LTC3407EMSE-2 Linear Technology, LTC3407EMSE-2 Datasheet - Page 12

• Manufacturer Part Number: LTC3407EMSE-2
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,TSSOP,10PIN,PLASTIC
• Manufacturer: Linear Technology

LTC3407-3
APPLICATIO S I FOR ATIO
with a load current of 800mA and an ambient temperature
of 70°C. From the Typical Performance Characteristics
graph of Switch Resistance, the R
main switch is 0.425Ω. Therefore, power dissipated by
each channel is:
The DFN package junction-to-ambient thermal resistance,
θ
regulator operating in a 70°C ambient temperature is
approximately:
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
mum of 800mA in active mode and 2mA in standby mode.
The output voltage is V
needs power in standby, Burst Mode operation is selected
for good low load efficiency.
First, calculate the inductor value for about 30% ripple
current at maximum V
12
JA
P
T
L
, is 40°C/W. Therefore, the junction temperature of the
J
D
=
= 2 • 0.272 • 40 + 70 = 91.8°C
= I
2 25
.
2
• R
MHz
IN
DS(ON)
1 8
.
= 2.8V to 4.2V. The load requires a maxi-
•
V
300
U
= 272mW
mA
IN
OUT
:
•
U
⎛
⎝ ⎜
1
= 1.8V. Since the load still
Figure 3. LTC3407-3 Layout Diagram (See Board Layout Checklist)
–
V
C
1 8
4 2
OUT2
OUT2
.
.
DS(ON)
V
V
W
V
IN
⎞
⎠ ⎟
=
resistance of the
1 5
.
BOLD LINES INDICATE
HIGH CURRENT PATH
µ
C
H
IN
U
L2
MODE/SYNC
SW2
V
RUN2
FB2
LTC3407-3
GND
V
Choosing a vendor’s closest inductor value of 2.2µH,
results in a maximum ripple current of:
For cost reasons, a ceramic capacitor will be used. C
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
A good standard value is 10µF. Since the output imped-
ance of a Li-Ion battery is very low, C
The PGOOD 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
and GND (exposed pad) as close as possible? This capaci-
tor provides the AC current to the internal power MOSFETs
and their drivers.
IN
C
∆ =
RUN1
OUT
I
SW1
POR
V
L
FB1
≈
2 25
.
1 8
.
L1
MHz
2 25
34073 F03
1 8
.
.
V
• .
MHz
2 2
C
V
OUT1
800
OUT1
IN
µ
•( % • .
H
connect to the power V
mA
5
•
⎛
⎝ ⎜
1
−
1 8
1 8
4 2
.
.
V
)
V
V
IN
=
⎞
⎠ ⎟
7 1
is typically 10µF.
=
.
208
µ
F
m
IN
A A
(Pin 3)
34073fa
OUT