Micrel
MIC2172/3172
2
inverting input is also possible.
(
4.75 ×0.623
)
L1≤
2×1.68×1×105
L1 ≤ 26.062µH (use 27µH)
Equation (3) solves for L1’s maximum current value.
IN TON
Voltage Clipper
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 breakdown (failure) of the
MIC3172’s power NPN Q1.
I
V
IL1(peak)
=
(3)
L1
Where:
Enable/Shutdown
TON = δ / fSW = 6.23×10-6 sec
4.75× 6.23×10−6
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 VIN or
to a TTL high voltage.
IL1(peak)
=
27×10−6
L1(peak) = 1.096A
I
Use a 27µH inductor with a peak current rating of at
least 1.4A.
Discontinuous Mode Design
Flyback Conversion
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.
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.
2POUT
δ ≥
(8)
Figure 12 shows a practical flyback converter design
using the MIC3172.
ICL
V
IN(min)
For a practical example let:
POUT = 5.0V × 0.25A = 1.25W
VIN = 4.0V to 6.0V
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.
I
CL = 1.25A when δ < 50%
Then:
δ ≥
2×1.25
1.25× 4
C1 (input capacitor) may be reduced or eliminated if the
MIC3172 is located near a low impedance voltage
source.
δ ≥ 0.5 (50%) Use 0.55.
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%.
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.
Calculate the maximum transformer turns ratio a, or
NPRI/NSEC, that will guarantee safe operation of the
MIC2172/3172 power switch.
Frequency Compensation
VCE FCE
V
IN(max)
a ≤
(9)
A simple frequency compensation network consisting of
R3 and C2 prevents output oscillations.
VSEC
Where:
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 conventional technique of connecting the
components from the error amplifier output to its
a = transformer maximum turns ratio
VCE = power switch collector to emitter maximum
voltage
M9999-041806
(408) 955-1690
April 2006
15
MICREL [ MICREL SEMICONDUCTOR ]
