LTC1707 Datasheet, PDF (11/16 Pages) Linear Technology – High Efficiency Monolithic Synchronous Step-Down Switching Regulator

LTC1707
APPLICATIO S I FOR ATIO
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
LTC1707. These items are also illustrated graphically in
the layout diagram of Figure 7. Check the following in your
layout:
1. Are the signal and power grounds segregated? The
LTC1707 signal ground consists of the resistive
divider, the optional compensation network (R_C and
C_C1), C_SS, C_REF and C_C2. The power ground consists of
the (–) plate of C_IN, the (–) plate of C_OUT and Pin 4 of the
LTC1707. The power ground traces should be kept
short, direct and wide. The signal ground and power
ground should converge to a common node in a star-
ground configuration.
2. Does the V_FB pin connect directly to the feedback
resistors? The resistive divider R1/R2 must be con-
nected between the (+) plate of C_OUT and signal ground.
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 sensitive small-
signal nodes.
C_REF
C_C2
OPTIONAL
R_C
C_C1
C_SS
1
I_TH
8
V_REF
2 _{RUN/SS SYNC/MODE} 7
LTC1707
3
V_FB
6
V_IN
4
GND
5
SW
BOLD LINES INDICATE HIGH CURRENT PATHS
+
L1
+
+
C_IN
R2
+
C_OUT
V_IN
R1
V_OUT
–
–
1707 F07
Figure 7. LTC1707 Layout Diagram
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