LTC3407-4 Linear Technology, LTC3407-4 Datasheet - Page 12

• Manufacturer Part Number: LTC3407-4
• Description: Dual Synchronous 800mA 2.25MHz Step-Down DC/DC Regulator
• Manufacturer: Linear Technology

LTC3407-4
APPLICATIO S I FOR ATIO
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 LTC3407-4 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
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 output voltage can now be programmed by choosing
the values of R1 and R2. To maintain high efficiency, the
current in these resistors should be kept small. Choosing
2µA with the 0.6V feedback voltage makes R1~300k. A
close standard 1% resistor is 280k, and R2 is then 887k.
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.
12
T
L
C
∆ =
J
OUT
I
=
L
= 2 • 0.272 • 45 + 70 = 94.5°C
2 25
.
2 25
≈
.
2 5
MHz
IN
.
MHz
2 5
= 2.8V to 4.2V. The load requires a maxi-
2 25
2 5
.
.
.
•
V
300
V
• .
U
MHz
2 2
mA
IN
800
µ
OUT
:
H
• ( %• . )
U
mA
•
5
•
⎛
⎜
⎝
⎛
⎝ ⎜
= 2.5V. Since the load still
1
1
–
−
2 5
2 5
4 2
2 5
4 2
.
.
.
.
V
W
V
V
V
V
IN
⎞
⎟ =
⎠
=
⎞
⎠ ⎟
is typically 10µF.
=
7 1
.
1 5
204
.
µ
F
µ
m
H
U
A A
OUT
Figure 3. LTC3407-4 Layout Diagram (See Board Layout Checklist)
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3407-4. 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.
2. Are the C
C
3. The resistor divider, R1 and R2, must be connected
between the (+) plate of C
terminated near GND (exposed pad). The feedback signals
V
traces, such as the SW line (Pins 4 and 7), and its trace
should be minimized.
4. Keep sensitive components away from the SW pins. The
input capacitor C
routed away from the SW traces and the inductors.
5. A ground plane is preferred, but if not available, keep the
signal and power grounds segregated with small signal
components returning to the GND pin at one point and
should not share the high current path of C
6. Flood all unused areas on all layers with copper.
Flooding with copper will reduce the temperature rise of
power components. These copper areas should be con-
nected to V
V
BOLD LINES INDICATE HIGH CURRENT PATHS
OUT2
OUT
FB
should be routed away from noisy components and
returns current to GND and the (–) plate of C
C
OUT2
V
OUT
IN
C5
R4
IN
or GND.
and L1 closely connected? The (–) plate of
IN
C
IN
R3
and the resistors R1 to R4 should be
L2
IN
MODE/SYNC
SW2
V
connect to the power V
RUN2
FB2
OUT
LTC3407-4
GND
V
IN
and a ground sense line
www.DataSheet4U.com
RUN1
SW1
POR
V
FB1
R1
L1
IN
C4
R2
or C
IN
3407 F03
(Pin 3)
OUT
IN
.
34074f
C
V
OUT1
.
OUT1