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參數(shù)資料
| 型號： | 83F5958 |
| 英文描述： | IC-1.25A SWITCHING REGULATOR |
| 中文描述： | 集成電路1.25A的開關(guān)穩(wěn)壓器 |
| 文件頁數(shù)： | 13/16頁 |
| 文件大小： | 129K |
| 代理商： | 83F5958 |
1997
4-25
MIC2172/3172
Micrel
4
Then:
δ
≥
2
×
1.25
1.25
×
4
δ
≥
0.5 (50%) Use 0.55.
The slightly higher duty cycle value is used to overcome
circuit inefficiencies. A few iterations of equation (8) may be
required if the duty cycle is found to be greater than 50%.
Calculate the maximum transformer turns ratio
a,
or
N
PRI
/N
SEC
, that will guarantee safe operation of the MIC2172/
3172 power switch.
(9)
a
≤
V
CE
F
CE
– V
IN(max)
V
SEC
Where:
a
= transformer maximum turns ratio
V
CE
= power switch collector to emitter
maximum voltage
F
CE
= safety derating factor (0.8 for most
commercial and industrial applications)
V
IN(max)
= maximum input voltage
V
SEC
= transformer secondary voltage (V
OUT
+ V
F
)
For the practical example:
V
CE
= 65V max. for the MIC2172/3172
F
CE
= 0.8
V
SEC
= 5.6V
Then:
a
≤
65
×
0.8 – 6.0
5.6
a
≤
8.2143
Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.
(10)
L
PRI
≤
0.5 f
SW
V
IN(min)
2
T
ON
2
P
OUT
Where:
L
PRI
= maximum primary inductance
f
SW
= device switching frequency (100kHz)
V
IN(min)
= minimum input voltage
T
ON
= power switch on time
Then:
L
PRI
≤
0.5
×
1
×
10
5
×
4.0
2
5.5
×
10
-6
1.25
L
PRI
≤
19.23
μ
H
Use an 18
μ
H primary inductance to overcome circuit ineffi-
ciencies.
)
2
To complete the design the inductance value of the second-
ary is found which will guarantee that the energy stored in the
transformer during the power switch on time will be com-
pleted discharged into the output during the off-time. This is
necessary when operating in discontinuous-mode.
(11)
L
SEC
≤
0.5 f
SW
V
SEC
2
T
OFF
2
P
OUT
Where:
L
SEC
= maximum secondary inductance
T
OFF
= power switch off time
Then:
L
SEC
≤
0.5
×
1
×
10
5
×
5.6
2
×
4.5
×
10
-6
1.25
L
SEC
≤
25.4
μ
H
)
2
Figure 12. MIC3172 5V 0.25A Flyback Converter
C4
470μF
D2
1N5818
C1
22μF
C2
1μF
R3
1k
R4*
V
OUT
5V, 0.25A
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
C3*
D1*
R1
3.74k
1%
R2
1.24k
1%
V
IN
4V to 6V
T1
Enable
Shutdown
1:1.25
L
PRI
= 100μH
MIC3172
V
IN
V
SW
FB
EN
GND
P1 P2 S
COMP
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