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TPS1685: Not starting when loaded.

Mickael BESSON1
Prodigy 25 points
Part Number: TPS1685


Hello,

I’m currently facing an issue with the TPS16850.
Specifically, whenever I apply a load above about 100 mA to the output of the TPS16850, the device won’t start up properly.

Too eliminate causes has much has possible I have follow the datasheet and disable OVP, UVP, overcurrent and fast-trip protection.

image.png

I’ve set the Enable pin separately with a 4 V supply, allowing me to control activation independently of the main input voltage. The input voltage itself is in the 30 V to 60 V range, and the normal operating current is between 2 A and 5 A in steady state.

I've tried different dv/dt cap values to see if there any difference and the result is always the same. The device won't start if a load is present at the initial startup. The load used is an SDL 1020X in CC mode between 100mA and 1A.

Test with the RPMGQ-12-20 mounted.

image.png

The initial problem come from the RPMGQ12-20 asking for current when the voltage is sufficient to him to startup but to make the debug easier and reproductible I use an electronic load.

I’m attaching the schematic and board layout to help clarify the setup. Hopefully, these details will make it easier to pinpoint what might be going on.

image.png

image.png

Test with : 
PSU Setup : 
- VIN 40V
- Current limit 8A
Electronic Load : 
- Constant Current 0.5A

image.png

PSU Setup : 
- VIN 40V
- Current limit 8A
Electronic Load : 
- Constant Current 0.07A

image.png

PSU Setup : 
- VIN 40V
- Current limit 8A
Electronic Load : 
- No Load

image.png

image.png

Thanks a lot for your help!

  • 0
    Prodigy 30 points

    I am having a similar issue. Let me know if you have additional findings.

  • 0
    Prodigy 30 points

     it seems eload in current limit mode has some inrush that caused your efuse to fail to startup. Any inrush current more than 0.5A during start up phase of Efuse would turn off the efuse. So downstream load must guarantee to not draw more than 500mA. Use power good indicator to gating the downstream load or delay ur load to only activate after your efuse startup. 

  • 0 in reply to hongen tu
    Prodigy 25 points

    This is why I have post the trace with this parameter : 
    PSU Setup : 
    - VIN 40V
    - Current limit 8A
    Electronic Load : 
    - Constant Current 0.07A

    We clearly see no inrush above 0.5A.

    I'm asking if it is the normal working condition or a failure of the components itself.
    If it is, the component is useless in our application and this kind of information should be write in the datasheet.

    Best regards. 

  • 0 in reply to Mickael BESSON1
    Prodigy 30 points

    I will let TI chime in. I do see ur first plot reach to 500mA. Following plot do have a spike in current, maybe need to zoom in to see if it get clipped due to sampling rate of ur scope. Don’t trust eload during startup, eload is good for steady state test. 

    u need to leverage PG to control downstream loads. Active load is loading during startup I suspect. 

  • 0
    TI__Mastermind 36751 points

    Hi Mickael,

    Sorry for the delay.

    Inrush current of cap + e-Load start-up current may have exceeded the 0.5A start-up current limit.

    Please make sure to use CR mode in e-load or actual resisatance.

    You can increase the dvdt cap so inrush current due to cap can be reduced and device can have a successful start-up.

    Thanks 
    Amrit