Role of resistors in the ICL7673 circuit

Discussion in 'General Electronics Chat' started by buzaiandras, May 1, 2012.

  1. buzaiandras

    Thread Starter Member

    Jul 18, 2011
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    0
    Hi,

    I came across the ICL7673 IC used for a battery backup system but I could not understand exactly what is the role of the R2 and R4 resistors in this circuit (marked with red in the attached image).
    So my question is: what role does the R2 and R4 resistors have?
    When Vp > Vs , P- is high and S- is low and when Vp < Vs, P- is low and S- is high. Since we have the R1 and R3 resistors to bias the base of the PNP transistors, why do we need the R2 and R4 resistors?
    If somebody could explain to me (in detail) I would greatly appreciate it.

    Thank you,

    Buzai
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,962
    1,097
    Goggle "pull-up resistor".
    This resistor ensure that both BJT are "OFF" when IC is in high state.
    And those resistors are necessary because ICL7673 is a open drain type output.
    So IC don't provide the true high state.
    They also speed-up turn-off time by discharging Ceb capacitor.
     
    Last edited: May 1, 2012
  3. buzaiandras

    Thread Starter Member

    Jul 18, 2011
    54
    0
    Could you please elaborate a little?
    What do you mean by the "IC" is high?
    As I understand only P- or S- can be high at a given time, not both.


    Thank you,

    Buzai
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,962
    1,097
    By IC I mean ICL7673.
    As you can see in the data sheet. ICL7673 is a open drain output type circuit.
    So we need those resistors to provide necessary path to close (cut-off) the bjt when P- or S- are in high state.
     
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