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LTC3544B

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 VIN 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

valueforabout35%ripplecurrent(100mAinthisexample)

at maximum VIN. Using a form of equation:

MHz

µH

2 5

2 25

100

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 CIN = 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:

OUT

0 8

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

C10

GND

PGND

3544B F04

VCC

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