MIC2141

Micrel

MIC2141

8

June 2000

Output Capacitor

If the availability of tantalum capacitors is limited, ceramic

capacitors and inexpensive electrolyics may be necessary.

Selection of the capacitor value will depend upon on the peak

inductor current and inductor size. MuRata offers the GRM

series with up to 10

µF at 25V, with a Y5V temperature

coefficient, in a 1210 surface-mount package. Low-cost

applications can use M-series leaded electrolytic capacitors

from Panasonic. In general, ceramic, electrolytic, or tantalum

values ranging from 10

µF to 47µF can be used for the output

capacitor.

r

e

r

u

t

c

a

f

u

n

a

Ms

e

i

r

e

Se

p

y

Te

g

a

k

c

a

P

a

t

a

R

u

mM

R

GV

5

Y

c

i

m

a

r

e

ct

n

u

o

m

e

c

a

f

r

u

s

y

a

h

s

i

V4

9

5m

u

l

a

t

n

a

tt

n

u

o

m

e

c

a

f

r

u

s

c

i

n

o

s

a

n

a

Ps

e

i

r

e

s

-

Mc

i

t

y

l

o

r

t

c

e

l

ed

e

d

a

e

l

Table 4. Capacitor Examples

Design Example

Given a design requirement of 12V output and 1mA load with

a minimum input voltage of 2.5V, Equation 1 can be used to

calculate to maximum inductance or it can be read from the

graph in Figure 4. Once the maximum inductance has been

determined, the peak current can be determined using Equa-

tion 2 or Figure 10.

V

I

V

SW

GND

FB

IN

VC

1

5

3

2

4

MIC2141

C4

0.1

µF

C1

10

µF

25V

CR1

BAT54HT1

L1

33

µH

C2

10

µF

25V

+5V to +15V

Return

+2.7V to +12V

Return

Figure 1. Basic Configuration

SW

GND

FB

IN

VC

4

5

3

1

2

MIC2141

C4

0.1

µF

R1

34.8k

R2

121k

C1

10

µF

25V

CR1

BAT54HT1

L1

22

µH

C2

10

µF

25V

+5V to +20V

Return

+2.7V to +12V

Return

V6V

1

R1

R2

IR1

I

15 A for V

15V

OUT

C

FB

FB(typ)

OUT

==++









++⋅⋅

====

µ

Figure 2. Gain-Boost Configuration

SW

GND

FB

IN

VC

1

5

3

2

4

MIC2141

C4

0.1

µF

C1

10

µF

25V

CR1

MBR0530

CR3

1N4148

CR2

1N4148

L1

4.7

µH

C2

10

µF

25V

+12V

Return

+2.7V to +4.7V

Return

Figure 3. Bootstrap Configuration

L

Vt

I

V

eff

V2

L17 H

MAX

IN(min)

ON(min)

O(max)

O

IN(min)

MAX

==

⋅⋅

−−⋅⋅ ⋅⋅

==

22

T

S(min)

µ

Select 15

µH ±10%.

I

tV

L

0.767 s

4.8V

13.5 H

I

272mA

PEAK

ON(max)

IN(max)

MIN

PEAK

==

⋅⋅

==

==

µ

µ

Select a BAT54 diode and CR32 inductor.

Always check the peak current to insure that it is within the

limits specified in the load line shown in Figure 10 for all

input and output voltages.

Gain Boost

Use Figure 2 to increase the voltage gain of the system. The

typical gain can easily be increased from the nominal gain of

6 to a value of 8 or 10. Figure 2 shows a gain of 8 so that with

2.5V applied to VC, V

Bootstrap

The bootstrap configuration is used to increase the maximum

output current for a given input voltage. This is most effective

when the input voltage is less than 5V. Output current can

typically be tripled by using this technique. See Table 4a. for

bootstrap-ready-built component values.