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TPS7A57: Output voltage not as expected

Arnav Shah
Prodigy 100 points
Part Number: TPS7A57

Hello,

We are implementing the TPS7A57 as follows: https://imgur.com/RVTk2RR

I measure the output voltage as 2.1V when our reference from the DAC MCP4728 is 0.93V. We have shorted the bias and input voltage to 3.3V although it doesn't matter the internal circuit should use the 3.3V input voltage as no bias is needed (I have tried enabling and disabling the CP with J105). Why is the output voltage 2.1V when we expect 0.93V as defined by the reference? 

Appreciate your feedback.

Thank you!

Arnav

  • +1
    TI__Genius 12511 points

    Hi Arnav,

    A few things,

    What is SNS tied to, is it VOUT? The schematic states V1 but that is not what VOUT is on the schematic. This would absolutely cause VOUT to be out of regulation if SNS is not tied closely to where you'd like to see the reference voltage (0.93V) at.

    The Vref pin is actually tied to a current source that is supposed to connect to a resistor to GND. If utilizing an external DAC then see section 8.1.17 to utilize voltage mode margining. During this operating the DAC is not tied directly to the pin, there is a resistance between the two points and additionally still a setpoint resistor to decide the voltage target and the DAC then adjusts that to be the precise voltage.

    Regards,

    John

  • 0 in reply to John Schneider
    Prodigy 100 points

    Hi John,

    V1 is connected to the 0.93V plane around 1 inch away. It is intended to remote sensing; are you saying it must be close by? Anyways, this should not affect the regulation if it is far away, just the noise on the line?

    Understand about vref, this implementation is not good. We need to add some resistance and default power voltage when the DAC is in reset. However, I have verified that the reference voltage is 0.93V with a multimeter, this should not be affecting operation either. 

    Thanks,

    Arnav

  • 0 in reply to Arnav Shah
    TI__Mastermind 18466 points

    Hey Arnav, 

    What is the current going into the REF pin?

    This part is special in that the output is set by a current, generating a voltage, on the REF pin. Does the problem persist if you connect a resistance to ground on the REF pin?

    Also, the SNS pin must be connected to the output; per the datasheet: 

    The unity-gain configuration is achieved by connecting SNS to OUT. Minimize trace inductance on the output and connect COUT as close to the output as possible.

    Best,

    Juliette

  • +1 in reply to Juliette
    Prodigy 100 points

    Hi Juliette,

    There is no current sinking into REF. I am using a voltage source DAC, not a current source. The 50uA current source at the REF is irrelevant since I generate the reference externally. I think I will still add the Rref resistor for when the DAC is in reset.

    What is the purpose of the 100Kohm resistor? 

    The SNS pin is connected to the output, just far away to remotely sense back the IR drop. I think this should be fine.

    Thanks,

    Arnav

  • 0 in reply to Arnav Shah
    Prodigy 100 points

    Never mind, I got it the 100Kohm resistor is to tune the current to Rref from 0-50uA to margin the voltage. Makes sense

  • 0 in reply to Arnav Shah
    TI__Mastermind 18466 points

    Hey Arnav, 

    was this able to fix your problem or are you still having the same issue?

    Best,

    Juliette

  • 0 in reply to Juliette
    Prodigy 100 points

    No the reference was always correct, but I found the problem. There was an open connection in SNS. After shorting it, I am able to regulate the output voltage. Thanks for your input, I think I will continue to use the DAC directly connected to ref, much easier implementation and more flexibility.