參數(shù)資料
型號(hào)： MIC3172YN
廠商： MICREL INC
元件分類： 穩(wěn)壓器
英文描述： 500kHz and 1MHz High Efficiency 1.5A Switching Regulators; Package: SO; No of Pins: 8; Temperature Range: -40?°C to 85?°C
中文描述： 3.5 A SWITCHING REGULATOR, 115 kHz SWITCHING FREQ-MAX, PDIP8
封裝： LEAD FREE, PLASTIC, DIP-8
文件頁(yè)數(shù)： 13/17頁(yè)
文件大?。?/td> 153K
代理商： MIC3172YN
April 2005
13
M9999-042205
MIC2172/3172
Then:
Micrel, Inc.
δ
≥
2
1.25
1.25
4
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
/N
, 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
a
8.2143
Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.
5.6
(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
≤
L
PRI
≤
19.23
H
Use an 18
μ
H primary inductance to overcome circuit ineffi-
ciencies.
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.
0.5
×
1
×
10
5
×
4.0
2
5.5
×
10
-6
1.25
(
)
2
(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
≤
L
SEC
≤
25.4
μ
H
0.5
×
1
×
10
5
×
5.6
2
×
4.5
×
10
-6
1.25
(
)
2
Figure 12. MIC3172 5V 0.25A Flyback Converter
C4
470μF
D2
1N5818
C1
22μF
C2
1μF
R3
1k
R4*
V
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
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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