12

LTC1627

possible? This capacitor provides the AC current to the

internal power MOSFETs.

4. Keep the switching node SW away from sensitive small-

signal nodes.

Design Example

As a design example, assume the LTC1627 is used in a

single lithium-ion battery-powered cellular phone applica-

down to about 2.7V. The load current requirement is a

maximum of 0.5A but most of the time it will be on standby

mode, requiring only 2mA. Efficiency at both low and high

load currents is important. Output voltage is 2.5V. With

this information we can calculate L using equation (1),

L

fI

V

V

V

L

OUT

OUT

IN

−









1

1

∆

(3)

f = 350kHz in equation (3) gives:

L

V

kHz

mA

V

V

H

)

−









=

25

350

200

1

25

42

14 5

..

.

.µ

A 15

µH inductor works well for this application. For good

efficiency choose a 1A inductor with less than 0.25

Ω

series resistance.

0.25

Ω. In most applications, the requirements for these

capacitors are fairly similar.

For the feedback resistors, choose R1 = 80.6k. R2 can then

be calculated from equation (2) to be:

R

V

Rk

OUT

2

08

1

1 171

=−









•=

.

; use 169k

Figure 8 shows the complete circuit along with its effi-

ciency curve.

0.1

µF

†

100

µF

6.3V

††

22

µF

16V

C1

0.1

µF

C2

0.1

µF

D2

D1

BAT54S**

15

µH*

* SUMIDA CD54-150

** ZETEX BAT54S

R2

169k

1%

R1

80.6k

1%

47pF

1

2

3

4

8

7

6

5

SYNC/FCB

SW

RUN/SS

GND

LTC1627

2.8V TO

4.5V

2.5V

0.5A

1627 F08a

+

+

OUTPUT CURRENT (mA)

1

100

1000

1627 F08b

10

100

95

90

85

80

75

70

65

60

55

50

45

Figure 8. Single Lithium-Ion to 2.5V/0.5A Regulator