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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. output voltage ripple on channel-1 and switching transistor emitter (pin-2 of MC34063A) voltage on channel-2 Parameter and component calculations Actual PCB layout

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  • \$\begingroup\$ Its possible that you didnt correctly choose the feedback resistors. What values did you choose? \$\endgroup\$ – Golaž Apr 8 '15 at 21:58
  • \$\begingroup\$ please show circuit \$\endgroup\$ – markrages Apr 9 '15 at 0:45
  • \$\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\$ – karanbanthia Apr 11 '15 at 8:23
  • \$\begingroup\$ @markrages Added the circuit and its calculations \$\endgroup\$ – karanbanthia Apr 11 '15 at 8:24
  • \$\begingroup\$ what are you powering the circuit with? please add a photograph of the circuit that you built. \$\endgroup\$ – markrages Apr 11 '15 at 20:26
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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.

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