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MC33063A as a step-up converter

Ali Burak Parım
Prodigy 50 points
Other Parts Discussed in Thread: MC33063A

Hi,

Could you please let me know my mistake(s) in my design?

I try to obtain 15V output voltage (with max. 30mV output voltage ripple) and 30-40mA max output current from 5V input voltage with MC33063A. I use the calculation table on datasheet. However, it does not switch!

I tried with some basic boost converter design equations related switching frequency, Iout, Vout, Vin, etc. And it gave me some irrelevant results like 6.4mH inductance or 0.56mA Iout ripple.

I cannot figure out my mistake(s). I guess removing the Rsc as short for increasing switching current may work but I afraid to burn it.

I reduced R_sc step by step by paralleling 1R resistors (2x, 4x, 6x, 8x1R) and changed my inductor as 150uH for closing the referance desing but I could not figure out.

Please help me a.s.a.p!

P.S.: You can see my values below!

Many thanks in advance for your help!

Best regards,

Ali Burak Parım

Values:

Vin= 5V ( from DC power supply)

Iout= 30-40mA

Vout=15V

freq_sw =>40kHz

Vsat=1V (because pin 1 and 8 connected with inductor)

V_F = 0.46V (I use SSA34 diode)

ton=18.08us, toff=6.916us, dutycycle=%72

C_T(timing cap.) = 723pF ( I use 1nF since I obtain 100Hz ramp signal with 720pF and apx. 42kHz with 1nF)

I_pk= 290mA and R_sc= apx. 1R

Lmin = 249uH

C_o1= 130uF (for 50mV ripple) or C_o2= 650uF (for 10mV ripple)

Finally, for 15V Vout

(R1,R2)= (1K, 11K) or (2.49K,27.4K)

  • 0
    TI__Guru**** 164406 points

    Ali Burak,

    In a boost design current only flows to the output capacitor for a small percentage of the switching cycle, therefore a large capacitance and low ESR will be required. Extra filters or a LDO can be used to decrease ripple noise.

    Your initial concern was “it does not switch!”. Does that means that pin 1 is near 5VDC with no switching to ground and the output voltage is about 4.5V?

    Assuming a 1V loss across RSC and the switch and a 0.5V loss on the output diode and RSC will result in a coil charging at 4V (5V-1V) and discharging at 10.5V (15V-5V+0.5V).  At the intersection of continuous and discontinuous coil current the duty cycle will be (10.5)/(10.5+4) = 72% as you said. For 40mA output current the peak coil current would be 2*40mA/(1-72%) = 286mA. The 1ohm RSC won’t have enough margin but o a 0.5 ohm RSC will.

    Can you provide pin1 waveform for L=150uH, RSC=0.5 ohms, CT = 1nF? Please state the input current, load current, and output voltage for the waveform.

  • 0 in reply to Ron Michallick
    Prodigy 50 points

    Hi Ron,

    Firstly, I must say it was not my mistake only. :) I changed the IC and every thing works now. I changed my inductor as 150uH and RSC as 0.2 ohm (before my first post). Almost every thing is fine. I testing the circuit with electronic load now. If you recommend changing RSC as 0.5 ohm for improve the efficiency or output ripple. However, I am cool with these values now.

    Thanks a lot for advice!

    Best wishes!

    Ali Burak Parım

  • 0 in reply to Ali Burak Parım
    TI__Guru**** 164406 points

    Ali Burak,

    Rsc primarily sets the maximum output current. I am glad that it is working now.