LTC3543_15 Datasheet, PDF (15/20 Pages) Linear Technology – 600mA Synchronous Step Down Buck Regulator with PLL, Soft-Start and Spread Spectrum

LTC3543
APPLICATIONS INFORMATION
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V_OUT immediately shifts by an amount
equal to (ΔI_LOAD • ESR), where ESR is the effective series
resistance of C_OUT. ΔI_LOAD also begins to charge or dis-
charge C_OUT, which generates a feedback error signal.
The regulator loop then acts to return V_OUT to its steady
state value. During this recovery time, V_OUT can be moni-
tored for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of the switching control
loop theory, see Application Note 76.
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 paral-
lel 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 follow-
ing checklist should be used to ensure proper opera-
tion of the LTC3543. These items are also illustrated
graphically in Figures 5 and 6. Check the following in
your layout:
1. The power traces, consisting of the GND trace, the SW
trace, and the V_IN trace should be kept short, direct
and wide.
2. Does the V_FB pin connect directly to the feedback volt-
age reference? Ensure that there is no load current
running from the feedback reference voltage and the
V_FB pin.
3. Does the (+) plate of C_IN connect to V_IN as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the switching node, SW, away from the sensitive
V_FB node.
5. Keep the (–) plates of C_IN and C_OUT as close as pos-
sible.
V_OUT
C_OUT
+–
GND
C_IN
L
SW
V_IN
V_IN
CAP
RUN
+
– ^C1
MODE
V_FB
–
R1 R2
C_FWD
+
3543 F05
Figure 5. LTC3543 Layout Diagram
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