MIC3172YN Micrel Inc, MIC3172YN Datasheet - Page 16

• Manufacturer Part Number: MIC3172YN
• Description: IC REG SW 100KHZ 1.25A 8-DIP
• Manufacturer: Micrel Inc
• Type: Step-Down (Buck), Step-Up (Boost), Inverting, Cukr

Specifications of MIC3172YN

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Current - Output: 1.25A
• Frequency - Switching: 100kHz
• Voltage - Input: 3 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• Power - Output: 302mW
• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Current: 3A
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +85°C
• Supply Voltage Range: 3V To 40V
• Termination Type: Through Hole
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: 576-2255; MIC3172YN

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%.
Where:
Micrel
April 2006
F
commercial and industrial applications)
V
V
V
F
V
a ≤ 8.2143
L
f
V
T
a
L
SW
PRI
CE
CE
ON
IN(max)
SEC
CE
SEC
IN(min)
PRI
≤
= device switching frequency (100kHz)
= safety derating factor (0.8 for most
= 65V max. for the MIC2172/3172
= 0.8
= power switch on time
= maximum primary inductance
65
= transformer secondary voltage (V
= 5.6V
≤
= minimum input voltage
×
= maximum input voltage
0.5
0.8
5.6
f
SW
±
6.0
V
P
IN(min)
OUT
2
T
ON
2
Figure 12. MIC3172 5V 0.25A Flyback Converter
OUT
+ V
F
)
(10)
16
Then:
Use an 18µH primary inductance to overcome circuit
inefficiencies.
To complete the design the inductance value of the
secondary is found which will guarantee that the energy
stored in the transformer during the power switch on
time will be completed discharged into the output during
the off-time. This is necessary when operating in
discontinuous-mode.
Where:
Then:
L
L
T
L
L
L
L
PRI
SEC
SEC
OFF
PRI
SEC
SEC
≤ 19.23µH
= maximum secondary inductance
= power switch off time
≤ 25.4µH
≤
≤
≤
0.5
0.5
0.5
×
×
f
1
SW
1
×
×
10
10
P
V
OUT
5
SEC
5
×
×
1.25
4.0
5.6
2
1.25
T
2
OFF
2
(
5.5
×
(
2
4.5
×
10
×
−
10
6
)
−
2
MIC2172/3172
6
[phone]
M9999-041806
)
2
(11)