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TPS737: The output voltage is not Zero when Enable pin is triggered

Aakash Dey
Intellectual 590 points
Part Number: TPS737

Hi,

I want to make the output voltage of TPS73733 Zero by triggering the Enable pin (EN_REG) of the IC. But when I triggered, I found the output voltage as 1.3V instead of 0V. 

  • 0
    TI__Mastermind 43630 points

    Hi Aakash,

    If the output is 1.3V when the TPS737 is disabled, something on the board must be pulling it up. 

    I notice that there is a 3.3V and 5V rail for the MCU. if the 5V rail still up when the TPS737 is disabled? Is there any chance there is leakage via the MCU or some other circuit on your board?

    The TPS737 has a ~80Kohm resistance from VOUT to GND so the pull-down is very weak.

    A device like the TPS746 has a much more aggressive pull-down when the device is disabled as shown below:

    I hope this helps.

  • 0 in reply to JCHK
    Intellectual 590 points

    Hi John,

    As you told I have 2 Rails, 5V_MCU and 3.3V_MCU. But, I want to disable the output rail of TPS73733 i.e. 3.3V rail. That means, when I disable the TPS73733, the voltage measured at the 5V_MCU rail is 5V (as expected), but the voltage measured at 3.3V_MCU rail is 1.3V (instead of 0V). 

    For the case of leakage, I have tested by changing R109 to 1M (earlier it is 4.7K as shown in schematic), but the result is found same.

    Moreover, I have found some suggestions in your forum (https://e2e.ti.com/support/power-management/f/196/t/662303?TPS737-when-EN-0-there-is-still-2-4V-on-the-output-pin ). Thus, I have tested the same with such active Pull down configuration. But the result is found same (1.3V instead of 0V in 3.3V_MCU rail). 

    Please check the below schematic (for Active pull-up configuration). Am I doing something wrong in this schematic? If yes, please help.

  • 0 in reply to Aakash Dey
    TI__Mastermind 43630 points

    Hi Aakash,

    If you monitor the EN_REG signal. is it actually going below 0.5V when you try to disable?

    Between 0.5 and 1.7V, the behavior of the TPS737 is undefined. 

    If the voltage at pin 5 is below 0.5V and you are still seeing 1.3V at the output, are you sure the current is coming from the TPS737?

    Regards,

  • 0 in reply to JCHK
    Intellectual 590 points

    Hi,

    I monitored EN_REG signal and found that it is going below 0.5V (measured as 0V). In this case, I have seen, the Power LED connected to the Output line of TPS73733 is not glowing. From this point, I can say that when the EN_REG is below 0.5V, the current output of TPS73733 is near about ZERO. 

    But, my question is why the voltage at the output pin is not going down to 0V.

    Moreover, I have found some suggestions in your forum (https://e2e.ti.com/support/power-management/f/196/t/662303?TPS737-when-EN-0-there-is-still-2-4V-on-the-output-pin ). Thus, I have tested the same with such active Pull down configuration. But the result is found same (1.3V instead of 0V in 3.3V_MCU rail). 

    Should I go  with the Active pull-up configuration?If yes, Please check the below schematic (for Active pull-up configuration). Am I doing something wrong in this schematic? If yes, please help.

  • 0 in reply to Aakash Dey
    TI__Mastermind 43630 points

    Hi Aakash,

    If you remove the TPS737 from the board. Can you still measure 1.3V at VOUT?

    There must be something else pulling this rail up.

    Please confirm.

  • 0 in reply to JCHK
    Intellectual 590 points

    Hi, 

    With load connected, the measured voltage is 1.3V. But with no-load, the voltage is going down to Zero.

    As in my load, a various capacitors are connected, If this 1.3V is due to the charge voltage of the capacitors. If yes, Will a active pulled down solve the issue?

    Please advise. 

  • 0 in reply to Aakash Dey
    TI__Mastermind 43630 points

    HI Aakash,

    It depends on the source impedance of whatever is pulling up the load. If this is very high, than yes, you can easily add an active pull-down. If it is low, then something might fail. 

    One possible means of testing this is when the LDO is not on the board, add a zero-ohm resistor from VOUT to GND. If something is getting hot, a pull-down will not likely help. You would need to somehow block the source of whatever is pulling up the rail. 

    I hope this helps.

  • 0 in reply to JCHK
    Intellectual 590 points

    Hi, 

    Thanks for your suggestion. Let me test whatever you mentioned above with LDO unconnected.I will get back to you soon with my test results.

    Thanks.