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TIDA-00751: About Resistance Calculate

Leo Yang
Expert 7235 points

Part Number: TIDA-00751

Hi team,

Could you explain how to calculate the value of R7 and R8 in the SCH. Thanks,

Leo

  • +1 in reply to Jim Carrillo
    TI__Prodigy 690 points

    The purpose of R8 is to limit the peak base current of Q2 when Q3 operates in saturation.  An equation for this resistor would be R8 = [V_tp6(on) - Vbe_Q2 - Vce_sat_Q3]/(I_base_Q2 + I_discharge_tp5), where I_discharge_tp5 can be derived from the turn off time spec and the ratio of the gate capacitance of the relay MOSFET's to C9.

    R7=R9 with C9 < (Cgs_Q1 + Cgs_q2)/20 with the added constraint that R7 || R9 sets the secondary side current in the On-state.  Ideally, TP5 should discharge much faster than TP6 so that the BJT pulldown gets triggered as soon as possible after the /EN signal goes high in order to ensure that the relay MOSFET's turn off quickly.

    An easier method to determine R8 would be to simulate the circuit after R7, R9, and C9 have been determined.  R8 can then be tuned to meet the turn off time spec of the relay.

  • 0 in reply to David Stout
    TI__Expert 7235 points

    Thanks for replying!