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LM5146-Q1: LM5146-Q1 high no load input current

Part Number: LM5146-Q1
Other Parts Discussed in Thread: LM5146

Hi,

I've got a problem with one of the designs based on LM5146. All seems to function correctly except for quite high input current as for no load (it consumes roughly 50-60 mA of current depending on Vin). We've manufactured a couple of prototypes and all seem to have precisely the same effect. After reading some other threads on the forum I've checked also supplying Vcc from the Vout and it seems it helped although just a bit. Is there something that we missed in the design process ? What would be worth checking ? I've attached schematics.

Best, BuckConverter12-8.pdf

  • Hello

    First you should be sure that the device is in diode emulation mode by grounding the SYNCIN pin, rather than leave it floating.

    You should also check the waveform at the SW node to be sure.

    If you still see a somewhat high input current, then your FETs may have larger gate capacitance than desired.

    You may also try shorting out the resistor in series with the boot capacitor; that may be slowing the transition's down to the

    point that it is impacting your light load efficiency.

    I assume that the snubber on the SW node is not being used.  If it is, you can try removing that as well.

    Thanks

  • Even though SYNCIN is not grounded I believe that the device is in diode emulation mode. Check out waveforms of SW, HS gate and LS gate attached. Snubber is not used and removing the resistor in series with the boot capacitor did not introduce any visible change. Any further ideas ?

  • Hello

    It does look like DCM.

    You can get the EVM for this device and check the input current and compare.

    Also, be sure the there is no load.  The above waveform would indicate some finite load.

    You may also need FETs with lower gate charge to reduce the power needed to drive the gates.

    A lower switching frequency may help.

    Thanks

  • Hi Frank !

    Thanks for your suggestions. I've checked out the EVM and official datasheet states 20mA input current at no load. I understand that my design with double HS mosfets automatically has doubled gate charge. Do you think that may have an effect of more than doubling the input current at no load ? It seems that I also use higher gate charge mosfet on the LS. Switching frequency is not that high though - it's 606 kHz.

    Thanks,

  • Hello

    You can temporarily remove one HS FET and test again at no load.

    If that helps enough, then maybe you could choose different FETs.

    Thanks

  • Hello

    I will close this post due to inactivity.

    You can re-open if you have any further questions.

    Thanks