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CD4093B-MIL: totem pole driven by CD4093 interface circuit

Ammanna Seepana
Prodigy 10 points
Part Number: CD4093B-MIL
Other Parts Discussed in Thread: UC1705

CD4093  output is fed to a npn-pnp based totem pole circuit to drive a Mosfet. A resistor of 1kohm is provided in between the cd4093output & base of the totem pole circuit. Now observed that the switching losses are becoming more and the Vgs amplitude becomes more sloppy..

Could you suggest us about the minimum resistance?

  • 0
    Guru 259920 points

    CD4000 devices have a quite high output impedance, especially at low supply voltages. It might be possible to use 0 Ω. What is your VDD?

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    VDD is 14V. 
    As this output driving the totem pole which needs approximately 20mAmpere base current for a few nano seconds(200ns) while turning on & off the mosfet.

    So, can We go ahead without resistance?

  • 0 in reply to Ammanna Seepana
    Guru 259920 points

    The IOL/IOH values specified in the table on p. 3 show that the device cannot drive 20 mA with a small voltage drop.

    Figures 6/7/8/9 show that the device can drive 20 mA with a voltage drop of 5 V in the typical case (correspondig to 250 Ω), but that this is not guaranteed, not even at 25 °C. The output impedance has very loose specifications, especially over temperature.

    Using no base resistor might be possible, if a higher base current is allowed. You have to decide yourself if you can afford the risk that 20 mA will not be reached in the worst case.

    I'd recommend an integrated gate driver like the UC1705.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Yes, exactly, we would have planned with a driver IC, but because of some other reasons we have not done. Now, to meet the datasheet values, there was a series resistance of 1kohm, to limit the momentary short circuit current to 14mA. But this is giving a little more rise time for the Vgs. The CMOS has to give lower current, but a period of ~200ns; So, If we decrease the resistance to around 300 Ohms, higher current and the time wil be shorter.

    Pls suggest..

  • 0 in reply to Ammanna Seepana
    Guru 259920 points

    To shorten the time of high current, put a capacitor in parallel with the resistor.

    I cannot suggest alternatives when I do not know the constraints on your design.