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TPS73201-EP: The overcurrent limit is over spec

Part Number: TPS73201-EP
Other Parts Discussed in Thread: TEST, TPS73201DRBEVM-518

Hi expert,

      I got a feedback from customer talking about overcurrent limit of TPS73201-EP.

      In the datasheet, there is a range of it, which is 250mA ~ 600mA.

      You can see the waveform captured by customer, it is limited by 634mA which is over spec.

      Can you explain how does it happen? It is a 5V to 3.3V design.

      Thanks a lot.

Best regards, Cage.

  • Hello Cage,

    The 250 - 600mA limit is tested for Vout at 90% of intended value. It sounds like this is only a maximum spec under these specific conditions. The short circuit current value only gives a typical value of 300mA (i.e. no max short circuit current).

    Can you describe the waveform to me a little more to better help me understand? It looks like Vout is at 2V when it should be set to 3.3V (as per your comments about the design). What is the output load at this time? Also, what causes Vout to drop to 0V? Any help understanding the circuit and what's changing during the waveform would be useful.

    Thanks,
    Kyle
  • Hi Kyle,

         It is tested by EVM. As below picture.

         And the Vout was 3.3V until customer applied a load on it.

        And Vout went down to 0V when the current increased to 634mA.

        Ch1 is Vout, and Ch4 is Iout.

        Thank you.

    Best regards, Cage.

  • Hi Cage,

    So to clarify, the waveform shows Vout sometime after the load has already been applied that causes too much current? Are you able to upload a waveform the first time the load is applied? In other words, can we see Vout at 3.3V and what happens after the load is applied.

    Thanks,
    Kyle
  • Hi Kyle,

         Here is the waveform at beginning of load increasing.

         For your reference.

    Best regards, Cage.

    CH1: Vout, CH4: Iout.

  • Hi Kyle,

    Sorry that the Vin should be 3.3V, and Vout is 3V.
    Customer is asking how do we specify the range of Over current limit in datasheet?
    In the datasheet, it shows Vout=0.9*Vout(normal), in this case, the Vout normal is 3V, so it should be limited when Vout drops to 2.7V.
    But in this case, Vout dropped to 2V to cause protection.
    Could you explain it?
    Thanks a lot for your help always.

    Best regards, Cage.
  • Hi Kyle,

    Do you have any update for this issue?
    Customer is pushing hard!
    Thank you.

    Best regards, Cage.
  • Hello Cage,

    Sorry for the delay. I'm still looking into this and I hope to have a response by tomorrow.

    Thanks,
    Kyle
  • Hello Cage,

    I would like to clarify what I previously said about the output voltage. The output current limit was tested with Vout set to 90% of Vout(nom). This is just a test condition, not something that will occur (i.e. Vout won't necessarily go to 90% of its nominal value).

    So the only potential concern we see in the waveforms is that the current is above 600mA at some point. Could you clarify how the customer is measuring it? Sometimes current probes have offsets that cause inaccurate measurements . Also where is the current being measured? Ideally it would be the current out of the device, not the device plus output capacitor. The capacitor could be adding current to the output that the IC doesn't sense.

    Thanks,
    Kyle
  • Hi Kyle,

         Customer use a current probe to measure the current after capacitors.

         It is a 3.3V to 3V design, which means the current limit should at 3V*0.9=2.7V.

         But from the below waveform, you can see the Vout didn't drop to 0 immediately when Iout is over 600mA.

         The current limit behavior of TPS73201-EP is latch or what kind of action when TPS73201-EP is at current limit condition?

         Thank you.

    Best regards, Cage.

    ps, I will also ask customer to remove capacitor to see if it improves or not.

    ps, if it is possible you can also take an EVM to duplicate it?

  • Hello Cage,

    Could you make sure that the current probe is properly calibrated? Oftentimes these probes have offsets and this could account for the current difference from 600mA.

    The current limit will not be set to 90%. That is just a test measurement condition. In other words, the current limit test was done when Vout is 90% of its nominal value. When current limit is hit Vout will drop to whatever it needs to in order to stay under 600mA.

    Unfortunately I do not believe there is an EVM for this device.

    Thanks,
    Kyle
  • Hi Cage,

    What EVM specifically is the customer using? Is it for the TPS73201-EP?


    Thanks,

    Kyle

  • Hi Kyle,

        Yes, TPS73201-EP, and EVM is TPS73201DRBEVM-518.

        Thank you.

    Best regards, Cage.

  • Hi Cage,

    Did the customer replace the TPS73201 on that EVM with the -EP version? If not you may want to reach out to the commercial team. Otherwise let me know the answers to the previous questions.

    Thanks,
    Kyle
  • Hi Kyle,

         Yes, customer replaced a TPS73201-EP on the EVM, which is TPS73201DRBEVM-518.

         For your reference.

    Best regards, Cage.

  • Hello Cage,

    Understood. Please let me know if the customer has been able to measure the over current condition without the capacitor. Also please confirm the current probes have been properly calibrated.

    Thanks,
    Kyle
  • Hey Cage,

    Was the customer able to take these measurements?

    Thanks,
    Daniel
  • Hi Daniel,

         Yes, the current probe is very accurate, and even scope is very accurate.

         After they removed output cap, the overcurrent limit is just slightly reduced from 624mA to 612mA.

         Customer just needs a explanation why it is over spec.

         Thank you.

    Best regards, Cage.

    ps, Customer also mentioned one thing, if the difference between Vin and Vout is small, the current limit will be easy to over 600mA, if it is big, the current limit will be about 580mA or smaller.

  • Hey Cage,

    Please show the customer footnote one in the datasheet for the table.

    (1) Minimum VIN = VOUT + VDO or 1.7 V, whichever is greater

    Have their tests been with <1.7 V between Vout and Vin?

    Thanks,

    Daniel

  • Hi Daniel,

         Yes, Vin=3.3V, Vout=3V.

         So the datasheet should need to add extra test conditions in the item of output current limit.

         Because in datasheet, the test conditions is only showing Vout=0.9xVout(nom).

         Thank you.

    Best regards, Cage.

  • Hey Cage,

    Vin=3.3 V and Vout = 3 V satisfies (1) Minimum VIN = VOUT + VDO or 1.7 V.

    The reason the customer is seeing it as higher is actually due to the test condition at the top of the table which says VIN = VOUT(nom) + 0.5 V.

    This is at the top of the table 6.5 on page 6 of the datasheet.

    Thanks,
    Daniel