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TPS61202 not shutting off when EN brought low

Allen Litton
Prodigy 50 points
Other Parts Discussed in Thread: TPS61202, TPS61020

I am using a TPS61202 to produce 5V from 2x series AAA Alkaline cells with a max load current of about 250-300mA. The enable line is normally pulled low and driven high to turn on the device.  My read of the data sheet is when the EN line is pulled low it is supposed to disconnect the battery source form the load with a max Iq of 55uA.


What we are seeing is even with EN tied hard low,  and PS and UVLO tied to VBat per the example schematics we are seeing voltage leak through to the output with about a 0.8V drop and an Iq of about 4mA - mostly due to the fact the processor on the load side is trying to run. When the EN line is asserted the switcher does start normally and output +5V

I really need a solution that has very low off current and is externally controlled to turn on. I chose this part because of the higher current capability and the "load disconnect" feature. I need to be able to deliver 5V at full load current ~300mA down to the Alkaline cutoff of about 1V per cell or 2V input.  Is there something I'm missing about how to use this device? I specifically excluded device like the TPS 6122x because of the ever present diode between input and output.


Thanks for any advice.
Allen



  • 0
    TI__Mastermind 46905 points

    could you share your schematic and test condition, or the waveform if possible. only providing enough information, i can help to find the root cause quickly.

    i would suggest you apply a EVM for reference. EVM will help in debugging the circuit.

    i think you are care about the power loss, then maybe you should connect PS pin to GND to enable power save mode.

    BSC team

    jasper li

  • 0 in reply to Jasper Li
    Prodigy 50 points

    Jasper,

    Thanks for the prompt reply.

    The schematic is shown below. It is a TPS61202 (+5V Fixed) despite the generic part number. FB is tied straight to the output.

    Signal PSSW is normally tied to a momentary NO push button tied to B+ (Battery Voltage)  to turn the supply on until the processor supplied PSEN signal is asserted to keep the power supply on and turn it off under control of the processor.

    Even with R2 removed to isolate the processor and R1 reduced to 1K we still see voltage at VOUT. There are no switching waveforms when EN is low.  It simply appears as if there is a forward diode between pin L and pin VOUT.  With fresh AAA Alkaline batteries and 3V B+ we see about 2.2V at VOUT with EN low - even grounded. There is no advertised "load disconnect"  happening.

    When we do have R2 in circuit and assert PSSW with the push button the switcher does start up and we do get +5V. The processor asserts PSEN before the switch is released and the +5V stays on until PSEN is de-asserted and the switcher stops, but we still have voltage at VOUT.

    This behavior is seen across multiple devices so I do not think it is due to a damaged device.

    We do not really need the low power mode during operation since the device is only used periodically and it is more important to have long battery life with the unit powered down.

    Allen

  • 0 in reply to Allen Litton
    TI__Mastermind 46905 points

    hi Allen

    i do some experiment at the TPS61202EVM, and didn't find the problem as you describe.  but it seem there is a problem in your schematic. the resistor R1 is not connected to the EN pin, you can see there is not dot as in other connection. so the EN will be floating

    and could you also try connecting the PS to GND if the above suggestion doesn't work.

    jasper li

  • 0 in reply to Jasper Li
    Prodigy 50 points

    Jasper,


    Actually the resistor is tied to EN. It doesn't show a dot in Pulsonix if the component pin is on the wire. It is tied to EN on the PCB.

    I can try grounding PS and see if it makes a difference.

    Allen

  • 0 in reply to Allen Litton
    Prodigy 50 points

    I did try grounding the PS pin and I am seeing the same results. I still see an output voltage .8V lower than the input and no switching when the EN is low. I am seeing about a 4mA current when EN is low from the battery.

  • 0 in reply to Allen Litton
    TI__Mastermind 46905 points

    it is very strange. are you sure the Vout and input current is caused by TPS61020, not other circuit in your system board? and i suggest to apply an EVM for reference. you can also try to replace the IC on the EVM using the sample you are using, to confirm if it is caused by the IC.

    Jasper

  • 0 in reply to Allen Litton
    TI__Prodigy 520 points

    Hi Allen,

     

    Suggest you apply some load to the output, like 20mA , then you pulled the EN low again, if the output is still 0.8V lower than Vin, then please show us the waveforms of SW pin, EN pin, Vout pin. That would help us to debug.

    Thanks

    Ming