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TS5A3166 isolation in powered-off mode

Tamas Ledeczi
Intellectual 390 points
Other Parts Discussed in Thread: TS5A3166, TS5A3167

I have designed a battery powered device where there is a need to measure the battery voltage (VBAT, 2xAAA batteries) during operation. The VBAT rail is connected through the TS5A3166 analog switch to a GPIO/ADC input of the applied microcontroller. There is a boost regulator in the design which provides the internal 3.3V power rail from the VBAT input. The TS5A3166 is powered from the internal 3.3V rail. The reason I selected the TS5A3166 for the job was the "Isolation in Powered-Off Mode, V+=0" feature highlighted in the datasheet. The theory behind was that if the unit is switched off (boost regulator shuts down and 3.3V -> 0V) the VBAT connected through the TS5A3166 (VBAT connected to NO, pin#1) will not feed the microcontroller through the GPIO/ADC pin. However, this is what I see. Assuming a 3V VBAT input, when the booster shuts down, the 3.3V rail starts to collapse exponentially but stops at around 1.6V and remains there. In this state the switch is open and I can see the VBAT voltage at the COM (pin#2) pin as well, despite the fact that the IN pin (pin#4) is solid '0'. Removing the switch, the 3.3V rail goes down to 0V when the booster shuts down.

Looks like to me there is protection diode between the NO (pin#1) and the power pin (pin#5) of the TS5A3166 and the VBAT input directly feeds back the 3.3V rail through the diode and doesn't allow the 3.3V rail going down to 0V. Please clarify if my understanding is correct.

Thanks:  Tom Ledeczi

  • 0
    TI__Expert 6395 points
    Hello Tom,

    Do you have a schematic showing the connections on the TS5A3166 so I can test the same setup on my bench? I think I have a good idea from your description but want to be sure.

    In addition is it possible for you to measure the leakage into the NO pin and out of the COM pin?

    Regards,
    Andrew
  • 0 in reply to Andrew_Mason
    Intellectual 390 points

    Hi Andrew,

    Thanks for your quick response. I have a pdf print of the simplified schematic how the TS5A3166 is connected. However, I don't see the right tool to attach a file, I can just insert a file. Should create an image file (eg. jpg) and insert? Or do you have a mail address I can send the pdf?

    Regards:  Tom

  • 0
    Intellectual 390 points

    Andrew - here is the extract from the schematic:

  • 0 in reply to Tamas Ledeczi
    TI__Expert 6395 points

    Hello Tom,

    Is it possible to do the following to try and isolate the TS5A3166 from the micro

    1. Depopulate R60
    2. Run the same test you were running and now check the voltage on:
      1. Pin 2(COM pin) of the TS5A3166
      2. Pin 16 of U13A

    Regards,

    Andrew 

  • 0 in reply to Andrew_Mason
    Intellectual 390 points

    Hi Andrew,

    I did what you asked but there is no change with respect to how the TS5A3166 behaves. After shutting down the U18 switcher, the 3.3V rail goes down to just around 1.6V and the 3.0V battery voltage appears on the disconnected COM pin (pin #2) as well. Pin#16 of U13 is around 900mV.

    Regards:  Tom

  • 0 in reply to Tamas Ledeczi
    Intellectual 390 points

    Hi Andrew,

    Some more information. After shutting down the regulator, if I short the 3.3V rail to GND for a very short time, the TS5A3166 goes to the right state with VBAT=3.0V at pin#1 and COM=0.2V at pin#2. After the quick short applied, the 3.3V rail remains at 0V. Current figures through VBAT are as follows:

    1. 43mA with switcher ON.

    2. 2.5mA with switcher OFF and with the wrong voltage on the 3.3V rail.

    3. 3uA with switcher OFF and 3.3V rail at 0V (after the quick short applied).

    By adding an extra 100R to the 3.3V rail the problem dissappears. And again, if I remove the TS5A3166 from the board, the 3.3V rail goes down to 0V when the switcher shuts down.

    Regards:  Tom

  • 0 in reply to Tamas Ledeczi
    TI__Expert 6395 points
    Hello Tom,

    Thanks for the additional information. I'm reviewing and will provide feedback when I have my setup complete.

    Regards,
    Andrew
  • 0 in reply to Andrew_Mason
    Intellectual 390 points

    Hi Andrew,

    Could you please give me a status update? I haven't received any feedback for a while.

    Thanks:

    Tom

  • 0 in reply to Tamas Ledeczi
    Intellectual 390 points

    Hi Andrew,

    Any status update on this? Looks like we dropped the ball. Please give me a feedback.

    Thanks:

    Tom

  • 0 in reply to Tamas Ledeczi
    TI__Mastermind 45565 points
    Tom,

    I apologize for dropping the ball. I will be supporting this device. Can you provide me some scope shots of the issue you are having with a marked up schematic where you are probing? This can quickly get me up to speed on what your issue.

    Thank you,
    Adam
  • 0 in reply to Adam Torma
    Intellectual 390 points

    Adam,

    First of all, thanks for getting back to me on this. I don't have any scope shots, I just used a simple multimeter to measure voltages. I attached a schematic pdf earlier and exactly described the problem. When I switches off the U18 switcher (PWR_ON goes high) the 3V3 rail starts to collapse but just to about 1.6V becasue the VBAT rail starts to feed the 3V3 rail through the U14 TS5A3166 switch (from pin#1 to pin#5). Removing the U14 switch solves the problem, the 3V3 rail goes down to 0V without problem. Moreover, adding stronger discharge path to the 3V3 rail (eg. 100R to GND) eliminates the problem as well with the TS5A3166 switch populated. Looks to me that if the discharge of the 3V3 rail is week, the current through the possible internal protection diode between pin#1 and pin#5 of the TS5A3166 is enough to maintain the 1.6V residue voltage at the 3V3 rail. Does this make sense? Is there any path (diode) between pin#1 and pin#5 internal to the chip?

    I can make further measurements next week if you think it is necessary.

    Thanks:

    Tom

  • 0 in reply to Tamas Ledeczi
    TI__Mastermind 45565 points
    Tom,

    I'm sorry that I do not have access to the TS5A3166 design files.
    I have contacted Shreyas Rao who should have access to the TS5A3166 design files to help investigate if there is an internal path from pin 1 to 5 that is enough to keep your 3.3V rail at 1.6 V when you remove the power to the rail.

    Thank you,
    Adam
  • 0 in reply to Adam Torma
    TI__Guru 61520 points
    Hi Tom ,

    There is internal positive diode to Vcc from NO and COM pins . This is inferred from the abs max table showing the abs max voltage for Com and NO pins to be Vc +0.5V indicating the voltage on them shouldn't exceed Vc .
  • 0 in reply to ShreyasRao
    Intellectual 390 points

    Hi Shreyas,

    Thanks for the feedback, this is what I have expected. The internal diode explains what I have experienced.

    Thanks again:

    Tom

  • 0 in reply to Tamas Ledeczi
    TI__Guru 61520 points
    Hi Tom ,

    Adding to this , there is no positive diode to Vc from the control signal (digital input). The signal shouldn't exceed the Vc for proper operation of device and must be limited to within 50mA when fwd biased .
    Please let know if there is anything else .
    Also , let know about how you get around the issue which could be helpful for me and for everyone in general .
  • 0 in reply to ShreyasRao
    Intellectual 390 points

    Hi Shreyas,

    According to the datasheet, the control input is 5.5V tolerant and the max leakage current is 20nA (Vi=5.5V or 0V) independently of the VCC supply voltage. This tells me that there is no clamping diode between the control input and the VCC (exactly as you wrote) and I can drive the control input with higher Vi voltage as that of the VCC without problem. Please nod if this statement is correct. Based on your feedback, I'll power the switch from the battery voltage (VBAT) which can go down to 1.8V but the control input will be drived with 3.3V logic (Vih=2.7V).

    It is clear that the voltage levels at the NO/COM pins should not exceed VCC+0.5V or the diodes will be forward biased. This is not an issue in my application.

    Based on the datasheet it looks to me that the NO/COM pins are symmetrical, therefore, it doesn't make any difference which side of the switch is connected to the battery voltage and which is to processor's ADC input. Please nod if this is true.

    Thanks:

    Tom

  • 0 in reply to Tamas Ledeczi
    TI__Guru 61520 points
    Hi Tom ,

    No issues when the Control input has higher voltage above the Vcc (although not recommended in general ).
    Yes , NO /COM both are symmetrical I/O , so once enabled both sides should have same potential with small drop across the switch because of Ron.
  • 0 in reply to ShreyasRao
    Intellectual 390 points

    Hi Shreyas,

    As you wrote, in general it is not recommended to drive the control input of the TS5A3166 switch above the supply voltage. Can it cause any degradation of the device when used this condition long term?

    In my application, the switch is used to isolate/connect the battery voltage from/to the ADC input of the applied microcontroller depending on the ON/OFF state of the equipment. In the current design, the switch is powered from the 3.3V rail. To eliminate the back-powering effect of the clamping diodes, in the revised scheme the switch will be powered directly from a 2xAAA battery. Therefore, as the battery decays the minimum supply voltage of the switch is 2V. Vih of the control input is a constant 3.2V when the equipment is switched ON. Therefore, worst case Vih is 1.2V higher than the VCC supply.

    Can this type of usage (Vih>VCC) influence the expected lifetime or failure rate of the switch? Changing the switch to the TS5A3167, I can use Vih<=VBAT as well, if this can reduce risk of posible malfunctioning of the switch. Please advise if this is the preferred way.

    Thanks:

    Tom

  • 0 in reply to Tamas Ledeczi
    Intellectual 390 points

    Hi Shreyas,

    Haven't received any reply yet for my latest questions above. Could you please give me a feedback?

    Thanks in advance:

    Tom

  • 0 in reply to Tamas Ledeczi
    TI__Guru 61520 points
    Hi Tom ,

    There is no degradation of the device as long as you limit the voltage within the recommended conditions . Beyond the abs max conditions , the degradation might occur.
    The control input doesn't have internal diode but I/Os have it . So you shouldn't be violating the abs max conditions on the I/Os (NO and COM) .