Datasheet

LTC3544B
14
3544bfb
applicaTions inForMaTion
Design Example
As a design example, consider using the LTC3544B as
a portable application with a Li-Ion battery. The battery
provides V
IN
ranging from 2.8V to 4.2V. The demand at
2.5V is 250mA necessitating the use of the 300mA output
for this requirement.
Beginning with this channel, first calculate the inductor
value for about 35% ripple current (100mA in this example)
at maximum V
IN
. Using a form of equation:
L
V
MHz mA
V
V
µH4
2 5
2 25 100
1
2 5
4 2
4 5=






=
.
. •
–
.
.
.
For the inductor, use the closest standard value of 4.7µH.
A 4.7µF capacitor should be sufficient 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 suffice 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
efficiency at very light loads, but totals on the order of
200k are a good compromise between efficiency 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
R k
OUT
8
0 8
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 efficiency curve and transient response for
the 300mA channel.
Figure 4
C1
C9
C10
GND
L1 L4
L2 L3
PGND
3544B F04
V
CC
C4
C3
C2