Datasheet

LTC3407-3
12
34073fb
APPLICATIONS INFORMATION
with a load current of 800mA and an ambient temperature
of 70°C. From the Typical Performance Characteristics
graph of Switch Resistance, the R
DS(ON)
resistance of
the main switch is 0.425Ω. Therefore, power dissipated
by each channel is:
P
D
= I
OUT
2
• R
DS(ON)
= 272mW
The DFN package junction-to-ambient thermal resistance,
θ
JA
, is 40°C/W. Therefore, the junction temperature of
the regulator operating in a 70°C ambient temperature is
approximately:
T
J
= 2 • 0.272 • 40 + 70 = 91.8°C
which is below the absolute maximum junction tempera-
ture of 125°C.
Design Example
As a design example, consider using the LTC3407-3 in
an portable application with a Li-Ion battery. The battery
provides a V
IN
= 2.8V to 4.2V. The load requires a maximum
of 800mA in active mode and 2mA in standby mode. The
output voltage is V
OUT
= 1.8V. Since the load still needs
power in standby, Burst Mode operation is selected for
good low load effi ciency.
First, calculate the inductor value for about 30% ripple
current at maximum V
IN
:
L 
1.8V
2.25MHz • 300m
A
•1–
1.8V
4.2V






=1.5μH
Choosing a vendor’s closest inductor value of 2.2μH,
results in a maximum ripple current of:
I
L
=
1.8V
2.25MHz • 2.2μ
H
•1
1.8V
4.2V






= 208mA
For cost reasons, a ceramic capacitor will be used. C
OUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
C
OUT
≈ 2.5
800mA
2.25MHz •(5% • 1.8V)
= 9.9μF
A good standard value is 10μF. Since the output impedance
of a Li-Ion battery is very low, C
IN
is typically 10μF.
The POR pin is a common drain output and requires a pull-
up resistor. A 100k resistor is used for adequate speed.
Figure 1 shows the complete schematic for this design
example.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3407-3. These items are also illustrated graphically
in the layout diagram of Figure 3. Check the following in
your layout:
1. Does the capacitor C
IN
connect to the power V
IN
(Pin
3) and GND (exposed pad) as close as possible? This
capacitor provides the AC current to the internal power
MOSFETs and their drivers.
Figure 3. LTC3407-3 Layout Diagram (See Board Layout Checklist)
RUN2 V
IN
V
IN
V
OUT2
V
OUT1
RUN1
POR
SW1
V
FB1
GND
V
FB2
SW2
MODE/SYNC
LTC3407-3
C
IN
L1
L2
C
OUT2
C
OUT1
34073 F03
BOLD LINES INDICATE
HIGH CURRENT PATH