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TPS23753: Using Vc pin as a supply for external op-amp?

Bryan McLaughlin
Expert 5055 points
Other Parts Discussed in Thread: TPS23753

A customer is using the TPS23753 PoE controller in an IP-webcam application. They would like to use an op-amp fomr something on the high-voltage (24-60V) side of the converter, and they are wondering if the Vc pin of the TPS23753 can supply the power for it.

The datasheet says the pin is a bias voltage for the DC/DC converter, but it also appears to play a part in startup timing. They would need 10mA from this pin.

I do not expect this to be feasible, but I wanted to run it by the experts first. I will likely just suggest they use an op-amp with high-voltage capability, or to consider using the op-amp on the low-voltage side of the converter.

Thanks!

  • 0
    TI__Mastermind 42795 points

    Refer to figure 1 in the datasheet. During converter start up, the Vc pin can source several mA (see Vc section of DS) to charge the Cvc capacitor until the converter starts up (~9V at Vc). When the converter starts, Vc node is powered through Dvc from the T1 auxiliary winding and a typical Vc running voltage is 12V. Maximum Vc is 19V.

    A typical auxiliary T1 output can furnish ~ 12V at 20mA which would be the op amp operating source.

  • 0 in reply to Eric Wright
    TI__Expert 7990 points

    As a follow-on to Eric's response, you might be able to connect a second R-diode-filter capacitor to the bias winding for opamp power. 

    It will only be powered while the converter is running and will not effect startup.  There are subtle interactions between many things in these circuits so you should build one and test.  Check startup, ond overload operation.