Datasheet

LT8641
13
Rev B
For more information www.analog.com
APPLICATIONS INFORMATION
Low EMI PCB Layout
The LT8641 is specifically designed to minimize EMI emis-
sions and also to maximize efficiency when switching at
high frequencies. For optimal performance the LT8641
requires the use of multiple V
IN
bypass capacitors.
Tw o small 1µF capacitors should be placed as close as
possible to the LT8641: One capacitor should be tied to
V
IN1
/GND1; a second capacitor should be tied to V
IN2
/
GND2. A third capacitor with a larger value, 2.2µF or
higher, should be placed near V
IN1
or V
IN2
.
See Figure 1 for a recommended PCB layout.
For more detail and PCB design files refer to the Demo
Board guide for the LT8641.
Note that large, switched currents flow in the LT8641
V
IN1
, V
IN2
, GND1, and GND2 pins and the input capacitors
(C
IN1
, C
IN2
). The loops formed by the input capacitors
should be as small as possible by placing the capacitors
adjacent to the V
IN1/2
and GND1/2 pins. Capacitors with
small case size such as 0603 are optimal due to lowest
parasitic inductance.
The input capacitors, along with the inductor and output
capacitors, should be placed on the same side of the
circuit board, and their connections should be made on
that layer. Place a local, unbroken ground plane under the
application circuit on the layer closest to the surface layer.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and RT nodes small so that the ground
traces will shield them from the SW and BOOST nodes.
The exposed pad on the bottom of the package should be
soldered to SW to reduce thermal resistance to ambient. To
keep thermal resistance low, extend the ground plane from
GND1 and GND2 as much as possible, and add thermal
vias to additional ground planes within the circuit board
and on the bottom side.
Figure 1. Recommended PCB Layout for the LT8641
V
V
V
V
V
1
6
11
16
GROUND PLANE
ON LAYER 2
20
R1
R2
C
VCC
C
BST
C
IN1
C
IN2
C
IN3
C
OUT
L
R
T
C
SS
C1 R
PG
17
7 10
22
21
GROUND VIA V
IN
VIA V
OUT
VIA OTHER SIGNAL VIAS
8641 F01
V
V
OUT
V
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