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TPS56121 enable/disable

Harisankar Aravindakshan
Prodigy 140 points
Other Parts Discussed in Thread: TPS56121, UCD90120A, UCD90120

Hi,

I want to control TPS56121 with UCD90120A.

The EN/SS pin has a 33nF cap to GND. In parallel to the cap I have an N channel MOSFET connected (BSS138) whose gate is connected to a GPIO from UCD90120A (GPIO configured active low).

Is this circuit correct? Is there anything else that I should take care of?

  • 0
    TI__Expert 4350 points

    Hi Harisankar,

    Thank you for your interest in TI parts.

    The description of your circuit sounds correct, except that you would want to consider what happens when the UCD90120 is in a reset state. If properly configured, this will allow you to turn the TPS56121 ON and OFF via the GPIO pin. But there are several GPIO pins and they do not all behave the same.

    Pins 17-to-24 will be active low when the UCD90120 is in the reset state, which means that during reset, the GPIO pin will be held low, the BSS138 will therefore be OFF, which will allow the TPS56121 to power up. If this is not what you want, then don't use any of the GPIO pins 17-to-24.

    Instead use one of the other GPIO pins which will go high impedance during reset. You will need to add a pullup resistor from the gate of the BSS138 to an appropriate bias V. These other GPIO pins can be configured as needed, but the pullup will be achieved by the additional resistor.

    Regards,

    MC.

  • 0 in reply to Martin Cardella
    Prodigy 140 points

    Hi Martin,

    thanks a lot for the help. I am using pin 12 as the enable input, and I have a 10K pull up to supply.

    So that means, it will be good to use pins 17-24 as enables for only those supply ICs whose EN is active high

  • 0 in reply to Harisankar Aravindakshan
    TI__Expert 4350 points

    Hi Harisankar,

    Yes, but keep in mind the reset logic on the UCD90120. When the UCD90120 itself is in reset state, pins 17-to-24 are actively held low, so with your inversion from the BSS138, you would obtain undesired logic. You might be able to omit the BSS138 and then the reset logic polarity would be correct, but you would need to configure the GPIO pin as high impedance during normal operation (and not active high), and you would still need the R pullup.

    This 17-to-24 pin field certainly can be used for active high EN pins, but if those EN pins are also SS pins, then the UCD90120 pin would be required to be high impedance when active. Otherwise the active high output would defeat the SS operation.

    Regards,

    MC.