10

© 2006 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC600

Applications Information (Cont.)

Comparison with Other Regulation Methods

In many instances, a charge pump regulator is the best

choice for portable power applications. These regulators

offer many advantages over switch mode regulators. A

smaller bill of materials, less layout area, lower compo-

nent height, less noise, no EMF, and less overall circuit

cost are typical reasons to use this type of regulation. In

some cases the efficiency of a charge pump regulator ex-

ceeds the efficiency of a switch mode regulator.

Inductors are often the largest and most expensive dis-

crete component in a design. Because there are no in-

ductors used in the SC600, cost, noise, layout area, as

well as the the EMF associated with the inductor, are

eliminated.

The SC600’s fixed frequency harmonics are an advantage

in portable communications equipment, such as cellular

telephones. The SC600 has distinct frequencies of op-

eration, so the harmonics are predictable. The harmonics

are not fixed in a switch mode regulator.

Ripple Performance

Examples of the output ripple, charge pump frequency

and capacitor size are listed in Table 2.

Switch mode regulators have harmonics which vary due to

the pulse width modulation used to regulate the output.

Varying harmonics can make it difficult to ensure accept-

able noise performance over the entire operating range.

Many switch mode regulators have increased voltage rip-

ple on the output during pulse skipping mode due to large

periods of time when no current is supplied to the output.

The SC600 supplies current to the output continuously,

so the voltage ripple is less than a switch mode regula-

tor, even with greatly reduced output capacitance. The

SC600 delivers a continuous current to the output during

1x, 1.5x and 2x modes. Most of the battery life requires

1.5x mode.

Frequency Selection

CX8 and CD4 are frequency select inputs; input from a µP

or other device may be used to change the charge pump

frequency at any time (as shown in Table 1). The opti-

mal frequency will depend upon the capacitor values, the

load current, and the acceptable amount of output ripple.

Lower frequencies will be more efficient, while higher fre-

quencies will support higher output currents with lower

ripple.

y

c

n

e

u

q

e

r

F

g

n

i

h

c

t

i

w

S8

X

C4

D

C

z

H

k

2

30

0

z

H

k

80

1

z

H

k

2

6

21

0

z

H

k

0

5

61

1

Table 1 -Frequency Selection Logic

.

o

N

t

r

a

P]

z

H

k

[

.

q

e

r

FI

T

U

O

]

A

m

[]

p

-

p

V

m

[

e

l

p

p

i

R

t

u

p

t

u

O

e

d

o

m

x

5

.

1

]

p

-

p

V

m

[

e

l

p

p

i

R

t

u

p

u

O

e

d

o

m

x

2

C

T

U

O

]

F

μ

[C

T

E

K

C

U

B

]

F

μ

[

V

0

.

5

A

0

0

6

C

S

85

5

65

11

1

2

30

20

5

15

21

1

2

6

20

65

20

21

1

0

5

60

60

15

11

1

0

5

60

60

25

23

3

.

01

Table 2 -Ripple Performance