LTC3544B_15 Datasheet, PDF (13/18 Pages) Linear Technology – Quad Synchronous Step-Down Regulator

LTC3544B
Applications information
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.
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.
V_OUT1
C13
V_OUT3
R5
C4
R6
V_OUT2
R8
C10
R11
V_CC
L4 ^{2.25V TO 5.5V}
C8
GNDA
R15
C15
R16 _RUN100
SW1000 V_CC GNDA V_FB200A
RUN100
V_FB200B
V_{FB100 VFB100}
RUN200A
RUN200A
LTC3544B
V_FB300 ^VFB300
RUN300
RUN300
L2 _SW200A
SW200A
RUN200B
RUN200B
C6
SW200B PGND PV_IN SW300
SW200B
C9
SW300 ^L1
V_FB200A
L3
C12
V_FB200B
PGND
PV_IN
2.25V TO 5.5V
R2
C1
R3
3544B F03
V_OUT4
C3
Figure 3. LTC3544B Layout Diagram
3544bfb
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