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Original question:

LM51551-Q1: LM51551-Q1

LM51551-Q1: Start up problem about LM51551

Manu Chang
Intellectual 2210 points
Part Number: LM51551-Q1
Other Parts Discussed in Thread: LM51551

Hi Team,

My customers are using LM51551-Q1 to generate a 18V voltage from a 12V battery. However, during start up when the Vin is only 6V the LM51551 cannot successfully start up and the output waveform is like triangle wave.

However when 12Vin start up and then turn the Vin down to 6V the device works normal. The input current during 6Vin start up is about 2.4W and the schematic is as below. 

Pls help to check out what may cause this kind of start up problem and how can we fix it, thanks!

BR,

Manu 

  • 0
    TI__Mastermind 31770 points

    Hi Manu,

    Thanks for using the e2e forum.
    This sound like this issue is related to the softstart. If the output waveform is triangular, the source might be the device trying to start up, but then going back into shutdown and resetting, because a protection like OCP got triggered.
    The current schematic shows a SS cap of 100nF (C498). The first recommendation would be to increase this capacitor to create a longer softstart time. E.g. they can try with 200nF and check if there is already an improvement.

    If there is no change, I would recommend to check for general design improvements. For checking if the inductor, sense resistance and compensation is well selected, please use our quickstart calculations tool:
    https://www.ti.com/tool/download/SNVR481

    Please let me know if additional help is needed.

    Thanks and best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Intellectual 2210 points

    Hi Niklas,

    Thanks for the reply. Customer also has done some measurements to start up the LM51551 at 6Vin, they added the SS capacitor it turns better but still cannot be started completely.

    At the same time, they found that with increased SS capacitor, when decreasing the frequency below 100kHz and RT over 220kohm it can be started.

    But they still have some concerns about using the LM51551 below 100kHz.

    So can the LM51551 normally work with a RT higher than 220kohm? would this bring extra problem?

    Thanks!

    BR,

    Manu Chang

  • 0 in reply to Manu Chang
    TI__Mastermind 31770 points

    Hi Manu,

    Thanks for the update.

    I would avoid setting the LM51551 device to a switching frequency below 100kHz.
    The device specifications state that 100kHz is the minimum frequency the device should be operated with, so forcing the fsw lower would drive the device out of spec, so we can no longer guarantee proper functioning of the device.

    I mentioned or quickstart calculator tool in my last message. Has customer used this tool?
    It would be helpful if they could send me the filled calculator, so I can make a general review of their design.

    Best regards,
    Niklas