Micrel, Inc.

MIC2171

May 2007

7

M9999-051107

The junction temperature for any semiconductor is

calculated using the following:

where:

For the practical example:

then:

This junction temperature is below the rated maximum of

150°C.

Grounding

Refer to Figure 5. Heavy lines indicate high current

paths.

MIC2171

IN

SW

FB

VC

GND

Single point ground

Figure 5. 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 MIC2171 or any

switching regulator from functioning properly.

Boost Conversion

Refer to Figure 1 for a typical boost conversion

application where a +5V logic supply is available but

+12V at 0.25A 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 mode.

Discontinuous 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 mode.

L1 is operating in continuous mode if it does not

discharge completely before the MIC2171 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(min)

CL

OUT

V

V

2

I

I

δ

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≤

(1a)

F

OUT

IN(min)

F

OUT

V

V

V

V

V

+

−

+

=

δ

where:

(Refer to Electrical Characteristics.)

δ = duty cycle

For the example in Figure 1.

δ = 0.662

then:

12

0.662

4.178

2

2.235

×

×

⎟

⎠

⎞

⎜

⎝

⎛

≤

This value is greater than the 0.25A output current

requirement, so we can proceed to find the minimum

inductance value of L1 for discontinuous operation at

(2)

SW

OUT

2

IN

f

2P

)

(V

L1

δ

≥