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TPS40170 startup with load

Gunter Biesenbruch
Prodigy 120 points
Other Parts Discussed in Thread: TPS40170, TPS40170EVM-597

Hi,

I am working with a TPS40170 in a 24V DC input and 6V/15A output application.

In the case I let the startup without load running and after the 6V are stable I connect

the load, everything is working ( I testet only 10A).

If I remove the 24V input voltage and switch on again (some seconds later) , the TPS40170 is going to overcurrent. Sometimes to SCP and sometimes to OCP (I measured and counted the HDRV line signals). 

The 24V are  "clean", the input C's are 2*100uF. The output c's are 2*220uF. I use a snubber with 4.7R and 1nF, direct connected near to source and drain between high and lowside FET. 

Is it possible to start the device under full-load, or I have to monitor the PGOOD and only after PGOOD is high, to switch on the load ? Maybe another controller is better for the load conditions ?

Regards

Gunter

  • 0
    TI__Mastermind 35165 points

    Hi Gunter,

    It should be possible to startup with a load on the output.

    During startup, in addition to the load, make sure to include the current needed to charge up the output capacitors. It is possible this causing it to reach the over current limit threshold. Also I recommend using a ceramic capacitor at the input placed very closely to the drain of the high-side FET and source of the low-side FET. This also helps to reduce overshoot and ringing at the SW node to improve the accuracy of the Rdson sensing.

    Could you post your schematic and layout? I suggest double checking the ILIM resistor value to make sure it includes margin for the current to charge up the output capacitors during startup. Also it would be good to compare your layout to the layout recommendations in the TPS40170EVM-597 user's guide located on page 10. Here's a link: http://www.ti.com/lit/ug/slvua14/slvua14.pdf

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 120 points

    Hi Anthony,

    thank you very much for your very fast answer. When I compare my layout with the layout in your link,

    I see I have some "bigger" issues to solve. Additional I didn't took care about the current needed to charge the output capacitors. Here I have to calculate the current for to charge the additional capacitors on the connected boards too. 

    As I have to rework the layout completly, is it neccessary to use a 4-layer pcb ? My customer wants everything yesterday with the lowest price what is possible........It is a consumer application.

    Best Regards

    Gunter

  • 0 in reply to Gunter Biesenbruch
    TI__Mastermind 35165 points

    4-layer PCB is not necessarily required but most I have seen do use 4-layers. If you share your layout I can help point out some of the main things I notice that could cause the early current limit.