I use NE555 & MC34063 to build a LED driver with dimmer function. The crcuit of NE555 , MC34063 & LED are bridged by a IRF640N NMOSFET. NE555 is responsible for adjusting the brightness and MC34063 is responsible for light up the LED.

Under 90% duty cycle, everything is fine. However, the LED start to flicker under 10% duty cycle.

Here are plot charts of 10% & 90% duty cycle & the circuit of MC34063. (The values in both charts are sampled by 10X probe, please multiple them by 10.)



I guess that the root cause might be the MC34063 entered DCM.

There are some triangles in 10% duty cycle snapshot. Are these evidence of DCM or just big ripple? Should I replace the inductor in MC34063 circuit with bigger one? What if I just replace the output capacitor?

  • \$\begingroup\$ Switching frequency? \$\endgroup\$ – Rohat Kılıç Oct 31 '16 at 19:48
  • \$\begingroup\$ Switching frequency is 50khz and the ne555 dimmer's is 1 KHz. \$\endgroup\$ – Bruce Hsu Nov 1 '16 at 1:04
  • \$\begingroup\$ Have you tried a second MC34063 IC to verify its not defective? Are your wires as short as possible? Is the circuit assembled on a bread-board/proto-board or is it soldered in a PCB? \$\endgroup\$ – Logic1 Nov 1 '16 at 2:20
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    \$\begingroup\$ It's quite normal for an LED to flicker under %10 DC @ low PWM frequencies such as 1kHz. You can solve this issue by increasing PWM frequency to, say, 5kHz or higher. \$\endgroup\$ – Rohat Kılıç Nov 1 '16 at 3:07

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