MC34063 issue, switching stops at regular intervals

KARAN BANTHIA

Other Parts Discussed in Thread: MC34063A

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

Switching frequency - max 100KHz

I have tried reducing the current limit resistor (Rsc), increasing and/or decreasing the load 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 after regular intervals and starts again which is unwanted and causing a low frequency ripple on the supply voltage and rest of the circuit.
Kindly help!

over 9 years ago

0 Ron Michallick over 9 years ago

TI__Guru**** 153836 points

Karan,

Can you send a waveform of the CT signal?
I suspect it may tell reason for the problem.

0 KARAN BANTHIA over 9 years ago in reply to Ron Michallick

Prodigy 40 points

Hi Ron,

Ct seems to be fine. Attached is its waveform.

0 Ron Michallick over 9 years ago in reply to KARAN BANTHIA

TI__Guru**** 153836 points

Karan,

The oscillator is running well. It is normal for the switching to stop when the output voltage gets too high. It is odd that the output drops 200mV before the switching starts again.

What values were chosen for CIN, COUT, diode, and two feedback resistors?
Does the feedback network have anything that would case lag?

0 KARAN BANTHIA over 9 years ago in reply to Ron Michallick

Prodigy 40 points

Hi Ron,

Cin = 100uf, Cout = 220uf, feedback resistors are 3.4K & 10.2K as shown in the attached MC34063 calculator that I have use. I have also attached my schematic (note: feedback resistor values are incorrect in the schematic, keep so for some other reason, but the actual values are in the ratio 1:4)

0 Ron Michallick over 9 years ago in reply to KARAN BANTHIA

TI__Guru**** 153836 points

Karan,

Is pin 5 a good 1/4 replica of VOUT?
Also check ground side of R61 to see if it is clean.

0 KARAN BANTHIA over 9 years ago in reply to Ron Michallick

Prodigy 40 points

Hi Ron,

Thanks for your help. As you had mentioned that since it is a switching regulator it switches OFF when the output voltage is greater than what it is designed for and this is exactly what I dont want. I have shifted my design to MC34166 which is a PWM DC-DC regulator and everything seems to be working fine. The output ripple voltage peak to peak is only around 10mV @ 5V o/p.

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MC34063 issue, switching stops at regular intervals