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LM60430: Ouput impedance of the converter when it is OFF

jerome tual
Prodigy 150 points
Part Number: LM60430
Other Parts Discussed in Thread: TPS2116

Hi,

I use the LM60430 to do a step down converter : 12V to 5V / 2A.
I need to add a super capacitor on the 5V power supply.

I would like to know if I have to add a serial diode between the SW ouput of the LM60430 and the super capacitor, for example when there is not a supply on the VIN input :
If the ouput impedance is high a serial diode is not required,
But, if the output of the LM60430 is shunted to GND, I have to add a serial diode in order to avoid to discharge the super capacitor.
Please can you give me the output impedance value of the LM60430 when it is not supplied

Thanks,

Jerome

  • 0
    TI__Expert 8130 points

    Hello Jerome,

    We do not recommend to place a diode in series with the output since that can disrupt the feedback. You might be able to put a diode in series with the input, but you have to be careful that the converter does not try to "boost" the input from the output. I would recommend to place some kind of disconnect switch at the output.

    See the following application note for more details:

    Regards,
    Alejandro

  • 0 in reply to iAlejandro
    Prodigy 150 points
    Hi, 
    Ok, I found in the datasheet of the LM60430:
    If this voltage is less than the output voltage of the regulator, the output capacitors discharge through the device back to the input. This
    uncontrolled current flow can damage the device.
    The input voltage must not be allowed to fall below the output voltage. In this scenario, such as a shorted input
    test, the output capacitors discharges through the internal parasitic diode found between the VIN and SW pins of
    the device. During this condition, the current can become uncontrolled, possibly causing damage to the device. If
    this scenario is considered likely, then a Schottky diode between the input supply and the output should be used.

    But ​: the voltage rating of SW to VIN will be exceeded due to the supercapacitor ? (The IC is disabled when there is no input voltage)

    You said I would recommend to place some kind of disconnect switch at the output. What are you thinking ?

    I was thinking a diode in series​ as below : but the forward voltage can be uncomfortable and the case of the diode (SMC) is not suitable for a layout with a critical density
    Thanks,
    Jerome
  • 0 in reply to iAlejandro
    Prodigy 150 points
    Hi, 
    Ok, I found in the datasheet of the LM60430:
    If this voltage is less than the output voltage of the regulator, the output capacitors discharge through the device back to the input. This
    uncontrolled current flow can damage the device.
    The input voltage must not be allowed to fall below the output voltage. In this scenario, such as a shorted input
    test, the output capacitors discharges through the internal parasitic diode found between the VIN and SW pins of
    the device. During this condition, the current can become uncontrolled, possibly causing damage to the device. If
    this scenario is considered likely, then a Schottky diode between the input supply and the output should be used.

    But ​: the voltage rating of SW to VIN will be exceeded due to the supercapacitor ? (The IC is disabled when there is no input voltage)

    You said I would recommend to place some kind of disconnect switch at the output. What are you thinking ?

    I was thinking a diode in series​ as below : but the forward voltage can be uncomfortable and the case of the diode (SMC) is not suitable for a layout with a critical density
    Thanks,
    Jerome
  • 0 in reply to jerome tual
    Prodigy 150 points

    About a output disconnect switch solution, I looked for a power multiplexer as the TPS 2116, it can be replaced the diodes D38 & D40 and I have to keep D39 to disconnect the IC (LM60430) ?

  • 0 in reply to jerome tual
    TI__Expert 8130 points

    Jerome,

    A power mux is one of the solutions. Please take a look at the following reports for extra details:

    1. 11 Ways to Protect Your Power Path
    2. TPS2116 Application Brief



    Regards,
    Alejandro

  • 0 in reply to iAlejandro
    Prodigy 150 points

    Hi,

    It looks like good.

    But the TPS2116 is not in production.

    When will it in production ?

    Thanks,

    Jerome.

  • 0 in reply to jerome tual
    TI__Expert 8130 points

    Hi Jerome,

    Unfortunately, I do not know. I personally am not involved in that team or department. I would suggest to create another post on the forum for the TPS2116. Therefore, the right point of contact will be able to provide you with the information.

    Kind regards,
    Alejandro