Datasheet

LTC1877
14
1877fb
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. Effi ciency 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 =
1
f
()
ΔI
L
()
V
OUT
1−
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
(3)
Substituting V
OUT
= 2.5V, V
IN
= 4.2V, ΔI
L
=120mA and
f = 550kHz in Equation (3) gives:
L =
2.5V
550kHz(120mA)
1−
2.5V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
= 15.3μH
A 15μH inductor works well for this application. For best
effi ciency choose a 1A inductor with less than 0.25Ω
series resistance.
C
IN
will require an RMS current rating of at least 0.15A
at temperature and C
OUT
will require an ESR of less than
0.25Ω. 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:
R2 =
V
OUT
0.8
–1
⎛
⎝
⎜
⎞
⎠
⎟
R1= 875.5k; use 887k
Figure 8 shows the complete circuit along with its ef-
fi ciency curve.
–
+
RUN
I
TH
V
FB
GND
PLL LPF
SYNC/MODE
SW
LTC1877
C
C2
C
C1
R
C
C
OUT
C
IN
1877 F07
L1
R2
V
IN
BOLD LINES INDICATE
HIGH CURRENT PATHS
1
2
3
4
8
7
6
5
OPTIONAL
R1
+
–
+
V
OUT
V
IN
+
V
OUT
2.5V
220pF
20pF
10μF**
CER
15μH*
887k
47μF***
412k
LTC1877
RUN
I
TH
V
FB
GND
8
7
1
2
6
5
3
4
PLL LPF
SYNC/MODE
V
IN
SW
V
IN
2.7V TO 4.2V
1877 F08a
+
*SUMIDA CD54-150
**TAIYO YUDEN CERAMIC LMK325BJ106MN
***SANYO POSCAP 6TPA47M
OUTPUT CURRENT (mA)
0.1
EFFICIENCY (%)
95
90
85
80
75
70
65
60
55
50
1.0 10
1877 F08b
100 1000
V
OUT
= 2.5V
L = 15μH
V
IN
= 3.6V
V
IN
= 4.2V
V
IN
= 3.0V
Figure 7. LTC1877 Layout Diagram
Figure 8. Single Lithium-Ion to 2.5V/0.3A Regulator from Design Example
APPLICATIONS INFORMATION