MIC2172/3172
= required output voltage
Micrel
V
Switch Operation
OUT
V = D1 forward voltage drop
During Q1’s on time (Q1 is the internal NPN transistor—see
block diagrams), energy is stored in T1’s primary inductance.
DuringQ1’sofftime,storedenergyispartiallydischargedinto
C4 (output filter capacitor). Careful selection of a low ESR
capacitor for C4 may provide satisfactory output ripple volt-
age making additional filter stages unnecessary.
F
For the example in figure 11.
I
I
= 0.14A
OUT
= 1.147A
CL
V
= 4.75V (minimum)
IN
δ = 0.623
= 12.0V
C1 (input capacitor) may be reduced or eliminated if the
MIC3172 is located near a low impedance voltage source.
V
OUT
V = 0.6V
Output Diode
F
Then:
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.
1.147
2
× 4.75 × 0.623
I
≤
OUT
12
Frequency Compensation
I
≤ 0.141A
OUT
A simple frequency compensation network consisting of R3
and C2 prevents output oscillations.
This value is greater than the 0.14A output current require-
ment so we can proceed to find the inductance value of L1.
High impedance output stages (transconductance type) in
the MIC2172/3172 often permit simplified loop-stability solu-
tions to be connected to circuit ground, although a more
conventional technique of connecting the components from
the error amplifier output to its inverting input is also possible.
2
V
δ
(
)
IN
L1 ≤
(2)
2 POUT fSW
Where:
P
= 12 × 0.14 = 1.68W
Voltage Clipper
OUT
5
f
= 1×10 Hz (100kHz)
Care must be taken to minimize T1’s leakage inductance,
otherwise it may be necessary to incorporate the voltage
clipper consisting of D1, R4, and C3 to avoid second break-
down (failure) of the MIC3172’s power NPN Q1.
SW
For our practical example:
2
4.75 × 0.623
(
)
L1 ≤
2 × 1.68 × 1×105
Enable/Shutdown
I
≤ 26.062µH (use 27µH)
L1
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
Equation (3) solves for L1’s maximum current value.
V
T
IN ON
I
=
(3)
L1(peak)
L1
required, simply connect EN to V or to a TTL high voltage.
IN
Where:
Discontinuous Mode Design
-6
T
= δ / f
= 6.23×10 sec
SW
When designing a discontinuous flyback converter, first de-
termine 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.
ON
-6
4.75 × 6.23 × 10
I
=
L1(peak)
-6
27 ×10
I
= 1.096A
L1(peak)
Use a 27µH inductor with a peak current rating of at least
1.4A.
2 P
OUT
(8)
Flyback Conversion
δ ≥
I
V
CL IN(min)
Flyback converter topology may be used in low power appli-
cations 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 (trans-
former primary) current can be continuous or discontinuous.
Discontinuous operation is recommended.
For a practical example let:
P
= 5.0V × 0.25A = 1.25W
OUT
V
I
= 4.0V to 6.0V
= 1.25A when δ < 50%
IN
CL
0.833 (2 – δ) when δ ≥ 50%
Figure 12 shows a practical flyback converter design using
the MIC3172.
4-24
1997
MICREL [ MICREL SEMICONDUCTOR ]
