Datasheet
LTC3544B
13
3544bfb
A second, more severe transient is caused by switching
in loads with large (>1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
OUT
, causing a rapid drop in V
OUT
. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the
load rise time is limited to approximately (25 • C
LOAD
).
Thus, a 10μF capacitor charging to 3.3V would require a
250μs rise time, limiting the charging current to about
130mA.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3544B. These items are also illustrated graphically in
Figures 3 and 4. Check the following in your layout:
1.
The
power traces, consisting of the PGND trace, the
GNDA trace, the SW traces, the PV
IN
trace and the V
CC
trace should be kept short, direct and wide.
applicaTions inForMaTion
2. Does each of the V
FBx
pins connect directly to the
respective feedback resistors? The resistive dividers
must be connected between the (+) plate of the cor-
responding output filter capacitor (e.g. C13) and GNDA.
If the circuit being powered is at such a distance from
the part where voltage drops along circuit traces are
large, consider a Kelvin connection from the powered
circuit back to the resistive dividers.
3.
Keep C8 and C9 as close to the part as possible.
4.
Keep the switching nodes (SWx) away from the sensi
-
tive V
FBx
nodes.
5. Keep the ground connected plates of the input and
output capacitors as close as possible.
6. Care
should be taken to provide enough space between
unshielded inductors in order to minimize any trans-
former coupling.
C6R5
R6
L2
C4
V
OUT3
C12R8
R11
L3
C10
V
OUT2
C3
C1
R2
V
OUT4
3544B F03
R3
SW200B
SW200B
SW300
SW300
PGND PV
IN
SW1000
RUN100
GNDA
C15R15
R16
RUN100
V
FB100
V
FB100
RUN200ARUN200A
SW200A
SW200A
V
FB200B
V
FB200A
V
FB200B
V
FB300
V
FB300
RUN300 RUN300
RUN200B RUN200B
V
FB200A
V
CC
GNDA
PGND
V
CC
2.25V TO 5.5V
PV
IN
2.25V TO 5.5V
C8
L4
L1
LTC3544B
C9
C13
V
OUT1
Figure 3. LTC3544B Layout Diagram