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TPS24750: Series resistor to the gate of external Reverse Voltage Protection MOSFET & Gate clamping

Part Number: TPS24750

Hi,

I am using CSD16570Q5BT as a reverse voltage protection FET. My application is 8V-1A and I need minimal resistance in the E-Fuse+FET. This MOSFET has 10-15nF Ciss. During turn-ON this does not affect my slew rate, but only delays the turn-ON slightly. When the TPS24750 fully enhances the MOSFET (13-15Vgs) there is ~200-250nC gate charge.

Datasheet suggests series resistor of at least 1kOhm to any external slew-limiting capacitor connected from TPS24750's gate pin to ground. I have 10nF-1kOhm at that pin. However I don't see a suggestion for series resistor going to the external MOSFET's gate. In the evaluation module's schematic (TPS24750EVM-546) TI does not use a series gate resistor either. I understand this resistor limits the current stress on TPS24750 during a turn-OFF case.

  1. Why is there a suggestion for series resistor to external capacitor and not also for other capacitive loads on that pin, while a following FET can also have considerable capacitance? Do I need a series resistor to the GATE of CSD16570Q5BT?
  2. Datasheet suggests MOSFET "Gate Clamp Diode" for applications with >14V input voltage. In an 8V application the Gate voltage can reach +23V. I think I don't need extra clamping diode since TPS24750 already clamps VCC to Gate at 13-15V. And if my input dies, the maximum load stress is 8V. Would you agree that I don't need clamping diodes since my application is below 14V, or is there a risk that in a hard short circuit case the gate-to-source of my MOSFET can see +23V stress? 

Kind regards,

Zeki

  • Hi Zeki,

    Let me get back to you with my comments by tomorrow.

  • Hi Zeki,

    1. The gate sinking current during fast trip event is 1A. So, it will take additional  225 ns for the gate sinking current to discharge 15nF cap (FET Ciss) from 15V to 0V. This is a very negligible amount of time.On the other hand, the external Gate cap can be very high depending on the application. Theoretically, there is no limit to the external gate cap value. Therfore, we had to mention in the datasheet that, 'A 1 kΩ placed in series with the external gate capacitor prevents it from slowing a fast-turnoff event'. 
    2. Yes, you will not need an extra clamping diode. During Fast-trip conditions, the Gate is discharged within microseconds of time. So, it shouldn't be  a problem.