June 2000

7

MIC2141

MIC2141

Micrel

Application Information

Predesigned circuit information is at the end of this section.

Component Selection

Boost Inductor

Maximum power is delivered to the load when the oscillator

is gated

on 100% of the time. Total output power and circuit

efficiency must be considered when determining the maxi-

mum inductor. The largest inductor possible is preferable in

order to minimize the peak current and output ripple. Effi-

ciency can vary from 80% to 90% depending upon input

voltage, output voltage, load current, inductor, and output

diode.

Equation 1 solves for the output current capability for a given

inductor value and expected efficiency. Figures 5 through 9

graph estimates for maximum output current, assuming the

minimum duty cycle, maximum frequency, and 85% effi-

ciency. To determine the required inductance, find the inter-

section between the output voltage and current and select the

value of the inductor curve just above the intersection. If the

efficiency is expected to be other than the 85% used for the

graph, Equation 1 can then be used to better determine the

maximum output capability.

(1)

I

Vt

2L

T

V

eff

V

O(max)

IN(min) ON

MAX

S

O

IN min

−

()

2

1

The peak inductor and switch current can be calculated from

Equation 2 or read from the graph in Figure 10. The peak

current shown in Figure 10 is derived assuming a maximum

duty cycle and a minimum frequency. The selected inductor

and diode peak current capability must exceed this value.

The peak current seen by the inductor is calculated at the

maximum input voltage. A wider input voltage range will result

in a higher worst-case peak current in the inductor. This effect

can be seen in Table 4 by comparing the difference between

the peak current at V

(2)

I

tV

L

PK

ON max

IN max

MIN

=

()()

DCM/CCM Boundary

Equation 3 solves for the point at which the inductor current

will transition from DCM (discontinuous conduction mode) to

CCM (continuous conduction mode). As the input voltage is

raised above this level the inductor has a potential for

developing a dc component while the oscillator is gated on.

Table 1 display the input points at which the inductor current

can possibly operate in the CCM region. Operation in this

region can result in a peak current slightly higher than

displayed Table 4.

(3)

VV

V

1 D

IN ccm

OUT

FWD

()

Table 2 lists common inductors suitable for most applica-

tions. Table 6 lists minimum inductor sizes versus input and

output voltage. In low-cost, low-peak-current applications,

RF-type leaded inductors may sufficient. All inductors listed

in Table 4 can be found within the selection of CR32- or

LQH4C-series inductors from either Sumida or muRata.

r

e

r

u

t

c

a

f

u

n

a

Ms

e

i

r

e

Se

p

y

T

e

c

i

v

e

D

a

t

a

R

u

mC

4

/

C

3

/

C

1

H

Q

Lt

n

u

o

m

e

c

a

f

r

u

s

a

d

i

m

u

S2

3

R

Ct

n

u

o

m

e

c

a

f

r

u

s

r

e

ll

i

M

.

W

.

JF

8

7d

e

d

a

e

l

l

a

i

x

a

t

f

a

r

c

li

o

C0

9d

e

d

a

e

l

l

a

i

x

a

Table 2. Inductor Examples

Boost Output Diode

Speed, forward voltage, and reverse current are very impor-

tant in selecting the output diode. In the boost configuration,

the average diode current is the same as the average load

current. (The peak current is the same as the peak inductor

current and can be derived from Equation 2 or Figure 10.)

Care must be take to make sure that the peak current is

evaluated at the maximum input voltage.

e

d

o

i

D

5

7C

°

V

D

W

F

t

a

A

m

0

0

1

C

°

5

2

V

D

W

F

t

a

A

m

0

0

1

m

o

o

R

.

p

m

e

T

e

g

a

k

a

e

L

V

5

1

t

a

C

°

5

7

e

g

a

k

a

e

L

V

5

1

t

a

e

g

a

k

c

a

P

0

3

5

0

R

B

MV

5

7

2

.

0V

5

2

3

.

0A

µ

5

.

2A

µ

0

9

3

2

1

D

O

S

T

M

S

8

4

1

4

N

1

V

6

.

0

)

C

°

5

7

1

(

V

5

9

.

0

A

n

5

2

)

V

0

2

(

A

µ

2

.

0

)

V

0

2

(

d

e

d

a

e

l

T

M

S

d

n

a

4

5

T

A

B

V

4

.

0

)

C

°

5

8

(

V

5

4

.

0

A

n

0

1

)

V

5

2

(

A

µ

1

)

V

0

2

(

T

M

S

5

8

T

A

B

4

5

.

0

)

C

°

5

8

(

V

6

5

.

0A

µ

4

.

0

A

µ

2

)

C

5

°

8

(

4

3

-

O

D

d

e

d

a

e

l

Table 3. Diode Examples

As can be seen in the “Typical Characteristics: Efficiency”

graph, the output diode type can have an effect on circuit

efficiency. The BAT54- and BAT85-series diodes are low-

current Shottky diodes available from On Semiconductor and

Phillips, respectively. They are suitable for peak repetitive

currents of 300mA or less with good reverse current charac-

teristics. For applications that are cost driven, the 1N4148, or

equivalent, will provide sufficient switching speed with greater

forward drop and reduced cost. Other acceptable diodes are

On Semiconductor’s MBR0530 or Vishay’s B0530, although

they can have reverse currents that exceed 1mA at very high

junction temperatures. Table 3 summarizes some typical

performance characteristics of various suitable diodes.

V

T

U

O

V

)

M

C

C

(

N

I

V

3

.

3V

4

0

.

3

V

0

.

5V

0

4

.

4

V

0

.

9V

0

6

.

7

V

0

.

2

1V

0

.

0

1

V

0

.

5

1V

4

.

2

1

V

0

.

6

1V

2

.

3

1

V

0

.

0

2V

4

.

6

1

V

0

.

2

2V

0

.

8

1

Table 1. DCM/CCM Boundary