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TPS27SA08: Voltage at output when load switch is off

Jerome Villapando
Intellectual 290 points
Part Number: TPS27SA08
Other Parts Discussed in Thread: HSS-MOTHERBOARDEVM,

Hello,

I have an inquiry about the TPS27SA08 load switch. When the load switch is off (EN = low), there seems to be a voltage appearing at the output that scales with the input voltage applied at VBB. I have tested this with the evaluation board (HSS-MOTHERBOARDEVM) and our custom board. At an input voltage of 28V, I am seeing around 6V at the output. This voltage seems to be a weak floating value as I was able to easily pull the output voltage down to GND with a pulldown resistor of 15kOhms. However, with our custom board, the output node is a high impedance node and it is only able to pull the floating voltage value down to about 4V.

Jerome

  • 0
    Intellectual 290 points

    I just realized that figure 9-8 of the TPS27SA08 says that when the load switch is off "VOUT depends on external conditions". What does this mean?

  • 0 in reply to Jerome Villapando
    TI__Genius 16390 points

    Hi Jerome,

    When switch is off, there should be no current flowing through the high side switch. If there's a pull up resistor for open-load detection, then the output will be pulled to Vbb level. In your case, it seems the output is at a lower level. It could be caused by output capacitor or some other circuitry at the output.

    Would you be able to share the full schematics and some screenshots you have for the output? Thanks!

    Regards,

    Yichi

  • 0 in reply to Yichi Zhang.
    Intellectual 290 points

    Hello Yichi,

    Since this is also occurring on the dev board HSS-MOTHERBOARDEVM, I'll use that schematic as reference for now. From the looks of it, the output of the load switch is simply connected to some zener diodes. Our custom board has a similar setup except with voltage dividers on the node to measure the VOUT through our MCU's ADC. Here is a link to the dev board's schematics: https://www.ti.com/lit/ug/slvubd4b/slvubd4b.pdf?ts=1628909261972&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FHSS-MOTHERBOARDEVM.

    Is having a pullup recommended? Technically, our voltage divider acts as a pulldown in this case, albeit being very weak as it its 1 MOhm in total. We prefer if there are no unknown voltage states at the output at any point because we are doing our own specific load detection through the value of the output voltage of the switch.

    Jerome

  • 0 in reply to Jerome Villapando
    Intellectual 290 points

    Hello Yichi,

    I think I have figured out what was happening. I had DIA_EN high all the time which enabled the internal pullup to VBB. When I have DIA_EN low, the output is behaving as it should hovering at a low value when the switch is off. As for why pullup pulls the output at around 4-6V instead of VBB (28V in our case) might be due to the fact that the internal pullup is quite weak.

    Do you have any thoughts?

    Jerome

  • 0 in reply to Jerome Villapando
    TI__Genius 16390 points

    Hi Jerome,

    Nice finding! Could you try a smaller pull up resistor like 10k to see if the voltage is closer to the Vbb level? There might be some leakage current even when the switch is off.

    Regards,

    Yichi

  • 0 in reply to Yichi Zhang.
    Intellectual 290 points

    I used the a 15kOhm resistor and it was able to pull it up to Vbb. However, I would like to debug this issue further as I'd rather use the internal pullup rather than implementing an external fet and pullup to do open-load detection while the load switch is off. Since a Vbb of 28.3V gives a 6.17V at the output, there must be around 22.13uA of output current leakage assuming the pullup is exactly 1Mohm... I even desoldered all the output components and the result is the same. Do you have any insight on this?

    Jerome

  • 0 in reply to Jerome Villapando
    TI__Genius 16390 points

    Hi Jerome,

    From the datasheet, it seems to be way too high even at high temp of 125 degC.

    One thing you can try is to decrease the Vbb to see if you see the output voltage goes down proportionally - that's an indicator that the leakage current is constant.

    Also another thing is to try to solder another part to see if you see consistent results.

    Please let me know your findings.

    Regards,

    Yichi