MIC2172/3172

Micrel

4-24

1997

V

V

For the example in figure 11.

I

I

V

δ = 0.623

V

V

Then:

I

1.147

2







 ×

4.75

× 0.623

12

I

This value is greater than the 0.14A output current require-

ment so we can proceed to find the inductance value of L1.

(2)

L1

≤

V

2 P

Where:

P

f

For our practical example:

L1

≤

4.75

× 0.623

2

I

Equation (3) solves for L1’s maximum current value.

(3)

I

V

L1

Where:

T

I

4.75

27

I

Use a 27

µH inductor with a peak current rating of at least

1.4A.

Flyback Conversion

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.

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 volt-

age 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 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.

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 break-

down (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 1

µA.

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

Discontinuous Mode Design

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.

(8)

2 P

OUT

I

For a practical example let:

P

V

I

0.833 (2 –

δ) when δ ≥ 50%