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TPS7A33: Vout and Vfb follow Vin

Benjamin Uhing
Intellectual 2135 points
Part Number: TPS7A33

Hi, I'm having trouble with a TPS7A33 in the RGW package. I'm trying to set Vout to -16.5V and have the ability to vary Vin from -19V to -26V. My schematic is below and I have R22 making the connection to Vin installed and R20 making the connection to a load installed. Also note that R24 is installed and that I have more capacitance at the load for a total of 40 uF.

I'm not able to get the correct Vout from the TPS7A33. If I supply Vin= -19V, I see Vfb = -1.22V and Vout = -17V. As I increase the amplitude of Vin up to -26V, Vfb and Vout also increase in amplitude up to Vin = -26V, Vfb = -1.76V, and Vout = -24V.

I've tried removing Cff C17 to no effect. Any other ideas I can try?

Thanks,

Ben

  • 0
    TI__Mastermind 38115 points

    Hey Ben,

    One potential issue I'm seeing is that the feedback resistors R23 and R25 might be a little too large. A general rule of thumb is that you want the current in the feedback path from VOUT to GND to be at least 100 times the leakage current into the feedback pin to avoid accuracy issues. The FB leakage current is listed as 100nA (max), so you would want to size the resistors such that the feedback current is at least 100nA * 100 = 10uA. This would bound the limit on the feedback resistors by R23 + R25 <= 19V / 10uA = 1.9Mohm. I chose the -19V scenario because the -26V scenario is covered by this bound. 

    If this does not solve the issue, try replacing the TPS7A33 with a part that is known to be good. If that still doesn't help then let me know and we can keep trying to troubleshoot the issue.

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Intellectual 2135 points

    Nick,

    Thank you for your response. I believe the issue is actually my VIN supply not providing enough current. I can get my TPS7A33 out of dropout mode using a bench supply. I'll close the thread.

    Regards,

    Ben