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LMR38020: LMR38020SDDAR

?? ?
Prodigy 10 points
Part Number: LMR38020

Tool/software:

LMR38020SDDAR has no output and can only be restored when the input terminal is completely powered off and then powered on again.

The input VIN and EN pins of LMR38020 are directly connected together; The input circuit of LMR38020 has added a pmos as an input control switch, which may cause leakage and result in a voltage of around 1.2V on the input VIN and EN pins of LMR38020 when the module is powered off (pmos is turned off).
Can you help analyze the situation where the output of LMR38020 is locked? Even if it is powered on or off again, it cannot be restored unless the voltage of around 1.2V at the input VIN and EN pins of LMR38020 is discharged during power off to restore it.

  • 0
    TI__Mastermind 29145 points

    Hi,

    Can you provide your schematic so that we can have a clearer view of what you are asking? What do you mean if the output is locked?

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 10 points

    Sorry, the schematic is a confidential document and I am unable to send it out here. I am using a simplified diagram here to illustrate.
    As shown in the above figure, after Q36 is disconnected, the point indicated by the arrow will have a voltage of about 1.2V due to PMOS leakage, and LMR38020 will have no output. At this point, the PMOS connection is controlled, and the arrow indicates a point of 48V, but LMR38020 still has no output, which is not a normal phenomenon.It's like the output state is locked to no output.If you want to restore normal output, you need to completely reduce the voltage at the point indicated by the arrow from 1.2V to 0V when the PMOS is disconnected, then reconnect the PMOS and restore normal output after the point indicated by the arrow becomes 48V.

  • 0 in reply to ?? ?
    TI__Mastermind 29145 points

    Hi,

    I think I understand. The reason you have to fully discharge the output is due to the device falling threshold in order to fully turn off the device, since there is a difference between Ven rising and Ven falling threshold. The device cannot (and should not) have the EN pin floating. One way to do this is to add a discharge path from EN back to ground. You can do so via a resistor divider or NPN/NMOS switch that turns on when Q36 is off. 

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 10 points

    Hello, after listening to your answer, I have a few more questions.
    1. Why does the device have no output when the voltage of Ven rises when Q36 is turned on, even if the device is not completely turned off?
    2. This failure does not occur 100%, there are only a few cases in the market. I am unable to conduct experiments in the laboratory to verify the reliability of your proposed solution. In fact, I simulated the same input conditions in the laboratory for half a month and still did not encounter the problem.


    I hope you can help propose an experimental plan that can stably reproduce the failure case, or explain the principle of the failure case. For example, why can EN pin floating cause abnormal output, or what impact will the difference between Ven's rising and falling thresholds have?

  • 0 in reply to ?? ?
    TI__Mastermind 29145 points

    Hi,

    Basically it is a function of the device logic.

    When device is first turned on, the EN pin voltage must be above the rising threshold. Afterwards, the EN pin voltage will have a different falling threshold. This is implemented using some hysteresis circuit. I believe what you're seeing is a case where your EN pin voltage is floating in between the falling and rising threshold, so the device is unsure of whether to shutdown or rise, even after turn on Q36 to raise the input voltage to 48V. As mentioned in the datasheet, the EN pin cannot be left floating. A consequence of this could even be the internal LDO is not enabled, which powers the internal circuits to switch. 

    In typical application, you can connect a resistor divider from VIN to GND with the tapping point at the EN pin, and that should be enough to discharge the EN pin once you turn the switch off. 

    Thanks,

    Richard