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MC34063A

DC-TO-DC Converter Control Circuits

Wing Shing International Group 

MC34063A

CALCULATION

DC-TO-DC Converter Control Circuits

Table 1 Design Formula Table

Calculation

Step-Up

Step-Down

Voltage-Inverting

ton/toff

(ton+toff)max

toff

ton

CT

Ipk(switch)

RSC

Vout + VF - Vin(min)

Vin(min) - Vsat

1

f

ton + toff

ton/toff + 1

(ton + toff) - toff

4.0×10 -5 ton

( ) ton

2Iout(max) toff + 1

0.3

IPK(switch)

Vout + VF

Vin(min) - Vsat - Vout

1

f

ton + toff

ton/toff + 1

(ton + toff) - toff

4.0×10-5 ton

2Iout(max)

0.3

IPK(switch)

|Vout| + VF

Vin - Vsat

1

f

ton + toff

ton/toff + 1

(ton + toff) - toff

4.0×10-5 ton

( ) ton

2Iout(max) toff + 1

0.3

IPK(switch)

L(min)

CO

(Vin(min) - Vsat) ton(max)

Ipk(switch)

≈

Ioutton

Vripple(p-p)

(Vin(min) - Vsat - Vout) ton(max)

Ipk(switch)

Ipk(switch)(ton + toff)

8Vripple(p-p)

(Vin(min) - Vsat) ton(max)

Ipk(switch)

≈

Ioutton

Vripple(p-p)

Vsat=Saturation voltage of the output switch.

VF=Forward voltage drop of the output rectifier.

The following power supply characteristics must be chosen:

Vin=Nominal input voltage.

Vout=Desired output voltage. Vout =1.25(1+R2/R1)

Iout=Desired output current.

fmin=Minimum desired output switching frequency at the selected values of Vin and Io

Vripple(pp)=Desired peak-to-peak output ripple voltage.In prictice,the calculated capacitor value

will need to be increased due to its equivalent series resistance and board layout.

The ripple should be kept to a low value since it will directiy affect the line and load

regulation.

13

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