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Original question:

LM27762: lm27762 negative voltage is not working

LM27762: LM27762 negative output start up problem

Keith Weller
Prodigy 10 points
Part Number: LM27762


Hi,

We're using an LM27762 on a new design to provide a negative bias rail of about -2.9V with a small current of around 2mA. 

With the values shown in the schematic below I find that the negative rail gets into some sort of start up cycle where it tries to reach the set voltage, shuts off, then tries again. Unfortunately I don't have access to the PGOOD signal.

When I change C12 to 100nF it starts up OK so I suspect some internal current limit is being tripped due to the ratio of C12 to C19.

The original C12 value of 1uF was based on the evaluation module (where C19 was 4u7) so maybe reducing C19 from 10uF would also fix it, but unfortunately the WEBENCH tool isn't yet available to guide me with the optimum part selection.

Does anyone have a guide to selecting C12 and C19 ('C1' an 'CP' in the data sheet) for a given load current?

Thanks,

  • 0
    Prodigy 10 points

    NB. R27 and R28 are incorrectly shown in the schematic - 300K is actually fitted.

  • 0 in reply to Keith Weller
    TI__Mastermind 26390 points

    Hi Keith,

    I am working on your request. I will provide a feedback by tomorrow.

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 26390 points

    Hi Keith,

    For LM27762, all five external capacitors need to be carefully chosen for proper operation. Unfortunately, there is no selection guide for the capacitors but the datasheet would be our best resource here. 

    My recommendation would be to start by choosing the required input and LDO output capacitors followed by recommended Cfly and Ccp. If you find issues with recommended capacitors, then we look into the root cause. 

    Firstly, I see that the output capacitor C13 is less than required. For stable operation we need at least 2.2uF output capacitor.

    The flying capacitor (C1 ) transfers charge from the input to the output. During first cycle C1 is charged and then in next cycle C1 transfers charge to Ccp. So changing Ccp may also require changing the flying capacitor or input capacitor to maintain good overall circuit performance. However, it is hard to define a ratio for this. 10uF Ccp is only required for higher-current applications. For a small current of around 2mA, 4.7uF is sufficient.

    I hope this helps.

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 10 points

    Hi Febin,

    Thanks for pointing out C13 should be increased to 2u2. I find that it starts up OK with a 100nF flying capacitor so I’ll stick with that for now, although it would be good to know what the optimal value is when the design information becomes available.

     Another quirky feature I found today relates to the use of the ‘EN-‘ negative supply enable; I find that the positive supply dips for around 200us when EN- is pulled low to turn off the negative rail even though EN+ is firmly tied to VIN. We’d like to be able to shut off the switching while our converter is sampling, but this positive supply dip unfortunately upsets the analog signal going to the converter. Any suggestions?

     Regards,

    Keith  

  • 0 in reply to Keith Weller
    TI__Mastermind 26390 points

    Hi Keith,

    Could you please share the scope plots of Vin, EN+, EN-, captured during this event?

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 10 points

    Hi Febin,

    I suspect the problem may have been due to a bad joint on the EN+ pin as after re-soldering it seems to have gone away (touch wood!).

    Many thanks for your help.

    Keith

  • 0 in reply to Keith Weller
    TI__Mastermind 26390 points

    Hi Keith,

    That's good news. :) Thanks for updating.

    I will go ahead and close this thread. Please hit Resolved if you agree.

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 10 points

    Thanks Febin, and hopefully some design guidance on selecting the optimum flying capacitor value will become available in due course.

  • 0 in reply to Keith Weller
    TI__Mastermind 26390 points

    Hi Keith,

    I will take the feedback to my team and discuss what can be done in this regards. As this is a very old part and we do not have much supporting data, this could take some time and therefore; I suggest to consider the datasheet as the primary design guide.

    Regards,

    Febin