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TPS548A28EVM: Operation under low VIN

Ella KIM
Expert 3932 points
Part Number: TPS548A28EVM
Other Parts Discussed in Thread: TPS548A28

Hi Team,

While customer is evaluating TPS548A28EVM, anunexpected operation is observed.

At 4VIN, TPS548A28 cannot drive 13~15A load. Vout is reduced and repeats ON and OFF. It doesn't seem EN pin threshold or IN UVLO is problematic now. 

It doesn't seem to violate min on/off time; ON 781ns (=1/800kHz*2.5V/4V) and OFF 469ns.

Can you advise how it happens?

  • +1
    TI__Mastermind 19150 points

    The behavior you are describing could be the en uvlo (resistors) or thermal shutdown.    

    To troubleshoot en uvlo. 

    Connect a 1.5V or greater voltage on pin 2 of J5 and repeat test or change R4 to 10kohm to lower the stop voltage

    As the output current increases, the input current increase and the input voltage on the board may droop if there is no kelvin sense to the lab supply

  • +1 in reply to David_Daniels
    TI__Expert 3932 points

    Hi David,

    It turns out it was because of current limit of input power supply. There's no issue when setting the supply correctly.

    Let me add one more question. The converter can go stable or instable depending on the input voltage and we usually need to make change in compensation values if VIN deviates from target value. It's impressive for me that EVM can operate such wide input range (4~16V) without stability issue. Does TPS548A28 have a special compensation scheme over another common converter's?

    Sincerely,

    Ella

  • 0 in reply to Ella KIM
    TI__Mastermind 19150 points

    Internally there is a ramp in the control loop that changes height with respect to the input voltage.   

    The ramp height is a gain in the control loop. 

    The input voltage is a gain in power stage. 

    When input voltage changes, the ramp height changes to minimize

    the gain change so it minimizes the effect in the overall control loop.  .