LTC2937 ON Pin control circuitry

Hi,

I have a question regarding the external control circuitry connected to the ON Pin off the LTC2937. The block diagram in the datasheet shows an internal 10µA current source pulling up the ON Pins potential to VDD (3.3V). The "usual" use case of the ON Pin should be as shown on the left of the picture below. In scenario (1) the ON Pin is floating and the internal current source can source up to 10µA (or 16µA max. occording to the datasheet) into the logic block (depicted as a "mystery load" Z). In the second scenario (2) the switch is closed, creating an low impedance path to GND, therefore the internal current source would not source any current to the mystery load Z.

Now to my question: In my application I would like to connect an Op Amp to the ON Pin (right circuit). Because the datasheet doesn't state any current limitations on the ON Pin I would try to recreate the situation on the left. Therefore I would place a series resistor R_s between the Op Amp and the LTC2937. The Op Amp delivers 3.5 V (for simplicity the internal resistance of the Op Amp is not considered). The ON Pin is rated for 3.3 V and because I would like to source up to 10µA max. into the internal logic block of the LTC2937 the resistance value of  R_s computes to 20k. So far so good. This constellation recreats the first scenario. But a problem arises (or maybe not ?!) when trying to recreate scenario (2). When the voltage on n1 is 0V the internal current source of the LTC2937 may not source the 10µA into the Op Amp (if the mystery load Z has a lower impedance than the series resistor R_s). A workaround would be to place a NMOS pull-down device on the ON Pin and control it through the Op Amp. But this solution is not desireable (due to very limited board space). Is it possible to overdrive the ON Pin by using a lower impedance series resistor (~ R100) and allowing the internal logic block to be able to sink more than 10µA?

Best regards

Parents
  • +1
    •  Analog Employees 
    on May 7, 2020 3:48 PM 11 months ago

    Greetings Konstantin,

       Thank you for the question.  I would not recommend using an op-amp as the input to the ON pin.  The ON pin is a digital logic pin and should be controlled through a digital signal.  For a mechanical solution, take a look at the LTC 2954 (www.analog.com/.../2954fb.pdf).  Another option would be to control the ON behavior through the ON_OFF_CONTROL register using I2C.

    Thank you and best regards,

    Brad

  • Hi blovell,

    thank you for your answer. The idea was to configure the op-amp as a comparator, therefore the signal on the ON pin would be digital. But the LTC2954 has a integrated nmos pull-down driver, which is exactly what the ON pin needs to be connected to (as expected). Therefore an op-amp + nmos or the LTC2954 seem to be a good solution for this application.

    Best regards

    Konstantin

Reply
  • Hi blovell,

    thank you for your answer. The idea was to configure the op-amp as a comparator, therefore the signal on the ON pin would be digital. But the LTC2954 has a integrated nmos pull-down driver, which is exactly what the ON pin needs to be connected to (as expected). Therefore an op-amp + nmos or the LTC2954 seem to be a good solution for this application.

    Best regards

    Konstantin

Children
No Data