mic3172 Micrel Semiconductor, mic3172 Datasheet - Page 13

• Manufacturer Part Number: mic3172
• Description: 100khz 1.25a Switching Regulators
• Manufacturer: 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%.
April 2005
PRI
/N
δ ≥ 0.5 (50%) Use 0.55.
a = transformer maximum turns ratio
V
F
V
V
V
F
V
a ≤ 8.2143
δ ≥
SEC
a ≤
a ≤
CE
CE
CE
IN(max)
SEC
CE
SEC
, 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
maximum voltage
commercial and industrial applications)
1.25 × 4
2 × 1.25
= transformer secondary voltage (V
= 5.6V
65 × 0.8 ± 6.0
V
CE
= maximum input voltage
F
CE
5.6
V
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
)
13
L
D1*
C3*
PRI
(10)
Where:
Then:
Use an 18µH 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
PRI
SEC
SEC
1N5818
ON
OFF
PRI
IN(min)
PRI
SEC
SEC
D2
= device switching frequency (100kHz)
= power switch on time
= maximum primary inductance
≤ 19.23µH
= maximum secondary inductance
= power switch off time
≤ 25.4µH
≤
≤
≤
≤
= minimum input voltage
470µF
0.5 f
0.5 × 1× 10
C4
0.5 f
0.5 × 1× 10
SW
SW
R1
3.74k
1%
R2
1.24k
1%
V
5V, 0.25A
V
P
P
V
OUT
IN(min)
OUT
OUT
SEC
5
5
2
× 4.0
1.25
× 5.6
2
T
T
OFF
1.25
ON
2
2
2
2
(
5.5 × 10
× 4.5 × 10
(
M9999-042205
-6
Micrel, Inc.
)
2
-6
)
2