LTC1877 datasheet(13/16 Pages) LINER

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LTC1877

LTC1877. The power ground traces should be kept

short, direct and wide. The signal ground and power

ground should converge to a common node in a star-

ground configuration.

2. Does the VFB pin connect directly to the feedback

resistors? The resistive divider R1/R2 must be con-

nected between the (+) plate of COUT and signal ground.

3. Does the (+) plate of CIN connect to VIN as closely as

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 LTC1877 is used in a

single lithium-ion battery-powered cellular phone applica-

tion. The input voltage will be operating from a maximum

of 4.2V down to about 2.7V. The load current requirement

is a maximum of 0.3A but most of the time it will be in

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),

APPLICATIONS INFORMATION

OUT

= () ∆()

−









(3)

Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL=120mA and

f = 550kHz in equation (3) gives:

kHz

=−









=µ

550

120

15 3

()

A 15

µH inductor works well for this application. For best

efficiency choose a 1A inductor with less than 0.25

Ω

series resistance.

CIN will require an RMS current rating of at least 0.15A at

temperature and COUT will require an ESR of less than

0.25

Ω. In most applications, the requirements for these

capacitors are fairly similar.

For the feedback resistors, choose R1 = 412k. R2 can

then be calculated from equation (2) to be:

k use

OUT

1 875 5

887









–. ;

Figure 8 shows the complete circuit along with its effi-

ciency curve.

–

RUN

ITH

VFB

GND

PLL LPF

SYNC/MODE

VIN

LTC1877

CC2

CC1

COUT

CIN

1877 F07

VIN

BOLD LINES INDICATE

HIGH CURRENT PATHS

OPTIONAL

–

VOUT

Figure 7. LTC1877 Layout Diagram

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