I am using MC34063 in step down configuration but the converter switches off every cycle. Following are the calculations and specifications:
Vin - 9 to 12V
Vout - 5V
Imax - 0.3A
Rsc - 0.36 ohms
L - 47uH 1.6A
Ct - 270pf
I have tried reducing the current limit resistor value (Rsc), but it didn't help. The usable current is below the designed rated current. I have attached an oscilloscope image showing the output voltage ripple on channel-1 and switching transistor emitter (pin-2 of MC34063A) voltage on channel-2.
Note that the transistor switches off completely causing a low frequency ripple in the output voltage which is unnecessary. Kindly help.
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\$\begingroup\$ Its possible that you didnt correctly choose the feedback resistors. What values did you choose? \$\endgroup\$– GolažCommented Apr 8, 2015 at 21:58
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\$\begingroup\$ please show circuit \$\endgroup\$– markragesCommented Apr 9, 2015 at 0:45
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\$\begingroup\$ @Golaž The feedback resistors values are correct since I am getting DC output as 5V, but the OFF period and AC ripple is my concern \$\endgroup\$– karanbanthiaCommented Apr 11, 2015 at 8:23
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\$\begingroup\$ @markrages Added the circuit and its calculations \$\endgroup\$– karanbanthiaCommented Apr 11, 2015 at 8:24
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\$\begingroup\$ what are you powering the circuit with? please add a photograph of the circuit that you built. \$\endgroup\$– markragesCommented Apr 11, 2015 at 20:26
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1 Answer
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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 don't 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 10-15 mV peak to peak @ 5 V o/p and no such 10 Hz ripple.
