mc33466 ON Semiconductor, mc33466 Datasheet - Page 12

• Manufacturer Part Number: mc33466
• Description: Fixed Frequency Pwm Micropower Dc-to-dc Converter
• Manufacturer: ON Semiconductor

The following converter design characteristics must be chosen:
NOTES: 1. V
Design Example − Step−down Application
1. Because this is a discontinuous mode design, D t
2. t on [ D
3. L t
4. I
5. ESR t
Choose C
Specified maximum ESR for each is 0.9 Ω.
The complete design schematic is shown in Figure 24.
Required: V
L(pk)
Choose L = 47 μH. Coilcraft part number DO3316P−473.
Calculation
V
(V
V
V
I
V
since it will directly affect line and load regulation. Capacitor C
designed for switching regulator applications.
D − Operating duty cycle = t
2. I
3. fs − Switching frequency, nominally 50 kHz.
4. R
5. Ns, Np − In flyback applications Ns is the number of turns of the secondary transformer winding; Np is the number of the
ripple(pp)
I
O
+
O
I
fs
L(avg)
in
O
ripple(pp)
in
L(pk)
t
D
on
− Desired dc output current
L
= two parallel AVX 330 μF tantalum chip capacitors. Part Number TAJE337M006.
− Nominal Operating dc input voltage
primary winding turns.
V
− Desired dc output voltage
+
in
(V
– V
sat
O
ripple(pp)
2I
− DC input switch.
I
in
in
L(pk)
− Load resistance. R
(50 kHz)
O
− Saturation voltage of the switching transistor.
O
= 8.0 V, an output voltage of 3.3 V at 300 mA is desired with an output ripple of less than 300 mVpp.
0.33
– V
− Desired peak−to−peak output ripple voltage. For best performance the ripple voltage should be kept to a low value
)(t on )
L
O
)(t on )
+
+
+ 6.6 μs.
(300 mV)
(660 mA)
(8 – 3.3)(6.6 μs)
+
[2(0.3)]
(8 – 3.3)(6.6 μs)
t
(V in * V
O
on
Step−Down
(V in – V
= V
(fs). This parameter must be chosen to be <0.5 for step−up and flyback applications.
(47 μH)
+ 0.455 W.
t
D
fs
O
I
/I
(V
(V
O
2I
L
O
O
O
O
in
O
.
)(t on )
+ 51.7 μH.
)
)
)(t on )
Figure 27. Design Equations
+ 660 mA.
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R
O
V
V
+
O
in
+ 3.3
I
V
I
L(pk)
O
O
12
8
+ 11 W
O
+ 0.41. Choose D = 0.33.
(V in * V sat )(t on )
should be a low equivalent series resistance (ESR) electrolytic
t
8fsC
t
Step−Up
(V
1
(V
D
fs
O
O
I
in
2I
(V
in
L
)(t on )
* V in )
in
2
O
) ( ESR )
)
2
1
2
t
(V in * V sat )(t on )
t
Ns
Np
Flyback
(V
D
fs
(V
I
in
in
2I
L
V
)(t on )
in
in
O
) ) V
O