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Original question:

LMR33620: Effects when device is unpowered but EN signal is present

LMR33620: Sequence between Vin and EN.

Shuhei Goto
Intellectual 2690 points
Part Number: LMR33620

Hi team,

I have questions regarding a situation that Vin=0V and EN=3.3V

  • Does this device need power sequence control?
  • In current system, EN=3.3V and Vin=0V can be happened. What irregular will be happen in this case?
  • Regarding the thread,
    What is purpose of "10k resistor be placed in series with the EN pin"?
    What ohm is better in the case of Vin=24V?

Could you give me any comment or answer?

Best regarads,

Goto

  • 0
    TI__Mastermind 20906 points

    Hi Shuhei

    • Please make sure EN is not higher than 'Vin+0.3V' as specified in datasheet. It's ok to power Vin by connecting EN to Vin or connecting RENT &RENB to set the Vin UVLO as seen in 9.2.2.10. 
    • Normally This should be avoided according to the section 7.1 Absolute Maximum Ratings in datasheet. There is a internal ESD diode which will conduct when EN>Vin+0.3V, and can be damaged. 
    • The purpose of the resistor is to limit the current through the ESD diode and avoid the damage. 
    • 10k is ok for Vin=24V. 

    Thanks and best regards,

  • 0 in reply to Gui He
    TI__Intellectual 2690 points

    Hi Gui,

    Thanks for your support.

    In this case, power line of Vin and EN are different.

    Then, Is there anything I can do to avoid "EN=3.3V and Vin=0V"? Or do I have to sequence control? 

    Best regards,

    Goto

  • 0 in reply to Shuhei Goto
    TI__Mastermind 20906 points

    Hi Shuhei

    Is Vin=0 or Vin is floating when EN=3.3V ?

    It's ok to power EN first with the 10k resistor if Vin is floating. 

    Thank you.

  • 0 in reply to Gui He
    TI__Intellectual 1400 points

    Today, we conducted an evaluation under the following conditions.
    In the system in which 3.3 V is applied to the EN terminal first, this circuit configuration is used.
    The VIN pin is set to Open and the power is turned on.

    As a result, the following image shows that the VIN is in the open state and the potential difference between EN and VIN: 0.8 V is maintained.
    It is understood that the internal ESD diode does not turn on up to a potential difference of up to 0.8V, so the VIN pin level is simply raised to 2.5V while maintaining that potential difference. Is it true that there will be no internal damage under this condition?

  • +1 in reply to Taku Murayama
    TI__Mastermind 20906 points

    Hi Taku

    I understand that the internal diode is already turned on. But with a 10k Resistor, the power dissipation is limited, so this should be ok. But still, you should do some validations about this, run for long time and see if the circuit operated well. Both under normal Ta and under high Ta.

    Thanks and best regards.