MIC2172 datasheet(11/16 Pages) MICREL

11 / 16 page

1997

4-23

MIC2172/3172

Micrel

Then:

SW = (0.625)

× 1 × 0.6

(SW) = 0.234W

(total) = 0.068 + 0.234

(total) = 0.302W

The junction temperature for any semiconductor is calculated

using the following:

J = TA + P(total) θJA

Where:

J = junction temperature

A = ambient temperature (maximum)

(total) = total power dissipation

JA = junction to ambient thermal resistance

For the practical example:

A = 70°C

JA = 130°C/W (for plastic DIP)

Then:

J = 70 + 0.30 × 130

J = 109°C

This junction temperature is below the rated maximum of

150

°C.

Grounding

Refer to figure 10. Heavy lines indicate high current paths.

MIC2172/3172

VIN

VSW

VIN

* MIC3172 only

EN*

GND

P1 P2 S

Single point ground

Figure 10. Single Point Ground

A single point ground is strongly recommended for proper

operation.

The signal ground, compensation network ground, and feed-

back network connections are sensitive to minor voltage

variations.

The input and output capacitor grounds and

power ground conductors 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

MIC2172/3172 or any switching regulator from functioning

properly.

Applications and Design Hints

Access to both the collector and emitter(s) of the NPN power

switch makes the MIC2172/3172 extremely versatile and

suitable for use in most PWM power supply topologies.

Boost Conversion

Refer to figure 11 for a typical boost conversion application

where a +5V logic supply is available but +12V at 0.14A is

required.

+5V

(4.75V min.)

1N5822

MIC2172

VIN

VSW

C1*

22µF

1µF

10.7k

470µF

VOUT

+12V, 0.14A

27µH

* Locate near MIC2172 when supply leads > 2

1.24k

SYNC

GND

P1 P2

N/C

COMP

Figure 11. 5V to 12V Boost Converter

The first step in designing a boost converter is determining

whether inductor L1 will cause the converter to operate in

either continuous or discontinuous mode. Discontinuous

mode is preferred because the feedback control of the

converter is simpler.

When L1 discharges its current completely during the

MIC2172/3172’s off-time, it is operating in discontinuous

mode.

L1 is operating in continuous mode if it does not discharge

completely before the MIC2172/3172 power switch is turned

on again.

Discontinuous Mode Design

Given the maximum output current, solve equation (1) to

determine whether the device can operate in discontinuous

mode without initiating the internal device current limit.

(1)

OUT ≤









IN δ

OUT

(1a)

δ =

OUT + VF – VIN

OUT +VF

Where:

CL = internal switch current limit

CL = 1.25A when δ < 50%

CL = 0.833 (2 – δ) when δ ≥ 50%

(Refer to Electrical Characteristics.)

OUT = maximum output current

IN = minimum input voltage

δ = duty cycle

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