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TPS2553D: FAULT signal not active during overcurrent

Faizul Bacchus
Intellectual 1895 points
Part Number: TPS2553D
Other Parts Discussed in Thread: TPS2553, LM5005

Hello Team

My customer is using the TPS2553 as a load switch in the circuit below with overcurrent set to nominal 266mA. They are testing their boards and found on some boards, the FAULT flag in not active even though the part is in current limit with output voltage collapsing as internal FET is in linear region. They are using resistive load and gradually dropping the resistance while monitoring the output voltage and FAULT flag. The FAULT flag only goes active when the output has collapsed <1V.

Vin = 5V

Load              Vout               Fault

15 Ohm          3.7V                High

13.5 Ohm       3.55                High

10 Ohm          3                      High

8.2 Ohm         2.8                   High

6.8Ohm          2.55                High

5.1Ohm          2.4                   High

3 Ohm            0.8                   Low

0 OHm            0.6                   Low

cid:image001.jpg@01D3C04B.D8E01280

Do you have any suggestions as to why this is happening. By the way once the FAULT flag is active and if they slowly increase the impedance going the other way, the FAULT flag is still active all the way until it comes out of over current limit.

Thanks,

Faizul Bacchus

  • 0 in reply to Faizul Bacchus
    TI__Expert 8980 points
    Hi, Faizul,

    TPS2553 fault will active when current is limited. I think maybe the setup have some problems. Can you help to confirm:
    1. What kind of resistor load are customer using, how they measure VOUT and resistor. The data is weird, Vout/Load is not 266mA.
    2. What's the power supply of Vin? DC-DC or DC source?
    3. Why did they add R192, just curious.

    Regards,
    Bob
  • 0 in reply to Bob Ma
    TI__Intellectual 1895 points

    Hi Bob,

    I agree with your statement that fault will be active when current is limited. That is the expected operation but in this case it does not seem to be doing this. In answer to your questions...

    1. 266mA is the nominal Ios calculated current limit per the equations on page 14. The full range with 100k Rilim resistor can range from 234mA to 303mA. I would say that the device is in current limit as the output voltage is dropping out signifying the internal FET is in the Linear region which happens while it is in current limit

    2. The +5V power supply is from an LM5005 buck converter

    3. R192 is in series to an I/O pin to the micro measuring the FAULT pin.

    I asked the customer to provide some waveforms of VIN, VOUT and FAULT signal. They have provided two cases, one at  6.8ohm load (First Picture) and the other at 0 ohm. In the pictures CH1 (yellow) is VIN, CH2 (blue) is VOUT and CH3 (purple) is FAULT. You can clearly see that while there is a big drop from VIN to VOUT in the first picture, the FAULT signal stays high and is not active. It is only when the output has collapsed totally that the FAULT is active (second picture)

    Thanks

    Faizul

  • 0 in reply to Faizul Bacchus
    TI__Expert 8980 points

    Hi, Faizul,

    I know the current limit range is from 234mA to 303mA when it's set at 266mA. But if your test condition(temperature, Vin...) not change, the variation of current limit will be very small. It should be close to 266mA, in my test it's around 260mA at 0ohm load, 4.7ohm load and 10ohm load. The fault assert at these three condition. Waveform are attached as below.

       But in your test condition, Vout/Load range is from 246mA(15ohm load) to 470mA(5.1ohm load). So I think the device is not in current limit mode. Best way to verify is to monitor current, you can't judge the device is in current limit mode because VOUT drop. By the way, the FET is in saturation region when it's in current limit.

       So I think there might be some problem with your test setup, but I can't get conclusion where the problem is. You can try to use our EVM to verify. If you don't have EVM, you can try to bypass other circuit and just leave 2553D to verify the problem(use DC source not DC-DC, just connect Vout to Load not to other circuit). Also, try to monitor the output current, to see whether it's working on current limit mode. Or you can tell me your set up or send a picture of your test setup, I will try you help you.

        Below are test result based on TPS2553DEVM, Rlimit=100Kohm, Vin=5V, EN and Fault are pull up by 100K resistors to Vin, Cin=1uf, Cout=2.2uf, Load=00hm,4.7ohm,10ohm.

    Load=0ohm:

    Load=4.7ohm

    Load=10ohm:

    Regards,

    Bob