DC-TO-DC Converter Control Circuits
CALCULATION
Table 1 Design Formula Table
Step-Up Step-Down
Calculation
Voltage-Inverting
Vout + VF
Vin(min) - Vsat - Vout
Vout + VF - Vin(min)
Vin(min) - Vsat
|
|
Vout + VF
t
on/toff
-
Vin Vsat
1
f
1
f
1
f
(ton+toff)
max
ton + toff
ton/toff + 1
ton + toff
ton/toff + 1
ton + toff
ton/toff + 1
t
off
on
(ton + toff) - toff
(ton + toff) - toff
4.0×10-5 ton
(ton + toff) - toff
4.0×10-5 ton
t
4.0×10-5 ton
ton
C
T
ton
toff
2Iout(max)
(
)
I
pk(switch)
+ 1
2Iout(max)
(
)
+ 1
2Iout(max)
toff
0.3
PK(switch)
0.3
0.3
R
SC
I
I
PK(switch)
I
PK(switch)
(Vin(min) - Vsat)
Ipk(switch)
(Vin(min) - Vsat - Vout)
Ipk(switch)
(Vin(min) - Vsat)
Ipk(switch)
ton(max)
ton(max)
ton(max)
L
(min)
Ioutton
Vripple(p-p)
Ipk(switch)(ton + toff)
8Vripple(p-p)
Ioutton
Vripple(p-p)
≈
≈
C
O
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.
Iout=Desired output current.
V
out =1.25(1+R2/R1)
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
Wing Shing [ WING SHING COMPUTER COMPONENTS ]
