MIC3172YN Micrel Inc, MIC3172YN Datasheet - Page 15

• 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

Equation (3) solves for L1’s maximum current value.
Where:
Use a 27µH inductor with a peak current rating of at
least 1.4A.
Flyback Conversion
Flyback converter topology may be used in low power
applications where voltage isolation is required or
whenever the input voltage can be less than or greater
than the output voltage. As with the step-up converter
the inductor (transformer primary) current can be
continuous or discontinuous. Discontinuous operation is
recommended.
Figure 12 shows a practical flyback converter design
using the MIC3172.
Switch Operation
During Q1’s on time (Q1 is the internal NPN transistor—
see block diagrams), energy is stored in T1’s primary
inductance. During Q1’s off time, stored energy is
partially discharged into C4 (output filter capacitor).
Careful selection of a low ESR capacitor for C4 may
provide satisfactory output ripple voltage making
additional filter stages unnecessary.
C1 (input capacitor) may be reduced or eliminated if the
MIC3172 is located near a low impedance voltage
source.
Output Diode
The output diode allows T1 to store energy in its primary
inductance (D2 nonconducting) and release energy into
C4 (D2 conducting). The low forward voltage drop of a
Schottky diode minimizes power loss in D2.
Frequency Compensation
A simple frequency compensation network consisting of
R3 and C2 prevents output oscillations.
High impedance output stages (transconductance type)
in the MIC2172/3172 often permit simplified loop-stability
solutions to be connected to circuit ground, although a
more
components from the error amplifier output to its
Micrel
April 2006
I
T
I
L1
I
I
L1
L1(peak)
L1(peak)
L1(peak)
ON
≤ 26.062µH (use 27µH)
≤
conventional
= δ / f
2
(
4.75
×
= 1.096A
1.68
=
=
SW
V
4.75
×
IN
0.623
= 6.23×10
×
L1
1
T
27
×
×
ON
10
6.23
×
)
2
technique
10
5
−
×
-6
6
10
sec
−
6
of
connecting
the
(3)
15
inverting input is also possible.
Voltage Clipper
Care must be taken to minimize T1’s leakage
inductance,
incorporate the voltage clipper consisting of D1, R4, and
C3 to avoid second breakdown (failure) of the
MIC3172’s power NPN Q1.
Enable/Shutdown
The MIC3172 includes the enable/shutdown feature.
When the device is shutdown, total supply current is less
than 1µA. This is ideal for battery applications where
portions of a system are powered only when needed. If
this feature is not required, simply connect EN to V
to a TTL high voltage.
Discontinuous Mode Design
When designing a discontinuous flyback converter, first
determine whether the device can safely handle the
peak primary current demand placed on it by the output
power. Equation (8) finds the maximum duty cycle
required for a given input voltage and output power. If
the duty cycle is greater than 0.8, discontinuous
operation cannot be used.
For a practical example let:
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
MIC2172/3172 power switch.
Where:
PRI
P
V
I
δ ≥ 0.5 (50%) Use 0.55.
/N
a = transformer maximum turns ratio
V
voltage
δ ≥
δ
a
CL
OUT
IN
CE
≥
≤
SEC
= 1.25A when δ < 50%
= 4.0V to 6.0V
= power switch collector to emitter maximum
I
1.25
2
V
CL
= 5.0V × 0.25A = 1.25W
, that will guarantee safe operation of the
×
CE
2
1.25
P
V
×
F
OUT
IN(min)
otherwise
CE
4
V
SEC
±
V
IN(max)
it
may
be
MIC2172/3172
[phone]
M9999-041806
necessary
IN
(8)
(9)
or
to