 # #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

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  ### 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. 