Fork me on GitHub

Project Notes

#667 MC34063 Buck Converter

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

Build

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

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)

Calculate Ton + Toff

Ton + Toff
= 1 / f
= 1 / 40kHz
= [25µs](https://www.wolframalpha.com/input?i=1%2F40kHz)

Calculate Toff

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) =

Calculate Ton

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

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)

Calculate Rsc

Rsc
= 0.3/Ipk
= 0.3/1.15
= [0.260Ω](https://www.wolframalpha.com/input?i=0.3%2F1.15

Calculate Cout

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
Co 35.94μF 47μF
L1 42.5μH 47μH
R1 2kΩ 2kΩ
R2 6kΩ 6.8kΩ
Vout 5V [5.5V(https://www.wolframalpha.com/input?i=1.25+%281+%2B+6.8%2F2%29) ]

Circuit Construction

bb

schematic

Testing

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

bb_build

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.

scope

Credits and References

About
Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.