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LM5010A: LM5010AMH/NOPB Start Up and UVLO

Luke Baer
Prodigy 20 points
Part Number: LM5010A
Other Parts Discussed in Thread: LM5010, LMR38020

Hello,

I'm working with LM5010AMH/NOPB. Side note - the E2E forum did not seem to recognize this as a valid PN, so I had to enter LM5010A.

Per the datasheet section 7.3.2 the UVLO is set to 5.25V. 

I have the part configured to regulate to 9.00V.

The part works perfectly fine when Vin exceeds the desired 9Vout.

The issue I'm seeing is if the input Voltage is ramped up from 0V. Once the UVLO of 5.25V is exceeded the part attempts to begin regulating, and we see an output starting at ~1.5V and this walks up slowly tracking Vin until we get into a stable mode where Vin exceeds the desired output.

I'd like to configure this part to output 0V until it is capable of putting out a solid 9.00V. In other words, is there a method to change the UVLO of the part to something like 9.5V? Any other ideas on approaches to keep the output "off" until we have sufficient Vin to be in a stable control mode?

Thanks in advance!!

The schematic is below

  • 0
    TI__Genius 9381 points

    Hi Luke,

    I see LM5010 don't have EN pin, so it can't change 5.25V UVLO level.

    Thanks

    Colin

  • 0 in reply to Colin Chen
    Prodigy 20 points

    Hi Colin,

    Thanks for the response.

    How about the VCC pin? Reading through the datasheet this is looking for 5.25V to turn the part on. Can this be externally driven with a Voltage divider? Reading through section 7.3.2 and the pin functions table looks like it might be possible.

    What about the RON/SD pin? Is there any way to configure this externally to allow operation once the input Voltage exceeds 9.5V?

    Sorry for being a pest but a workaround is desperately needed for this part.

    Thanks,
    Luke

  • 0 in reply to Luke Baer
    TI__Genius 9381 points

    Hi Luke,

    Sorry for this device for not have this function. VCC pin is the output from the startup regulator. An external voltage (7.5 V
    to 14 V) can be applied to this pin to reduce internal dissipation instead of seting UVLO.

    RON/SD pin is for set Ton or Shunt down.

    Maybe you can use LMR38020 to set UVLO.

    Colin

  • 0 in reply to Colin Chen
    Prodigy 20 points

    Hi Colin,

    How about the Ron/SD pin?

    Pulling this low shuts off the part, correct?

    Can I connect an external FET to pull this low anytime the input is below 9.5V, and then release it once the input exceeds 9.5V?

    This seems like a method to externally set the UVLO.

    Thanks,
    Luke

  • 0 in reply to Luke Baer
    TI__Genius 9381 points

    Hi Luke,

    In your application with an external FET, I can provide an idea for your reference as below:

    From datasheet, we can know that LM5010Ax can be shut down by forcing the RON/SD pin below 0.7 V, so we can set a voltage divider on 0.7V.

    Firstly with Q1 on and when V_Ron/SD>0.7V, the device begin to enable. Then we can provide logic signal to let Q1 off, and 124k is used for setting Fsw.

    Hope this can help you, thank you.

    Colin