LTC3411 Linear Technology, LTC3411 Datasheet - Page 15

• Manufacturer Part Number: LTC3411
• Description: 1.25A/ 4MHz/ Synchronous Step-Down DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
current in these resistors should be kept small. Choosing
2 A with the 0.8V feedback voltage makes R1~400k. A
close standard 1% resistor is 412k and R2 is then 887k.
The compensation should be optimized for these compo-
nents by examining the load step response but a good
place to start for the LTC3411 is with a 13k and 1000pF
filter. The output capacitor may need to be increased
depending on the actual undershoot during a load step.
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
LTC3411. These items are also illustrated graphically in
the layout diagram of Figure 6. Check the following in your
layout:
1. Does the capacitor C
and power GND (Pin 5) as close as possible? This
U
IN
connect to the power V
U
R2
Figure 6. LTC3411 Layout Diagram (See Board Layout Checklist)
C4
PGOOD
V
R1
IN
W
BOLD LINES INDICATE HIGH CURRENT PATHS
R5
R3
C3
PV
SV
PGOOD
V
I
TH
FB
U
IN
IN
IN
(Pin 6)
LTC3411
C
IN
SYNC/MODE
SHDN/R
PGND
SGND
SW
capacitor 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
near SGND (Pin 3). The feedback signal V
routed away from noisy components and traces, such as
the SW line (Pin 4), and its trace should be minimized.
4. Keep sensitive components away from the SW pin. The
input capacitor C
C
routed away from the SW trace and the inductor L1.
5. A ground plane is preferred, but if not available, keep the
signal and power grounds segregated with small signal
components returning to the SGND pin at one point which
is then connected to the PGND pin.
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 one of the input supplies: PV
SGND.
T
OUT
ITH
PS
and all the resistors R1, R2, R
returns current to PGND and the (–) plate of C
R
V
BM
T
L1
IN
OUT
and L1 closely connected? The (–) plate of
IN
, the compensation capacitor C
C
OUT
V
OUT
3411 F06
OUT
and a ground line terminated
T
, and R
IN
LTC3411
, PGND, SV
FB
C
should be
should be
sn3411 3411fs
15
C
IN
IN
and
.
or