L6731B

Device description

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

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

MOSFET begins to switch and the output voltage starts to increase. The L6731B can always

sink or source current. If an over current is detected during the soft-start phase, the device

provides a constant-current-protection. In this way, in case of short soft-start time and/or small

inductor value and/or high output capacitors value and so, in case of high ripple current during

the soft-start, the converter can start in any case, limiting the current (see 5.8: Monitoring and

protections) but not entering in HICCUP mode. During normal operation, if any under-voltage is

detected on one of the two supplies, the SS pin is internally shorted to GND and so the SS

capacitor is rapidly discharged.

5.7

Driver section

The high-side and low-side drivers allow using different types of power MOSFETs (also multiple

time control avoids MOSFETs cross-conduction maintaining very short dead time duration in

the range of 20ns. The control monitors the phase node in order to sense the low-side body

diode recirculation. If the phase node voltage is less than a certain threshold (-350mV typ.)

during the dead time, it will be reduced in the next PWM cycle. The predictive dead time control

doesn’t work when the high-side body diode is conducting because the phase node doesn’t go

negative. This situation happens when the converter is sinking current for example and, in this

case, an adaptive dead time control operates.