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TPS27081A: Cgs of p-ch fet

Pongo
Intellectual 270 points
Part Number: TPS27081A
Other Parts Discussed in Thread: CSD17484F4

Hello,

In your load switch, TPS27081A, what is the gate capacitance(or charge at voltage) of the high side p-channel fet. A pullup resistor(R1) is required to turn off the p-ch fet, and I need to be able to estimate the time for its gate capacitance to discharge whenever the low side n-ch fet turns off. The closest parameters that I see in the datasheet specify the times for the load capacitance to discharge, which is not what is needed.

Thanks

  • 0
    TI__Genius 15780 points

    Hi Pongo,

    This is an older device, so I would need to do some investigation to find the Cgs of the FET.

    What are your application requirements (input voltage, current load)? Also, are you adding C1 to your circuit for a controlled output rise time? If so, then we have a lot of other load switches that may be more applicable. These other devices offer a controlled rise time and a very fast (microseconds) FET turn off.

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    Intellectual 270 points

    Hello Alek,

    No, C1 would not be added. Fast switching is desired so there would be no intentional slowing of transitions. The reason this load switch was even being considered is because it has a rather wide input voltage range. Finding fast devices with decent output power that can also operate at a wide input voltage(Vcc) range is the main difficulty right now.

    Low side logic will be used. The input voltage("Vcc") will vary between 1.2 and a maximum of around 9-10 volts. I realize the load switch we're examining is only rated to max 8 Volts. There are some undesirable workarounds to this max, so a higher max V would definitely be better. Fast turn off is very important, and I need turn off times at least less than 1microsecond at all input voltages. Ideally turn off would be less than 300ns. The output must be able to quickly drive a femtofet such as CSD17484F4, or alternatively its output should be able to be "telescoped" to higher power levels. It is not necessary that the solution be a monolithic load switch, and the design options are wide open ranging from a discrete components load switch to a hack of functions from another device. Worst case, I could make a custom load switch using a TI fet with low Cgs, but I'm not aware of any fets with low enough Cgs to enable a high valued pull up resistor(10k or higher); only low valued resistors enable a fast turn off.

    Thanks

  • 0 in reply to Pongo
    TI__Genius 15780 points

    Hi Pongo,

    You mention that TI FETs do not have a low enough Cgs for your application, but if this is the case then the TPS27081 would not be suitable either. Our standalone FETs have lower Cgs values than our load switches (including TPS27081).

    Thanks,

    Alek Kaknevicius