startup (left) vs non-lsless startup (right)
Doc ID 12933 Rev 4
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