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LM74722-Q1: How PD remains ON during short interruption?

Vishwas H R
Prodigy 240 points
Part Number: LM74722-Q1

How PD/ MOSFET Q2 remains ON during short interruption.

The PD is pulled down when voltage at VS pin drops below the V(VS POR) falling threshold during short interruption right?

Therefore, the source and gate is shorted and PD will be off right?

  • 0
    TI__Guru 57737 points

    Hi Vishwas,

    The VS or the C pin which supplies power to the internal circuits of the IC is connected  to the common drain point. So, when the input falls below output voltage during short interruption, the DGATE is immediately turned OFF and does not allow C pin voltage to discharge. Remember, the C pin is powered from Vout (through Q2 or its body diode) when Vin starts dropping below Vout.

    So, the C pin is powered as long as the output capacitor is above V(C) POR Falling threshold. 

  • 0 in reply to Praveen GD
    Prodigy 240 points

    Hi,

    So, the PD remains ON means the current is flowing through body diode right during short interruption?

  • 0 in reply to Vishwas H R
    TI__Guru 57737 points

    As PD is ON, the current will flow from OUT to C pin through the FET itself.

  • 0 in reply to Praveen GD
    Prodigy 240 points

    How PD remains ON (how the current direction changes suddenly from VOUT to C) and what will be the voltage across the PD?

  • 0 in reply to Vishwas H R
    TI__Guru 57737 points

     During input short interruptions,  the DGATE is OFF for reverse current blocking and the C pin which is connected to common drain point is disconnected from input. The PD remain ON because, the C pin voltage is powered from Vout when Vin drops to 0V during short interruptions.

  • 0 in reply to Praveen GD
    Prodigy 240 points

    okay the voltage across PD will be then VOUT + boost regulator output (max 15.5V).

    The maximum voltage across the gate will be 15.5V + the voltage across VOUT. So, why do we need 18V zener why can't we use a normal diode during PD pulled low condition?

    Also, any recommendation for RPD value for 12V battery system?

  • 0 in reply to Vishwas H R
    TI__Guru 57737 points

    A zener is required because during conditions like output transients/short circuit, the Vout can drop to zero quickly but the PD will not be able to discharge so fast resulting in GATE-SOURCE voltage of the FET exceeding the Vgs rating of the FET which can damage it. To avoid damage to the FET in such conditions, a zener across PD to FET Source is recommended.