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LMQ66410-Q1: Buck not working

Stas Shamanin
Intellectual 450 points
Part Number: LMQ66410-Q1

Tool/software:

Hi!

This is my schematic:

I have strange behaviour. I measure on input 24V as specified. Buck gives low output (0.6v) and warms up. Problem repeats on multiple boards.

There is no SC, if I connect external 5V0 source after buck - board works and consumption on 5V0 is 50mA.

Looks like schematic problem, but I don't see it.

Have any ideas?

Thanks.

  • +1
    TI__Mastermind 24115 points

    Hi,

    The boot pin has an internal capacitor and does not need a capacitor connected between it and ground. It is not recommended for a 24 V to 5 V application that a boot capacitor is connected externally. Should a capacitor be added externally, that capacitor should be connected between boot and SW, not boot and ground. 

    Additionally, connecting an external voltage after the buck may have incurred damage to the device. Please be advised that an external voltage is not meant to be placed on the device's output. 

    Finally, as you are utilizing the device with a fixed output, CFF is not recommended to be connected on feedback.

    Thank you,

    Joshua Austria

  • 0 in reply to Joshua Austria
    Intellectual 450 points

    Hi!

    Thanks, I don't know from where came this Cboot1 to GND, this completely doesn't make sense.

    So stupid mistake to post it here.

    I removed Cboost1 and it started to work.

    Regarding Cff - I see it on evaluation board - you sure I don't need it?

    Regarding connection voltage after buck - I have almost anywhere diodes or low load before buck, so no high current may go back in system, that is why I felt safe to do it. But thanks to mention it.

  • +1 in reply to Stas Shamanin
    TI__Mastermind 24115 points

    Hi,

    In a fixed application, that Cff is not necessary. We provided that pad on the eval board as you can use an external resistor ladder to set output voltage with this device as well. In this case, you can use Cff to improve your phase margin.

    About the external voltage on the device's output side, I just wanted to make sure you are aware of the risks associated with it, but if you have your protection circuits in place to mitigate and accept that risk, then I have no further advice.

    Thank you,

    Joshua Austria