L6731D

Device description

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

programmed value. The L6731D 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

predictive dead time control avoids MOSFETs cross-conduction maintaining very short

dead time duration in the range of 20 ns. 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 (-350 mV typ.) during the dead time, it will be reduced in the next PWM cycle. The

predictive dead time control does not work when the high-side body diode is conducting

because the phase node does not go negative. This situation happens when the converter

is sinking current for example and, in this case, an adaptive dead time control operates.

5.8

Monitoring and protections

The output voltage is monitored by means of pin FB. If it is not within ±10 % (typ.) of the

programmed value, the power good (PGOOD) output is forced low. The device provides

over-voltage-protection: when the voltage sensed on FB pin reaches a value 20 % (typ.)

greater than the reference, the low-side driver is turned on as long as the over voltage is

detected (see Figure 7.).

It must be taken into account that there is an electrical network between the output terminal

and the FB pin and therefore the voltage at the pin is not a perfect replica of the output

voltage. However due to the fact that the converter can sink current, in the most of cases the

Figure 7.

OVP

LGate

FB