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TPS22811: Short Circuit Problem

Liam McDonough
Prodigy 10 points
Part Number: TPS22811

Tool/software:

hello,

I am using the TPS22811 as a switch in my circuit. I have a filtered 12V input to the IN(pin 5) of the chip. The EN_N/OVLO(pin 2) is the 12V input stepped down using a voltage divider(the voltage divider brings in the input down to 1.07V. The EN/UVLO(pin 2) is a 3.3V signal coming from a voltage sequencer chip(LTC2977). The DV/DT(pin 7) is connected to a 3300pF cap. The IMON(pin 9) is connected to a 1kΩ resistor that is tied to GND. The output has a 10uF cap on the output. 

This circuit normally has a 5A load on it and behaves as normal. However I need to have short circuit protection for this chip and I observe two issues when I short the output to GND.

First issue: When I short the output to GND I observe 37A from the output to GND before the output shuts off due to short circuit protection. The datasheet says the output should latch off but after the short is removed after about ~6 seconds the output turns on even though I did not toggle the EN/UVLO pin off/on or cycle power. Why is output not latching off?

Second Issue: When I briefly apply a short I see the output shut off. However after the output has shutoff I toggle the EN/UVLO pin off then on again which should turn the output back on. However I observe that the output turns on ~2 seconds after the enable signal goes high. Its almost as if the unit is in some of reset mode after the short.

Any advice would be appreciated.

Thank you

  • 0
    TI__Mastermind 39315 points

    Hi Liam,

    Is your input voltage dropping when you are performing this short. Did you capture any waveforms. How much input cap do you have?

    Waveform showing both issue (probe Vin, Vout, EN and IMON or current) in single capture.

    Best regards,
    Arush

  • 0 in reply to Arush Gupta
    Prodigy 10 points

    Hey Arush,

    My input voltage drops 110mV so down to 11.89V during a short. For my input caps I have a 100uF tantulum cap, a 10uF ceramic and a 0.1uF ceramic cap.

    Here is a scopeshot of what is happening. CH1(yellow) is the output voltage of the chip(pin 6). CH4(red) is the signal going to the EN/UVLO pin. CH2(green) is the output current. The short is applied and the 12V output drops. The short is then removed. I then toggle the EN/UVLO pin off then back on. This should turn the output back on. However there is a ~300ms delay from the EN/UVLO pin going high to the output turning on.

    Here is another scopeshot below. CH1(yellow) is the output voltage of the chip(pin 6). CH4(red) is the EN/UVLO pin. The same thing occurs as the previous scope shot but the delay was measured to be almost 2 seconds.

    Let me know if you can think of why there is such a long delay.

    Thank you

  • +1 in reply to Liam McDonough
    TI__Mastermind 39315 points

    Hi Liam,

    During output short scenarios, one thing we should ensure is Vout should not go below abs min of the device. In the waveform, I am unable to see the voltage. By any chance is it going below -0.3V. This can result in some unexpected behavior. Also in your case, device performing delayed start is not happening some time, right? So I suspect this abs min violation to be more probable cause. 

    Regarding your first issue of short circuit protection being latch off fault. Yes, it is latch off fault. When the current exceeds I_FT, fast trip turns off the device. The device will perform a retry after a deglitch time of few microseconds. If the short is false short and just a transient, the retry will be successful and device should recover but in case of true short, the device will fail the retry and remain latched off.

    Just curious, are you seeing these two issue in multiple devices.

    Best Regards,
    Arush

  • 0 in reply to Arush Gupta
    Prodigy 10 points

    Hey Arush,

    The output ended up going negative during a short circuit event. I added a diode on the output of the circuit and that prevented the output from going negative and fixed the issues I was seeing. Thank you for your help.

    Thank you,

    Liam