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AP2141 issue

Discussion in 'Electronic Design' started by Marco Trapanese, Apr 6, 2013.

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  1. I'm using a AP2141:

    to disconnect a circuit from another.
    It is connected as following:

    IN 4V
    EN 5V
    OUT 10k pull-down (for test purpose)

    On the OUT pin I read the same voltage applied to the IN pin, 4V.
    I checked it really is an AP2141 (active low).

    I can't understand why I read 4V instead of 0V.
    There are several boards on my desk, all with the same behavior.

    Perhaps I misunderstood something in the ds?

  2. Il 06/04/2013 19:40, Jim Thompson ha scritto:

    I connected a 180 ohm resistor across output terminals...

    Unless the label on the package is wrong, I do have a 2141 ;)

  3. Il 06/04/2013 21:59, Jim Thompson ha scritto:

    There is no mention of the contrary on the ds.

    I noticed that too ;) I tied them toghther.

    Two "frog" problems on a single board! Double croak :(

  4. Il 06/04/2013 21:01, Marco Trapanese ha scritto:

    Very strange.
    Some other tests:

    1) I disconnected the outputs and the enable pins from the circuit.
    Loaded the output ones with a 1k resistor.

    Powered the board and after that I forced the voltage on the enable pin
    with a piece of wire. It works opposite: applying a high level leads to
    conduct (output voltage = input voltage). Bring the enable down to zero
    volt disconnect the load (output voltage = 0V).

    With 4V at input I can rise the enable voltage up to 5V without any problem.

    2) I changed my board adding an n-mos to invert the enable signal. But
    again it doesn't switch off even with 0V on the enable pin. I removed
    the mos and did again the test with the wire *after* power up and it
    works (anyway like 2151).

    Bottom line: I'm afraid the datasheet doesn't tell the truth also this
    time. I can't understand at all the polarity of the enable signal. The
    figure 1 at page 6 is very clear though.
    Perhaps there are some timings to respect at power up. I suspect because
    the enable voltage will rise before the input, it may likely latch in
    some undesirable condition.

  5. Il 07/04/2013 17:01, Jamie ha scritto:

    Do you mean the enable pin? Or the voltage input pin?
    If you're talking about the enable pin at first I had a pull-down
    resistor, then I used a mos to tie to gnd the enable pin.

  6. Jamie

    Jamie Guest

    Maybe you need a pull down R on the input pin?

  7. Il 07/04/2013 19:52, Joe Chisolm ha scritto:

    I'm talking about the marks on the case. Shall I trust them?

    I measured 4 mV and about 5V. But I tried also from 3V with an external
    power supply.

  8. Jamie

    Jamie Guest

    Looking at the data sheet, which I just did, your device P.N. is
    designated to be active in the low state. This means you should
    have a pull UP not DOWN, R on the EN input. And pulling the EN
    pin low via the processor or a open collector/Drain device with
    pull up R still attached should work.

  9. Il 07/04/2013 22:46, Jamie ha scritto:

    Perhaps I didn't explain well.
    I have a pull-down because I want the device is ON by default.
    When I plug an external power supply the enable pin will be forced to 5V.

    I'd expect the device will disconnect its load.
    But it doesn't happen, the load is still connected.

    Inverting the logic, that is pull up the enable pin to turn on the
    device and tie to 0V to disconnect the load will work only if I power
    the board first and *after* I toggle the pin.

  10. Il 08/04/2013 16:49, Jim Thompson ha scritto:

    I've just ripped off those switches... and I make the boards work using
    standard mosfet :)

  11. Jamie

    Jamie Guest

    I think you picked the wrong part to use, it looks like a one shot
    switch. Which is common in power management start up systems because
    you only need to do this once. In other words, it latched in. To turn
    off such a device, you need to kill the front end, which is doable via
    a pmos being reversed biased from the uC for example.

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