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Extending CSD95372AQ5M max duty cycle

Brett Kutscher
Prodigy 130 points

Hello,

Is there any way to run the NexFET power stages at 100% duty cycle?  One option that I am considering is to apply a higher voltage to the Boot pin instead of using the boot cap.   For the sake of simplicity, assume I am switching with Vdd = 5V, Vin = 4V, and that I have a 9V supply available on the board -- can I apply this directly to the boot pin? 

Part of my confusion stems from the fact that the datasheet states that the absolute maximum rating of BOOT to BOOT_R can only be Vdd+0.3.  In the scenario that I outlined above, the BOOT to BOOT_R voltage would be 9V when the lower FET is turned on, which would exceed this rating.  However, the part is rated to have a Vin of up to 16V.  If were using it in this configuration (with the standard boot cap instead of my alternative power supply), the BOOT and BOOT_R pins would be as high as 21 and 16V respectively, but the chip would be operating in a safe zone.

Is there a diode not shown in the functional block diagram? 

Thanks,

Brett

  • 0
    TI__Mastermind 18200 points
    This power stage was not designed to accept an external supply on the boot pin because we do not have an internal boot diode. Instead we use a P-FET to charge the bootstrap cap (refer to electrical characteristics table).

    Why are you proposing this strategy as opposed to just using a bootstrap cap.
  • 0 in reply to Brett Barr1
    Prodigy 130 points
    Hi Brett,

    Thanks for your reply. I proposed this strategy because I was hoping to extend the duty cycle of the device (ideally to 100%). I was operating under the assumption that often bootstrap-driven devices cannot hold their top FET on indefinitely because the bootstrap capacitor will eventually discharge if the half bridge is not switching regularly (e.g. giving the cap a chance to recharge when the bridge output is low). If the boot pin was externally powered, this would not be an issue.

    But I digress... I guess the question that still remains is: is there any way to extend the duty cycle to 100%?
  • 0 in reply to Brett Kutscher
    TI__Mastermind 18200 points
    Yes, you're correct in that these parts should not be extended to 100% duty cycle.

    General feedback from our applications team was that they were curious as to why you would want to, as if the high side is always on, wouldn't that defeat the purpose of the FETs + driver combo?
  • 0 in reply to Brett Barr1
    Prodigy 130 points
    The short answer is that we're trying to use this part as a generic half bridge (because it's so small and efficient), but not necessarily as a buck converter.