19
LTC1553
APPLICATIONS INFORMATION
WUU
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disturbances in the LTC1553 and prevents differences
in ground potential from disrupting internal circuit
operation. This connection should then tie into the
ground plane at a single point, preferably at a fairly quiet
point in the circuit such as close to the output capaci-
tors. This is not always practical, however, due to
physical constraints. Another reasonably good point to
make this connection is between the output capacitors
and the source connection of the low side FET Q2. Do
not tie this single point ground in the trace run between
the low side FET source and the input capacitor ground,
as this area of the ground plane will be very noisy.
3. The small signal resistors and capacitors for frequency
compensation and soft start should be located very
close to their respective pins and the ground ends
connected to the signal ground pin through a separate
trace. Do not connect these parts to the ground plane!
4. The V
CC
and PV
CC
decoupling capacitors should be as
close to the LTC1553 as possible. The 10µF bypass
capacitors shown at V
CC
and PV
CC
will help provide
optimum regulation performance.
5. The (+) plate of C
IN
should be connected as close as
possible to the drain of the upper MOSFET. An addi-
tional 1µF ceramic capacitor between V
IN
and power
ground is recommended.
6. The SENSE pin is very sensitive to pickup from the
switching node. Care should be taken to isolate SENSE
from possible capacitive coupling to the inductor switch-
ing signal. A 0.1µF is required between the SENSE pin
and the SGND pin next to the LTC1553.
7. OUTEN is a high impedance input and should be
externally pulled up to a logic HIGH for normal
operation.
8. Kelvin sense I
MAX
and I
FB
at Q1 drain and source pins.
RATED V
OUT
V
OUT
15%
5%
–5%
t
PWRBAD
t
PWRGD
t
FAULT
FAULT
PWRGD
1553 F11
Figure 11. PWRGD and FAULT
LAYOUT CONSIDERATIONS
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1553. These items are also illustrated graphically in
the layout diagram of Figure 12. The thicker lines show the
high current paths. Note that at 10A current levels or
above, current density in the PC board itself is a serious
concern. Traces carrying high current should be as wide
as possible. For example, a PCB fabricated with 2oz copper
requires a minimum trace width of 0.15" to
carry 10A.
1. In general, layout should begin with the location of the
power devices. Be sure to orient the power circuitry so
that a clean power flow path is achieved. Conductor
widths should be maximized and lengths minimized.
After you are satisfied with the power path, the control
circuitry should be laid out. It is much easier to find
routes for the relatively small traces in the control
circuits than it is to find circuitous routes for high
current paths.
2. The GND and SGND pins should be shorted right at the
LTC1553. This helps to minimize internal ground
