#667 MC34063 Buck Converter

Design and test a basic 9v to 5v step-down regulator circuit using the MC34063A.

Notes

The MC34063A is a Buck / Boost / Inverting Regulator with a minimum number of external components.

• Operation from 3.0 V to 40 V Input
• Output Switch Current to 1.5 A
• Frequency Operation to 100 kHz
• Precision 2% Reference

Circuit Design Calculations

The MC34063A datasheet provides the design guidelines and calculations for step-down converter.

OnSemi also provide a MC34063A Design Worksheet to assist.

Circuit Design Calculations for 9V to 5V Converter

I’m going to work this through from scratch.

Some givens and assumptions

• Vf = 0.6V ; 1N5819 forward voltage
• Vsat = 1V ; 1N5819 saturation voltage
• Vripple = 100mV ; chosen peak ripple voltage
• Design for 1A and 30% inductor ripple (typical inductor use 20-40% of the average output current)
  • Il(avg) = 1A
  • Iripple = 1A * 30% = 0.30A
  • Ipk = Il(avg) + Iripple/2 = 1 + .30/2 = 1.15A
• 40Khz switching frequency

Calculate Ton/Toff = 1.87

Ton/Toff
= (Vout + Vf) / (Vin - Vsat - Vout)
= (5 + 0.60) / (9 - 1 - 5)
= 1.87

Calculate Ton + Toff = 25µs

Ton + Toff
= 1 / f
= 1 / 40kHz
= 25µs

Calculate Toff = 8.71µs

Toff
= (Ton + Toff)/(Ton/Toff + 1)
= 25µs/(1.87 + 1)
= 8.71µs

Calculate Ton = 16.29 μs

Ton
= (Ton + Toff) – Toff
= 25µs –  8.71µs
= 16.29 μs

Calculate timing Capacitor Ct to produce the desired frequency. Result: Ct = 652pf

Ct
= 4.0 * 10^-5 * Ton
= 4.0 * 10^-5 * 16.29μs
= 652pf

Calculate the minimum inductor value Lmin = 42.5μH

Lmin
= (Vin - Vsat - Vout)/Ipk x Ton
= (9V - 1V - 5V)/1.15A * 16.29µs
= 42.5μH

Calculate Rsc = 0.260Ω

Rsc
= 0.3/Ipk
= 0.3/1.15
= 0.260Ω

Calculate Cout. Let’s calculate the output capacitor values, we can choose a ripple value of 100mV (peak to peak) from the boost output. Result: Cout = 35.94μF

Cout
= Ipk (Ton + Toff)/(8 * Vripple)
= 1.15A * 25µs/(8 * 100mV)
= 35.94μF

Calculate feedback resistors R1 and R2, given R1 = 2kΩ and Vout = 1.25 (1 + R2/R1). Result: R2 = 6kΩ

R2
= R1 * (Vout/1.25 - 1)
= 2kΩ * (5/1.25 - 1)
= 6kΩ

Finally, selecting available components close to the theoretical:

Circuit Construction

Testing

The output voltage I’m seeing is 5.6V, close to the expected 5.5V with these components:

The ripple (captured AC-coupled on the following scope trace) appears to be running at around

• 200mV peak-peak
• 600µs period i.e. 1.7kHz

That’s quite a bit off from the design parameters; probably worth re-testing with a protoboard/PCB version of the circuit and perhaps a load closer to the design current.

Credits and References

• MC34063 datasheet
• MC34063A datasheet - OnSemi
• MC34063A Design Worksheet - OnSemi
• 1N5819 datasheet - OnSemi
• MC34063A Buck, Boost, Inverting Regulator
• 12V to 5V Buck Converter Circuit using MC34063