Voltage divider output not the expected

Discussion in 'General Electronics Chat' started by nickagian, Mar 3, 2011.

  1. nickagian

    Thread Starter Active Member

    Mar 12, 2010
    I have a problem using the PFI/PFO# function of the TPS3619-33. My schematic is shown here attached. This is part of a bigger schematic, all constructed on the same PCB.

    The R1/R2 resistors are intended to set a sensing threshold so that when the VBAT voltage falls below around 2.6V, the PFO# output goes to LOW. However, although the battery is around 3.3, PFO# is not HIGH but LOW. When I measured the output voltage of the divider, I found it to be 0.7V. I have removed the R1 resistor (it is indeed 570k, no error in this) and the resistance between VBAT and the PFI pin is found to be around 660k.

    Well, something very weird is happening here... Do you think is it something to do with the TPS3619-33 or some bug of the PCB?


    EDIT : I forgot to mention that although VBAT is intended to be the voltage from a Li-ion battery, the above behaviour is seen even with an input voltage from a power supply and even without any voltages on the PCB, the values of the resistors R1 and R2 are not their expected.
    Last edited: Mar 3, 2011
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    What is the value of Vmain?
  3. JMac3108

    Active Member

    Aug 16, 2010
    Nick, I checked the datasheet for the part and your math is correct.

    Here is how I would troubleshoot the problem.

    (1) Remove the chip or lift the PFI pin. Now set VBAT with a power supply and measure the mid-point of the voltage divider to see if it correct.

    (2) If it is correct then you may have a blown chip that is pulling down the line.

    (3) Another suggestion is that you are using resistors that are too large. With this large of value of resistors any small leakage currents can completely change your results. Try reducing both resistors by a factor of 10 and see if your results change.

    Hope this helps.