MIC2171 datasheet(11/24 Pages) MICROCHIP

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

2022 Microchip Technology Inc. and its subsidiaries

DS20006355A-page 11

MIC2171

EQUATION 5-4:

For the example in the Typical Application Circuits:

IOUT = 0.25A

ICL = 1.67 (2 – 0.662) = 2.24A

VIN(MIN) = 4.18V

δ = 0.662

VOUT = 12.0V

VF = 0.36V (@ 0.26A, 70°C)

Then:

EQUATION 5-5:

This value is greater than the 0.25A output current

requirement, so one can proceed to find the inductance

value of L1 for discontinuous operation at POUT.

EQUATION 5-6:

For our practical example:

EQUATION 5-7:

Equation 5-8 solves for L1’s maximum current value.

EQUATION 5-8:

EQUATION 5-9:

Use a 15 µH inductor with a peak current rating greater

than 2A.

5.6

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’s primary winding) current

can be continuous or discontinuous. In this particular

case, discontinuous operation is recommended.

The Typical Application Circuits shows a practical

flyback converter design using the MIC2171.

5.6.1

SWITCH OPERATION

During Q1’s on time (Q1 is the internal NPN transistor,

see the Functional Block Diagram, 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 voltage ripple

making additional filter stages unnecessary.

C1’s value (input capacitor) may be reduced or it can

be eliminated if the MIC2171 is located near a low

impedance voltage source

IOUT

ICLIM

---------------







 V

IN MIN







VOUT

----------------------------------------------------





VOUT VF VIN MIN





–

VOUT VF

-----------------------------------------------------------

Where:

ICLIM =

Internal switch current limit

ICLIM =

1.25A when δ < 50%

ICLIM =

1.67 (2 – δ) when δ ≥ 50%

IOUT =

Maximum output current

VIN(MIN) = Minimum input voltage = VIN – VSW

δ =

Duty cycle for boost converter in CRM

VOUT =

Required output voltage

VF =

Diode forward voltage drop

IOUT

2.235

-------------







 4.178



---------------------------------------



IOUT 0.389A



VIN 



ICLIM fSW



------------------------------- L1

VIN



2 



2 POUT fSW



----------------------------------------



Where:

POUT =

12 x 0.25 = 3W

fSW =

1.105 Hz (100 kHz)

4.178 0.662



2.235 1 105



------------------------------------- L1

4.178



2

0.662



2 3.0 1 105



-----------------------------------------



12.38H L1 19.26H



(Use 15 µH)

IL1 PEAK





VIN tON



-------------------------

Where:

tON =

δ / fSW = 6.62 × 10-6 sec.

IL1 PEAK





4.178 6.62 10 6

–



15 10 6

–



------------------------------------------------

1.096A

IL1 PEAK





1.84A

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