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TPS7A39: Feedforward capacitors and feedback pin protection

Marko Wuensch
Prodigy 110 points
Part Number: TPS7A39

Hi,

due to noise reduction and better PSRR as mentioned in the datasheet I want to use feedforward capacitors of 0.01 µF each. As described in chapter 2.3 of https://www.ti.com/lit/an/sbva042/sbva042.pdf I can protect FBP pin against undervoltage.

But how should I do this with the FBN pin? This pin has typically 3.7 mV and can reach 10 mV or down to -10 mV. The Absolute Maximum Rating for FBN is 0.3 V. A schottky diode against GND could compromise feedback loop, I think.

How to protect FBN against overvoltage when negative input voltage and negative output voltage collapse?

Thank you,

Regards,
Marko

  • 0
    TI__Guru 50025 points

    Hello,

    I have begun looking into your question and will provide a response tomorrow (7/9/20).

    Regards,

    Nick

  • 0
    TI__Guru 50025 points

    Hi Marko,

    Cff forms an RC circuit with R1 (the first feeback resistor from OUT to FBN) with tau = Cff*R1. If Cff*R1 is large enough, Cout can discharge faster than Cff and can push the FBN pin to a positive voltage. 

    To avoid this issue, put a Schottky diode (with active forward bias < 300mV) with anode at FBN and cathode at GND. During normal operation, FBN is close to 0V as you pointed out, so the Schottky diode will have nearly 0V forward bias and it is in an "off" state. If FBN goes positive during a shutdown event, the Schottky diode becomes forward biased and a a new path to ground opens to get rid of the excess charge.

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 110 points

    Hi Nick,

    thank you for your clarification.

    Best regards,

    Marko