Datasheet

16
LTC1879
1879f
APPLICATIO S I FOR ATIO
WUUU
Design Example
As a design example, assume the LTC1879 is used in a
dual lithium-ion battery-powered cellular phone applica-
tion. The V
IN
will be operating from a maximum of 8.4V
down to about 2.65V. The load current requirement is a
maximum of 0.7A 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)
Substituting V
OUT
= 2.5V, V
IN
= 8.4V, ∆I
L
= 210mA and
f = 550kHz in equation (3) gives:
L
V
kHz mA
V
V
H=






=µ
25
550 210
1
25
84
15 2
.
•
–
.
.
.
An 15µH inductor works well for this application. For good
efficiency choose a 1.5A inductor with less than 0.125Ω
series resistance.
C
IN
will require an RMS current rating of at least 0.35A at
temperature and C
OUT
will require an ESR of less than
0.125Ω. In most applications, the requirements for these
capacitors are fairly similar.
For the feedback resistors, choose R2 = 412k. R1 can then
be calculated from equation (2) to be:
R
V
R k use k
OUT
1
08
1 2 875 5 887=






=
.
–• .,
Figure 9 shows the complete circuit along with its effi-
ciency curve.