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Inrush current using LM22675

Ben Carr
Prodigy 35 points
Other Parts Discussed in Thread: LM22675, LM22671

Hi,

I am having a problem with inrush current using the LM22675. 

The LM22675 is set up to provide a 5V Vout from 24V Vin. The output capacitor is 150uF and the output inductor is 330uH.

The inrush current measured using my scope is coming out about 600mA however the load current is only 30mA. It would be good if we could limit the inrush current to less than 150mA.

Are there any ways I can limit the inrush current for this chip?

Thanks in advance for your advice

Regards

Ben

  • 0
    TI__Guru 57555 points

    I think that the LM22671 is a better choice in this application if you only need 30mA of load.  This device has a lower current limit and an adjustable soft-start feature.  Both of these will help with inrush current.  I have attached a Webench design for the LM22671.  The output capacitor is smaller than what you are using, so that should also help the inrush.  You can experiment with the soft-start cap and see if you can get the inrush current down to what you need.   I have also included a design with the LM22675 if you wish to continue with that device; the output cap is also smaller in that design. 

    If it would help I can send you an EVM with the LM22671 and you can substitute the values in the Webench design and experiment.

    Also, send us your full schematic and the input current waveforms, and we will look to see if there is anything strange.

    FD

    webench_design_433359_112_129534696.pdfwebench_design_433359_113_446796437.pdf

  • 0 in reply to Frank De Stasi
    Prodigy 35 points

    Frank,

    Thanks for your reply.

    The schematic is below:

    As you can see we have 2 of these chips to provide the 5V and -5V power rails.

    C72 was used to delay the starting of the -5V rail.

    The startup waveform is shown below

    Zooming in on the startup section:

    As you can see there are 2 peaks separated by about 35ms corresponding to the delayed start of the -5V rail however the inrush current is very high.

    Do you think this is abnormal?

    Would the LM22671 be a direct replacement? we are quite far into the design and don't really want to do any dramatic layout changes.

    Thanks for your help

    Regards

    Ben

  • 0 in reply to Ben Carr
    TI__Guru 57555 points

    It looks like the inrush is about 1A from the negative reg; that is less than the current limit on of the LM22675 so it is probably normal for an inverter.

    The schematic says 350mA but you mentioned 30mA?  If all you need is 30mA then the LM22671 should be OK; it is in the same package as the LM22675.

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 35 points
    Hi Frank,

    The current draw from the negative reg will be dominated by the minimum load resistor (maximum 30mA)
    The current draw from the positive rail will be a bit higher (about 150mA maximum)

    The total current draw from the 24V rail is about 30mA

    I will try placing a LM22671 and experimenting with the soft start feature.

    Thanks for your help

    Regards

    Ben