#667 MC34063 Buck Converter
Design and test a basic 9v to 5v step-down regulator circuit using the MC34063A.
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
Ton/Toff = (Vout + Vf) / (Vin - Vsat - Vout) = (5 + 0.60) / (9 - 1 - 5) = [1.87](https://www.wolframalpha.com/input?i=%285+%2B+0.60%29%2F%289+-+1+-+5%29)
Ton + Toff
Ton + Toff = 1 / f = 1 / 40kHz = [25µs](https://www.wolframalpha.com/input?i=1%2F40kHz)
Toff = (Ton + Toff)/(Ton/Toff + 1) = 25µs/(1.87 + 1) = [8.71µs](https://www.wolframalpha.com/input?i=25%C2%B5s%2F%281.87+%2B+1%29) =
Ton = (Ton + Toff) – Toff = 25µs – 8.71µs = [16.29 μs](https://www.wolframalpha.com/input?i=25%C2%B5s+%E2%80%93++8.71%C2%B5s)
Calculate timing Capacitor Ct to produce the desired frequency.
Ct = 4.0 * 10^-5 * Ton = 4.0 * 10^-5 * 16.29μs = [652pf](https://www.wolframalpha.com/input?i=4.0+*+10%5E-5+*+16.29%CE%BCs)
Calculate the minimum inductor value
Lmin = (Vin - Vsat - Vout)/Ipk x Ton = (9V - 1V - 5V)/1.15A * 16.29µs = [42.5μH](https://www.wolframalpha.com/input?i=%289V+-+1V+-+5V%29%2F1.15A+*+16.29%C2%B5s)
Rsc = 0.3/Ipk = 0.3/1.15 = [0.260Ω](https://www.wolframalpha.com/input?i=0.3%2F1.15
Step 12:- Let’s calculate the output capacitor values, we can choose a ripple value of 100mV (peak to peak) from the boost output.
Cout = Ipk (Ton + Toff)/(8 * Vripple) = 1.15A * 25µs/(8 * 100mV) = [35.94μF](https://www.wolframalpha.com/input?i=1.15A+*+25%C2%B5s%2F%288+*+100mV%29)
Calculate feedback resistors R1 and R2, given
R1 = 2kΩ and
Vout = 1.25 (1 + R2/R1)
R2 = R1 * (Vout/1.25 - 1) = 2kΩ * (5/1.25 - 1) = [6kΩ](https://www.wolframalpha.com/input?i=2k%CE%A9+*+%285%2F1.25+-+1%29)
Finally, selecting available components close to the theoretical:
|Ref||Design Value (Ideal)||Selected|
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.