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| 型號： | AN-22 |
| 英文描述： | OBSOLETE when inventory is depleted. 10% tolerance no l |
| 中文描述： | 設(shè)計多路輸出電源的的TOPSwitch |
| 文件頁數(shù)： | 19/24頁 |
| 文件大?。?/td> | 569K |
| 代理商： | AN-22 |
C
5/98
AN-22
19
Appendix B
3.3 V and 5 V Outputs
An increasing number of applications require that both 3.3 V
and 5 V outputs in multiple output power supplies, both
requiring
±
5% regulation to supply digital control circuitry.
Several commonly used techniques to achieve this
performance are described below.
Linear Regulator
The simplest, though least efficient technique, is to design
only a 5 V output winding with wire capable of supplying the
RMS current for both the 5 V and 3.3 V outputs. A linear
regulator is then placed on this 5 V output, regulating down to
3.3 V as shown in Figure 1. Integrated 3.3 V regulators are now
available from a number of suppliers with varying current
capabilities. A simple emitter follower regulator could also be
employed using discrete components.
The disadvantage of this technique is reduced power supply
efficiency, although it simplifies the transformer construction
and reduces the number of output pins.
Transformer Turns Ratio
Two techniques are commonly used to design separate
transformer windings for each output. Each has the required
turns ratio relationship to provide the regulation required.
1. Copper wire
If 3 turns are defined for the 3.3 V output and an ultra fast PN
junction diode is specified for this output, the calculation of
the volts per turn provides a solution where 4 turns are used
with a Schottky diode for the 5 V output.
V
V
V
N
PT
O
D
S
=
+
(
)
(1)
If
V
O
=
3 3 V
V
D
=
0 7 V
N
S
=
3 turns
Then from (1) find
V
PT
=
1 33
.
V per turn
rearranging (1) to calculate the turns required for the 5 V output
yields:
N
V
V
V
S
O
D
PT
=
+
(2)
If
V
S
=
5 V
V
D
=
0 4 V
Figure 1. Derivation of 3.3 V Output from 5 V with Linear Regulator.
PI-2133 -121597
5 V
RTN
3.3 V
Linear Regulator
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