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TPS2511: CS PIN no pulling low causes output voltage is wrong

Ken Zhang
Intellectual 2785 points
Part Number: TPS2511

HI, ALL

My customer had found some failure of TPS2511 in MP status. 

During tests, we find that CS/ pin of the defective TPS2511  can’t be pulled low  even the output current is more than half of the current setting by Ilim_set resistor, and the output voltage will be unstable.  if manually pulling low CS/ PIN, output can change from unstable to stable.  What is possible reason of CS/ pin impacting output stability in defective TPS2511?

The sink current of CS pin with defective TPS2511 is pulse shape, but for good TPS2511, the sink current can be continuous.  which one of CS pin sink current mode is right? the detail test waveforms are attached.TPS2511-OUTPUT ERROR-30-sep.pdf

  • 0
    TI__Genius 9630 points

    Hi Ken,

    Please try using a larger resistor for your pullup. Figure 23 in the datasheet shows a typical application circuit using a 100k resistor for the CS pin. Let me know how this changes your testing. 

    Regards,

    Emma

  • 0 in reply to Emma Hoying
    TI__Intellectual 2785 points

    Emma,

    I guess you misunderstand the test report. let me clarify it.

    The CS pin sink current listed in datasheet is 2mA to 10mA continuous current. I can't understand why you suggest to change pull-up resistor from 47Kohm to 100Kohm.

    From the test report, the sink current from CS/ pin of defective TPS2511 is 25mA pulse and  only for 500nS,  but that for good TPS2511 can be as high as 35mA continuous current. Therefore, my question is if the sink current with defective tps2511 is right or not.

    If the pulse sink current waveform with 25mA for 500ns is NOT right, it means that the defective tps2511 maybe damaged, I will apply for failure analysis from CQE team.

    As currently, a 100nF capacitor is connected with CS pin  to GND on customer board. If the pulse sink current of CS PIN is right, I will let customer remove the 100nF capacitor because the 25mA pulse current on cs/  pin can't let voltage across 100nF capacitor drop low. Removing 100nF capacitor is already tested and can solve the issue. Then we need to explain to customer:  The CS/ pin is a open-drain output type of pin.  even CS/ PIN can't be pull low, why the output of TPS2511 will be unstable and seems to be caused by CS/ pin?

  • 0 in reply to Ken Zhang
    TI__Genius 9630 points

    Hi Ken,

    Please note that you are trying to pull a current of 35mA. This is above the absolute maximum rating for this pin. Using the device outside of its absolute maximum ratings will damage the device, which is what probably what happened to the devices you sent data for. 

    One way to limit the current is to use a weak pull up resistor instead of a strong one. The reason I asked you to change from a strong pull up resistor to a weak pull up resistor was to limit the output current of CS. The schematic you had at the top of your report shows the pullup as 10k. 

    Also, you do not need a 100nF capacitor connected to CS. 

    If you are looking for expected function of the CS pin, please check out the application curves in the TPS2511 datasheet. There you will find the expected behavior of the CS pin under different applied loads. Figure 32 shows that CS should match Iout:

    Regards,

    Emma