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Original question:

LMZ34002: Can the output capacitor value be reduced to less than 100uF when the output current is 150mA with -12V?

LMZ34002: Could you tell me how to determine resistors for EN?

Kazuya Nakai59
Guru 21595 points
Part Number: LMZ34002
Other Parts Discussed in Thread: LMZM23601

 Hello guys,

 One of my customers was concidering using LMZ34002. But the device output capacitor needs 100uF ceramic.

 100uF capacitor is difficult to buy for them recently. So they wanted to reduce the output capacitor value.

 Then I recommended them to use LMZM23601 like application note

http://www.ti.com/lit/an/snva807a/snva807a.pdf

because the device can operate with smaller output capacitor than LMZ34002 base on the folllwing thread.

https://e2e.ti.com/support/power-management/f/196/t/784216

 The customer has a question about EN function in this application note use case.

Q. Could you tell me how to determine resistor values of RENT/RENB connected to EN pin in Figure 33

on page 25 in LMZM23601 datasheet when we use LMZM23601 like the application note?

   

 I think the resistor vallues to start LMZM23601 can be determined by Equation3 in the datasheet

because the device GND is 0V before switching starting.

 But I don't know how to determine the resistor value of RENT/RENB  for LMZM23601 output stopping.

 Could you please tell me how to determine the resistor values?

 Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai.

  • 0
    TI__Mastermind 19891 points
    Hello Nakai-san,

    Normally you would pick the resistor divider based on the Vstart requirement for VIN.
    The VIN stop threshold is a function of the built in hysteresis.

    With the inverting configuration, the negative output will affect the stop threshold since the IC is referenced to the negative rail.

    You are right, the VIN start threshold will not change since VOUT will be at 0V prior to startup.
    Once the regulator starts up and VOUT ramps negative, the IC will be sitting between VIN and -VOUT.
    The VIN stop threshold will effectively shift down by the negative output voltage.

    So if Vout is -5V, the stop threshold for the VIN voltage will now be shifted lower by 5V.

    Cheers,
    Denislav
  • 0 in reply to Denislav Petkov
    Guru 21595 points
    Hello Denislav,

    Thank you very much for your reply.

    Do you have any equation to calculate the resistor values connected to EN pin?

    Thank you again and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 19891 points
    Hello Nakai-san,

    Yes, if you are using the LMZM23601 for inverting application, you can use equation 3 in the datasheet to pick the Vstart threshold. As I mentioned above, the Vstop threshold will be shifted down by the negative output voltage setting in an inverting configuration.

    Cheers,
    Denislav
  • 0 in reply to Denislav Petkov
    Guru 21595 points
    Hello Denislav,

    Thank you for your reply. I will tell the customer your comments.

    Best regards,
    Kazuya Nakai.