LTC3419
13
3419fa
APPLICATIONS INFORMATION
Figure 2. LTC3419 Layout Diagram (See Board Layout Checklist)
Figure 3. LTC3419 Suggested Layout
V
IN
RUN2 RUN1
LTC3419
V
FB2
SW2
SW1
V
FB1
C
F2
C
F1
GND
V
IN
2.5V TO 5.5V
V
OUT2
V
OUT1
3419 F02
R3 R1
R4
L2 L1
R2
C
OUT2
C1
C
OUT1
BOLD LINES INDICATE HIGH CURRENT PATHS
MODE
L1
3419 F03
L2
C
IN
C
OUT1
C
OUT2
V
OUT2
V
OUT1
GND
R2 R1 R3 R4
C
F1
C
F2
V
FB1
RUN1
MODE
SW1
V
FB2
RUN2
SW2
V
IN
VIA TO GND
VIA TO V
IN
Design Example
As a design example, consider using the LTC3419 in a
portable application with a Li-Ion battery. The battery
provides a V
IN
ranging from 2.8V to 4.2V. The load on
each channel requires a maximum of 600mA in active
mode and 2mA in standby mode. The output voltages are
V
OUT1
= 2.5V and V
OUT2
= 1.8V.
Start with channel 1. First, calculate the inductor value
for about 40% ripple current (240mA in this example) at
maximum V
IN
. Using a derivation of Equation 1:
L
V
MHz mA
V
V
1
25
2 25 240
1
25
42
18=−
⎛
⎝
⎜
⎞
⎠
⎟
=
.
.•()
•
.
.
.77μH
For the inductor, use the closest standard value of
2.2μH.
A 10μF ceramic capacitor should be more than suffi cient
for this output capacitor. As for the input capacitor, a
typical value of C
IN
= 10μF should suffi ce, as the source
impedance of a Li-Ion battery is very low.
The feedback resistors program the output voltage. To
maintain high effi ciency at light loads, the current in these
resistors should be kept small. Choosing 10μA with the
0.6V feedback voltage makes R1~60k. A close standard
1% resistor is 59k. Using Equation 2.
R
V
Rk
OUT
2
06
1 1 187=−
⎛
⎝
⎜
⎞
⎠
⎟
=
.
•
An optional 22pF feedback capacitor (C
F1
) may be used
to improve transient response.
