Datasheet
LTC3544
14
3544fa
APPLICATIONS INFORMATION
Beginning with this channel, fi rst calculate the inductor
value for about 35% ripple current (100mA in this example)
at maximum V
IN
. Using a form of Equation 1:
L
V
MHz mA
V
V
µH4
25
2 25 100
1
25
42
45=
⎛
⎝
⎜
⎞
⎠
⎟
=
.
.•
–
.
.
.
For the inductor, use the closest standard value of 4.7µH.
A 4.7µF capacitor should be suffi cient for the output ca-
pacitor. A larger output capacitor will attenuate the load
transient response, but increase the settling time. A value
for C
IN
= 4.7µF should suffi ce as the source impedance of
a Li-Ion battery is very low.
The feedback resistors program the output voltage.
Minimizing the current in these resistors will maximize
effi ciency at very light loads, but totals on the order of
200k are a good compromise between effi ciency and im-
munity to any adverse effects of PCB parasitic capacitance
on the feedback pins. Choosing 10µA with 0.8V feedback
voltage makes R7 = 80k. A close standard 1% resistor is
76.8k. Using:
R
V
Rk
OUT
8
08
1 7 163 2=
⎛
⎝
⎜
⎞
⎠
⎟
=
.
–• .
The closest standard 1% resistor is 162k. An optional
20pF feedback capacitor may be used to improve transient
response. The component values for the other channels
are chosen in a similar fashion.
Figure 5 shows the complete schematic for this example,
along with the effi ciency curve and transient response for
the 300mA channel.
16
4
1
L2
4.7μH
L1
10μH
L4
4.7μH
C6
20pF
C9
4.7μF
C10
4.7μF
C5
20pF
C8
20pF
C2
4.7μF
C1
4.7μF
V
OUT1
1.2V
C4
10μF
3544 F05a
V
OUT2
2.5V
R3
93.1k
V
OUT2
1.5V
V
SUPPLY
3.6V
V
OUT3
0.8V
R4
107k
R1
59k
R2
118k
R7
162k
R8
76.8k
3
5
2
12
15 7
13
11
9
8
10
14 6
L3
4.7μH
C3
4.7μF
R6
100k
RUN200B
V
CC
LTC3544
PV
IN
GNDA PGND
SW200B
SW200A
V
FB200B
V
FB200A
RUN200A
RUN100
SW100
SW300
V
FB100
V
FB300
RUN300
Figure 5. Design Example
LOAD CURRENT (A)
30
EFFICIENCY (%)
90
100
20
10
80
50
70
60
40
0.0001 0.01 0.1 1
3544B F05b
0
0.001
V
IN
= 2.7V
V
IN
= 3.6V
V
IN
= 4.2V
V
OUT
= 2.5V
T
A
= 25°C
Effi ciency vs Output Current—300mA Channel,
All Other Channels Off
Transient Response
V
OUT300
50mV/DIV
AC COUPLED
I
L
250mA/DIV
I
LOAD
250mA/DIV
20μs/DIVV
IN
= 3.6V
V
OUT
= 2.5V
T
A
= 25°C
LOAD STEP = 300μA TO 300mA
3544B F05c