MIC2171 datasheet(10/24 Pages) MICROCHIP

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MIC2171

DS20006355A-page 10



2022 Microchip Technology Inc. and its subsidiaries

EQUATION 5-2:

From the Typical performance Characteristics:

RSW = 0.37Ω

Then:

PSW = (2.21)2 × 0.37 × 0.662

PSW = 1.2W

P(TOTAL) = 1.2 + 0.1

P(TOTAL) = 1.3W

The junction temperature for any semiconductor is

calculated using the following:

EQUATION 5-3:

For the practical example:

TA = 70°C

θJA = 45°C/W (for TO-220)

Then:

TJ = 70 + (1.3 x 45)

TJ = 128.5°C

This junction temperature is below the rated maximum

of 150°C.

5.4

Grounding

Refer to Figure 5-3. Heavy lines indicate high-current

ground paths.

FIGURE 5-3:

Single Point Ground.

A single point ground is strongly recommended for

proper operation.

The signal ground, compensation network ground, and

feedback network connections are sensitive to minor

voltage variations. The input and output capacitor

grounds and power ground tracks will exhibit voltage

drop when carrying large currents. Keep the sensitive

circuit ground traces separate from the power ground

traces. Small voltage variations applied to the sensitive

circuits can prevent the MIC2171 or any switching

regulator from functioning properly.

5.5

Boost Conversion

Refer to the Typical Application Circuits for a typical

boost conversion application where a +5V logic supply

is available and a +12V at 0.25A output is required.

The first step in designing a boost converter is

determining whether inductor L1 will cause the

converter to operate in either continuous or

discontinuous conduction mode. Discontinuous

conduction mode is preferred because the feedback

control of the converter is simpler.

When L1 discharges its current completely during the

MIC2171 off-time, it is operating in discontinuous

conduction mode.

L1 is operating in continuous conduction mode if it does

not discharge completely before the MIC2171 power

switch is turned on again.

5.5.1

DISCONTINUOUS CONDUCTION

MODE DESIGN

Given the maximum output current, solve Equation 5-4

to determine whether the device can operate in

discontinuous conduction mode without triggering the

internal device current limit.

PSW

ISW



2

RSW 



Where:

δ =

Duty cycle



VOUT VF VIN MIN





–

VOUT VF

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

For boost converter,

Where:

VIN(MIN) =

VIN – VSW

VSW =

ICLIM x RSW

VOUT =

Output voltage

VF =

D1 forward voltage drop at IOUT

TJ TA P TOTAL



 



Where:

TJ =

Junction temperature

TA =

Ambient temperature (maximum)

P(TOTAL) =

Total power dissipation

θJA =

Junction to ambient thermal resis-

tance

MIC2171

VIN

GND

Single point ground

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