Soft start and disable

L6726A

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Soft start and disable

L6726A 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 sources a 10µA soft start

current from COMP, linearly charging the compensation network capacitors. The ramping

COMP voltage is compared to the oscillator triangular waveform generating PWM pulses of

increasing width that charge the output capacitors.

When the FB voltage crosses 800 mV, the output voltage is in regulation: soft start phase

will end and the transconductance error amplifier output will be enabled closing the control

loop.

In the event of an over current during soft start, the over current logic will override the soft

start sequence and will shut down the PWM logic and both the high side and low side gates.

This condition is latched, cycle VCC to recover.

The device sources soft start current only when VCC power supply is above UVLO

threshold and over current threshold setting phase has been completed.

6.1

Low-Side-Less start up (LSLess)

L6726A performs a special sequence in enabling LS driver to switch: during the soft-start

phase, the LS driver results disabled (LS = OFF) until the HS starts to switch. This avoids

the dangerous negative spike on the output voltage that can happen if starting over a pre-

charged output and limits the output discharge (amount of output discharge depends on

programmed SS time length: the shorter the programmed SS, the more limited the output

discharge).

If the output voltage is pre-charged to a voltage higher than the final one, the HS would

never start to switch. In this case, LS is enabled and discharges the output to the final

regulation value.

Figure 5.

LSLess startup (left) vs. Non-LSLess startup (right)

6.2

Enable / Disable

The device can be disabled by pushing COMP / DIS pin under 0.4V (min). In this condition

HS and LS MOSFETs are turned off, and the 10

pin. Setting free the pin, the device enables again performing a new SS.