# #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:

Ref | Design Value (Ideal) | Selected |
---|---|---|

Co | 35.94μF | 47μF |

L1 | 42.5μH | 47μH |

R1 | 2kΩ | 2kΩ |

R2 | 6kΩ | 6.8kΩ |

Vout | 5V | 5.5V |

### 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.