20
LTC1775
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1775. These items are also illustrated graphically in
the layout diagram of Figure 12. Check the following in
your layout:
1) Connect the TK lead directly to the drain of the topside
MOSFET. Then connect the drain to the (+) plate of C
IN
.
This capacitor provides the AC current to the top
MOSFET.
2) The power ground pin connects directly to the source of
the bottom N-channel MOSFET. Then connect the source
to the anode of the Schottky diode (if used) and (–) plate
of C
IN
, which should have as short lead lengths as
possible.
3) The LTC1775 signal ground pin should connect to the
(–) plate of C
OUT
. Connect the (–) plate of C
OUT
to power
ground at the source of the bottom MOSFET. All small-
signal components (R2, C
SS
, C
C
, etc.) should return
directly to SGND.
Figure 12. LTC1775 Layout Diagram
+
+
C
SS
OPTIONAL 5V EXTV
CC
CONNECTION
M1
M2
D1
C
VCC
BOLD LINES INDICATE HIGH CURRENT PATHS
1775 F12
C
IN
C
OUT
V
IN
V
OUT
INTV
CC
OPEN
EXT
CLK
R1
OUTPUT DIVIDER
REQUIRED
WITH V
PROG
OPEN
R2
C
C1
R
C
C
B
D
B
+
L1
L
TRACE
+
+
–
–
V
IN
TK
EXTV
CC
LTC1775
SYNC
V
PROG
SW
TG
BOOST
INTV
CC
BG
RUN/SS
FCB
I
TH
SGND
V
OSENSE
PGND
2
1
3
4
5
6
7
8
16
15
14
13
12
11
10
9
4) Keep the switch node SW away from sensitive small-
signal nodes. Ideally the switch node should be placed
on the opposite side of the power MOSFETs from the
LTC1775.
5) Connect the INTV
CC
decoupling capacitor C
VCC
closely
to the INTV
CC
pin and the power ground pin. This
capacitor carries the MOSFET gate drive current.
6) Does the V
OSENSE
pin connect as close as possible to
the load? In adjustable applications, the resistive di-
vider (R1, R2) must be connected between the load and
signal ground. Place the divider near the LTC1775 in
order to keep the high impedance V
OSENSE
node short.
7) For applications with multiple switching power con-
verters connected to the same V
IN
, ensure that the input
filter capacitance for the LTC1775 is not shared with the
other converters. AC input current from another con-
verter will cause substantial input voltage ripple that
may interfere with proper operation of the LTC1775. A
few inches of PC trace or wire (L
TRACE
≈ 100nH)
between C
IN
and the V
IN
supply is sufficient to prevent
sharing.
APPLICATIO S I FOR ATIO
WUUU
