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mc34063 problem

Jose Morales
Prodigy 60 points

hello I´m using a MC34063 to elevate 24v to 32v DC and my maximum output current is 80 mA, But when I put a load that consumes 11 mA,  the MC34063  got very hot, is this behavior normal?

  • 0
    TI__Guru*** 140106 points

     Jose,

    That is not normal. Check pin 1 to see switching quality.
    Did you use a Rsc? Is the diode a Schottky?

    Please list the support components used.
    If possible, pin 1 waveform would be helpful.

     

  • 0 in reply to Ron Michallick
    Prodigy 60 points

    yes I'm using Rsc = 1 ohm, and my diode is the same that the datasheet proposed (1n5819).these are my design parameters

    Vin = 24.00 ±26% V
    Vout = 32.00 V
    Iout = 0.08 A
    Vripple = 0.10 V


    I attach the components that I have calculated  and the components that I've used:
    L=680uH
    Rsc= 1 ohm
    Ct = 470pf
    R1= 1.2 k
    R2= 30k
    C0 = 100 uF

    and I don't have a image from the waveform of the pin 1, the resistence that is conected in  the pin 8 got hot, the input current when I put the load was 42 mA and the output current was 11mA. Thanks for answer

     

  • 0 in reply to Jose Morales
    TI__Guru*** 140106 points

     Jose,

    I believe the base of Q1 is being overdriven creating long turn off time (see notes on page 4 of data sheet).

    I suggest changing pin 8 resistor to 18V/22mA = 820 ohm. This gives 22mA to Q1 and Q1 base resistor with minimum VIN. 22mA=7mA+(0.3V/1ohm)/20.

  • 0 in reply to Ron Michallick
    Prodigy 60 points

    Ok. Thanks for answer, I will make that change in the circuit

  • 0 in reply to Jose Morales
    Prodigy 45 points

    I have come across the problem posted by Jose Morales  (Sep 06 2013)  and an answer posted by gone Ron Michallick  (Sep 09 2013). It is about a year now since their communication and hopefully Jose might have solved the problem. However, I wish to add that the circuit should function better by connecting Q1 and Q2 in Darlington form as suggested in the literature. This will reduce the turn off time of Q1 that has to switch the maximum portion of peak current.

    Also 1N5819 should be replaced by schottky diode rated for 1 Amp, PIV 100 Volts. Look for diodes from MBR series or SR series.