[SOLVED] MC34063 problem. Switches off in every cycle

[karanbanthia]

Hi All,
I am using MC34063A in step down configuration, but it switches OFF during every cycle. Following are the parameters for which it is designed:
Vin - 9 to 12V
Vout - 5V
I max - 0.3A
Rsc - 0.36 ohms
L - 47uH 1.6A
Ct - 270pf
I have tried reducing the current limit resistor (Rsc), but it didn't help, the current is well below the designed limit. I have attached an oscilloscope image showing output voltage ripple on channel 1 and switching transistor emitter (pin-2) voltage on channel 2. Note that the switching stops every cycle which is unwanted and causing a low frequency ripple on the supply voltage and rest of the circuit.
Kindly help!

 

[betwixt]

I calculate different optimal values, possibly you are running above the maximum switching frequency allowed for that device.

Try:
Rsc = 0.5 Ohms
Ct = 370pF

that should give you a switching frequency of about 70KHz.

Brian.

 

[Vbase]

Hi All,
I am using MC34063A in step down configuration, but it switches OFF during every cycle. Following are the parameters for which it is designed:
Vin - 9 to 12V
Vout - 5V
I max - 0.3A
Rsc - 0.36 ohms
L - 47uH 1.6A
Ct - 270pf
I have tried reducing the current limit resistor (Rsc), but it didn't help, the current is well below the designed limit. I have attached an oscilloscope image showing output voltage ripple on channel 1 and switching transistor emitter (pin-2) voltage on channel 2. Note that the switching stops every cycle which is unwanted and causing a low frequency ripple on the supply voltage and rest of the circuit.
Kindly help!
**broken link removed**

Try L - 220uH, when the inductor is too low the peak current reaches prematurely and the output shuts off.

 

[karanbanthia]

Hi Brian,
I had used the MC34063A universal calculator for calculating the components and parameter values. I cross-checked those and all the calculations seems to be fine. I am using switching frequency as 100 KHz. Attached is the screenshot of calculations.


I tried increasing Ct to 270+47pf but it didn't help.

I tried increasing the inductor value but it increases the OFF time even more.

Regards,
Karan

 

[betwixt]

I'm not sure what the problem is but you did specify a minimum switching frequency of 100KHz which is actually the maximum the device is designed to work at. I noticed the calculations have returned a frequency slightly below 100KHZ, maybe this is why the negative ESR result has been given. I suggest using a lower frequency, the devices are characterized between about 25KHz and 75KHz.

Brian.

 

[karanbanthia]

I tried increasing the inductor value from 220uH upto 600uH but it has no effect. It turns out that as per calculations, 40uH is the minimum inductor value required, above that it doesn't matter until the current is within limit.

 

[karanbanthia]

Hi Brian,
I increased the Ct to 540pf (270+270pf parallel) and the same problem exists with the reduced switching frequency. Attached is its screenshot.



Regards,
Karan

 

[betwixt]

I have no idea what is causing it then. I use lots of MC34063A ICs in various circuits and have never had a problem with them.

Can you post a photograph of your construction, maybe that will give a clue.

Brian.

 

[Vbase]

I agree with Brian. It is possible that the 100KHz causing 10Hz oscillation on your layout. It could be also your load.
In real world you will never get 5mV ripple with output current of 0.3A, even if you use the best cap and you are an expert in layout. At 100KHz 5mV at the input of the error amp will give 50mV on the cap because of the 10K resistor with the input capacitance of the amp.
If you need 5mV ripple for analogue circuit you will have to filter the 5V with extra coil and cap.

You should recalculate your values to 50KHz and ripple of 50mV.
Test your circuit with a resistive load before connecting it to your circuit.

 

[karanbanthia]

Hi Brain,
I have attached the top and bottom side of my PCB containing the switching regulator. Also, I have attached my schematic (calibration resistors values are different) and layout pic.





This is the third time, I have used this IC. We generally tend to check only the ripple and output voltage. This time I recheck my previous designs and Voila!!, the same problem exists in both of them, even though they are working great till date. It's not a question of ripple voltage alone, why should the switching regulator switch OFF for such a long time?? If you have any of your previous designs ready at hand, can you post its image of emitter signal (pin no 2 on MC34063A), where switching is actually visible??

regards,
Karan

 

[betwixt]

I just happen to have one on my bench at the moment. It is part of a portable LED lighting system and not finished (the frequency is actually too high at ~130KHz at the moment) but you can see the waveform I get at pin 2. The input voltage is 6V from a Pb battery and the output is ~3.7V @ 100mA.

I think it highly unlikely but there is a chance you have magnetic coupling between the two inductors.

Brian.

 

[andre_luis]

Working at so high frequency I always put a ceramic capacitor near to the electrolytic. I'm not able to determine the expected effect of its inherent inductance at 100 KHz, but presume it is not negligible.

 

[karanbanthia]

Hello all,
Thank you for your suggestions. Turns out that since it is a switching regulator, it switches OFF completely when the output voltage is more than what it is designed for which is exactly what I dont want. I have shifted my design to MC34166 which is a PWM controlled DC-DC converter and everything seems to be working fine. I am getting an output voltage ripple of 10-15mv peak to peak @5V o/p and no such 10Hz ripple.

Regards,
Karan

 

[FvM]

MC34063 has hysteretic control. Although it's working quite well in many products, it's sensitive to parasitic circuit elements.

It may be still a good choice for a large quantity, lowest cost product where the time spent for circuit tuning during development doesn't count. For other applications, be happy if it works straight away by chance, or choose a state-of-the-art switcher.