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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

Fizz
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karanbanthia
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  • Its possible that you didnt correctly choose the feedback resistors. What values did you choose? – Golaž Apr 08 '15 at 21:58
  • please show circuit – markrages Apr 09 '15 at 00:45
  • @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 – karanbanthia Apr 11 '15 at 08:23
  • @markrages Added the circuit and its calculations – karanbanthia Apr 11 '15 at 08:24
  • what are you powering the circuit with? please add a photograph of the circuit that you built. – markrages Apr 11 '15 at 20:26
  • @markrages Using 3 cell li-ion battery(12.6V 2300mAh) to power the circuit – karanbanthia Apr 13 '15 at 11:36
  • When you selected an inductor did you account for the fact that the inductance drops off at higher currents - if you look in the datasheets there is usually an I vs L graph, and if you didn't choose wisely you may find the inductance at 0.6A is less than you need. – Tom Carpenter Nov 15 '15 at 23:37
  • Do you have a load on the output? That looks suspiciously like 'burst mode' operation where the output voltage is rising until it exceeds the feedback reference and the switch is shut down until the feedback is back within normal range. – Peter Smith Nov 16 '15 at 09:41

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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.

m.Alin
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karanbanthia
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