MC34166 MC33166
Table 1. Design Equations
Calculation
Step–Down
Step–Up/Down
Voltage–Inverting
ton
toff
(Notes 1, 2)
) Vout VF
* * Vin Vsat Vout
) ) Vout VF1 VF2
* * Vin VsatQ1 VsatQ2
) |Vout| VF
* Vin Vsat
ton
ǒ ) Ǔton
toff
fosc
ton
toff
1
ǒ ) Ǔton
toff
fosc
ton
toff
1
ǒ ) Ǔton
toff
fosc
ton
toff
1
Duty Cycle
(Note 3)
IL avg
ton fosc
Iout
ton fosc
ǒ Ǔ) Iout
ton
toff
1
ton fosc
ǒ Ǔ) Iout
ton
toff
1
Ipk(switch)
L
Vripple(pp)
Vout
) IL avg
DIL
2
ǒ Ǔ * * Vin
Vsat
DIL
Vout ton
Ǹǒ Ǔ ) DIL
12
8foscCo
(ESR)2
ǒ Ǔ) Vref
R2
R1
1
) IL avg
DIL
2
ǒ Ǔ * * Vin
VsatQ1
DIL
VsatQ2 ton
ǒ ǓǸǒ Ǔ ) ton
toff
1
) 1 2
foscCo
(ESR)2
ǒ Ǔ) Vref
R2
R1
1
) IL avg
DIL
2
* Vin Vsat
ǒ ǓDIL
ton
ǒ ǓǸǒ Ǔ ) ton
toff
1
) 1 2
foscCo
(ESR)2
ǒ Ǔ) Vref
R2
R1
1
NOTES: 1. Vsat – Switch Output source saturation voltage, refer to Figure 7.
2. VF – Output rectifier forward voltage drop. Typical value for 1N5822 Schottky barrier rectifier is 0.5 V.
3. Duty cycle is calculated at the minimum operating input voltage and must not exceed the guaranteed minimum DC(max) specification of 0.92.
The following converter characteristics must be chosen:
Vout – Desired output voltage.
Iout – Desired output current.
∆IL – Desired peak–to–peak inductor ripple current. For maximum output current especially when the duty cycle is greater than 0.5, it is suggested
that ∆IL be chosen to be less than 10% of the average inductor current IL avg. This will help prevent Ipk(switch) from reaching the guaranteed
minimum current limit threshold of 3.3 A. If the design goal is to use a minimum inductance value, let ∆IL = 2 (IL avg). This will proportionally
reduce the converter’s output current capability.
Vripple(pp) – Desired peak–to–peak output ripple voltage. For best performance, the ripple voltage should be kept to less than 2% of Vout. Capacitor CO
should be a low equivalent series resistance (ESR) electrolytic designed for switching regulator applications.
14
MOTOROLA ANALOG IC DEVICE DATA