ltc3851-1 Linear Technology Corporation, ltc3851-1 Datasheet - Page 22

• Manufacturer Part Number: ltc3851-1
• Description: Synchronous Step-down Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

LTC3851-1
APPLICATIONS INFORMATION
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If the ratio of
C
should be controlled so that the load rise time is limited
to approximately 25 • C
require a 250μs rise time, limiting the charging current
to about 200mA.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3851-1. These items are also illustrated graphically
in the layout diagram of Figure 9. Check the following in
your layout:
1. Are the board signal and power grounds segregated?
22
LOAD
The LTC3851-1 GND pin should tie to the ground plane
close to the input capacitor(s). The low current or signal
ground lines should make a single point tie directly to
to C
OUT
is greater than 1:50, the switch rise time
47pF
R
C
LOAD
0.1mF
R
C
FREQ
SS
1000pF
C
C
. Thus a 10μF capacitor would
C
C2
1
2
3
4
5
6
7
8
MODE/PLLIN
FREQ/PLLFLTR
RUN
TK/SS
I
V
SENSE
SENSE
TH
FB
LTC3851-1
–
+
Figure 9. LTC3851-1 Layout Diagram
PGOOD
BOOST
INTV
GND
SW
V
BG
TG
CC
IN
16
15
14
13
12
11
10
9
R1
R2
+
R
+
4.7μF
PGOOD
2. Does the V
3. Are the SENSE
D
C
OUT
B
the GND pin. The synchronous MOSFET source pins
should connect to the input capacitor(s) ground.
tors? The resistive divider R1, R2 must be connected
between the (+) plate of C
47pF to 100pF capacitor should be as close as possible
to the LTC3851-1. Be careful locating the feedback
resistors too far away from the LTC3851-1. The V
line should not be routed close to any other nodes with
high slew rates.
with minimum PC trace spacing? The fi lter capacitor
between SENSE
possible to the LTC3851-1. Ensure accurate current
sensing with Kelvin connections as shown in Figure 10.
Series resistance can be added to the SENSE lines to
increase noise rejection and to compensate for the ESL
of R
V
PULL-UP
C
SENSE
B
M1
R
L1
.
SENSE
FB
pin connect directly to the feedback resis-
–
M2
D1
+
and SENSE
and SENSE
C
IN
OUT
38511 F09
+
–
and signal ground. The
V
should be as close as
leads routed together
V
OUT
+
–
–
+
IN
38511f
FB