MIC2172 MICREL [Micrel Semiconductor], MIC2172 Datasheet - Page 13

• Manufacturer Part Number: MIC2172
• Description: 100kHz 1.25A Switching Regulators Preliminary Information
• Manufacturer: MICREL [Micrel Semiconductor]

MIC2172/3172
Then:
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
3172 power switch.
(9)
Where:
For the practical example:
Then:
Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.
1997
PRI
/N
a = transformer maximum turns ratio
V
F
V
V
V
F
V
a
SEC
a
a
CE
CE
CE
IN(max)
SEC
CE
SEC
0.5 (50%) Use 0.55.
8.2143
, that will guarantee safe operation of the MIC2172/
= safety derating factor (0.8 for most
= 0.8
= power switch collector to emitter
= 65V max. for the MIC2172/3172
1.25
maximum voltage
commercial and industrial applications)
= transformer secondary voltage (V
= 5.6V
2
V
65
CE
= maximum input voltage
1.25
F
CE
0.8 – 6.0
5.6
V
4
SEC
– V
IN(max)
Shutdown
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
Enable
4V to 6V
R3
1k
V
Figure 12. MIC3172 5V 0.25A Flyback Converter
IN
22µF
C2
1µF
C1
COMP
EN
P1 P2
MIC3172
GND
V
IN
OUT
S
V
FB
SW
+ V
R4*
F
)
4-25
L
C3*
D1*
PRI
(10)
Where:
Then:
Use an 18 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.
(11)
Where:
Then:
1:1.25
= 100µH
T1
L
f
V
T
L
L
T
L
L
L
L
L
SW
PRI
ON
PRI
SEC
OFF
SEC
1N5818
IN(min)
PRI
PRI
SEC
SEC
D2
= device switching frequency (100kHz)
= power switch on time
= maximum primary inductance
= maximum secondary inductance
= power switch off time
19.23 H
25.4 H
= minimum input voltage
470µF
0.5 f
0.5
C4
0.5 f
0.5
SW
SW
R1
3.74k
1%
R2
1.24k
1%
1 10
1 10
V
V
5V, 0.25A
P
P
V
OUT
IN(min)
OUT
OUT
SEC
5
5
2
1.25
2
T
4.0
T
OFF
1.25
5.6
ON
2
2
2
2
5.5 10
4.5 10
-6 2
-6 2
Micrel
4