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CSD95375Q4M: Buck Power Stage at 100% duty cycle

Martin Clemons
Prodigy 80 points
Part Number: CSD95375Q4M

Hello,

I'm very impressed with the performance and integration level of the Synchronous Power Stage. I would like to use the CSD95375Q4M in the following configuration, which is a little different than the typical application in the datasheet.

The switching topology is a buck+boost conveter, with 2x CSD95375Q4M ICs in a bridge-like configuration with the inductor in the "middle" of the bridge. So IC1 Vin is connected to Vbat (0.85-3.9V), Pgnd is connected to Vbat-neg(0V), and Vsw is connected to L1(terminal 1). IC2 Vin is connected to Vout (0-3.7V), Pgnd Vbat-neg(0V), Vsw L1 (terminal 2).

Vdd is supplied externally at 5.0V nominal.

To achieve this configuration I need 100% duty cycle on the non-switching side of the bridge (IC2 High in buck mode, IC1 High in boost mode). The control of the PWM and SKIP# pins is achieved through digital control.

Here are my questions:

1. The datasheet specifies a maximum on-time duty cycle of 85% ("6.2 Recommended Operating Conditions"). I assume that this is required by the bootstrap capacitor design. I'd like to provide my own high-side gate drive supply voltage ("Vgd") via a schottky diode into BOOT pin (thus replacing the BOOT capacitor). Assuming this is provided, can I drive the PWM pin to provide 100% high-side on duty cycle?

2.  Assuming 1) is possible, what should Vgd be to not exceed the Vgs rating of the internal MOSFET? Nominal operating voltage range on the Vsw pins will be in the range of 0-3.9V. So would a Vgd of 7V for example be safe?

Thank you.

  • 0
    TI__Mastermind 18200 points
    Hello Martin,
    I am going to need to do a little digging on this. I hope to have a response back to you by tomorrow.
  • 0 in reply to Brett Barr1
    TI__Mastermind 18200 points
    Martin,
    See the response from our apps team below:

    "Since this is a buck-boost, the two HS FETs will either work at switching mode or at 100% duty cycle, the internal BOOT switch will be on and off, Vgd and VDD will be shorted when the internal switch is on so that different Vgd and VDD voltages will not work.

    With VDD connecting to BOOT with a shottky diode, it will work for a buck-boost with low Vsw voltage. However, the customer has a max Vsw voltage at ~3.7V to 3.9V, the boot voltage won’t be high enough to make the driver work properly, this method will not work either."