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TPS62750: Reverse current protection not working?

Part Number: TPS62750

I am currently bringing up a new design that is using the TPS62750 as the main PMIC. The application is a USB device that has a 470mF supercap (not shown) and we are using the TPS62750 to bring down the 5V input voltage to 3.85V that is both charging the supercapacitor and supplying our system load.

When we disconnect the input power source, we are seeing the input voltage remain high from what seems to be back driving from the output supercap into the input voltage. We are seeing a 550mV drop, likely the forward voltage of High Side MOSFET body diode. Due to this and the voltage being sufficiently high to keep the enable pin high, the output capacitor discharge function isn't activated either. Green waveform is input voltage and Yellow waveform is output voltage.

Has anyone had a similar experience with this IC? Any way to avoid this?

  • Hi David,

    As stated in the DS page 16 the reverse blocking and the discharge function will be active when the device gets disabled. (EN=low)
    The best way would be to control the EN pin discrete and pull it low when the input source is disconnected.
    Can you try this and see if this works for you?

    Best Regards,
  • Hi Michael,

    I modified the circuit to do just that. There still seems to be a back driving effect, although it has been reduced. In the following screenshot, Yellow is VIN, Green is VOUT and Purple is EN. As VIN drops, the enable circuits detects the drop and forces enable low. However, VIN remains roughly 1V lower than VOUT.

    We can also see the discharge load of the TPS62750 is being applied as the VOUT is slowly discharging - so the discharge function is working. But with 470mF of output capacitance, this will be extremely long.

  • Hi Michael,

    The only other possible source of back driving between VOUT and VIN in this application is a protection clamp on the USB signals to VIN. I measured both USB signals and they are both LOW. So the back driving is definitely coming from the TPS62750.
  • Hi David,

    I did some bench tests to reproduce your scope plot.

    Could you please state your desired requirements and concerns when you disconnect your design from the USB port?

    Here are the requirements for the different functionality of the IC:
    - Vout discharge: EN low and Vin >UVLO
    - Vin discharge: not available
    - reverse current blocking: EN low (leakage current of few mA possible)

    Best Regards,