Datasheet

LTM8031
13
8031fb
applicaTions inForMaTion
4. Place the C
IN
and C
OUT
capacitors such that their
ground currents flow directly adjacent or underneath
the LTM8031.
5. Connect all of the GND connections to as large a copper
pour or plane area as possible on the top layer. Avoid
breaking the ground connection between the external
components and the LTM8031.
6. Use vias to connect the GND copper area to the board’s
internal ground plane. Liberally distribute these GND vias
to provide both a good ground connection and thermal
path to the internal planes of the printed circuit board.
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8031. However, these capacitors
can cause problems if the LTM8031 is plugged into a live
or fast rising or falling supply (see Linear Technology
Application Note 88 for a complete discussion). The low
loss ceramic capacitor combined with stray inductance in
series with the power source forms an under-damped tank
circuit, and the voltage at the V
IN
pin of the LTM8031 can
ring to twice the nominal input voltage, possibly exceed-
ing the LTM8031’s rating and damaging the part. A similar
phenomenon can occur inside the LTM8031 module, at the
output of the integrated EMI filter, with the same potential
of damaging the part.
If the input supply is poorly controlled or the user will be
plugging the LTM8031 into an energized supply, the input
network should be designed to prevent this overshoot. Fig-
ure 4 shows the waveforms that result when an LTM8031
circuit is connected to a 24V supply through six feet of 24-
gauge twisted pair. The first plot (4a) is the response with
a 2.2µF ceramic capacitor at the input. The input voltage
Figure 3. Layout Showing Suggested External Components,
GND Plane and Thermal Vias
GND
C
OUT
C
IN
8031 F03
V
IN
FIN
RUN/SS
SYNC
PGOOD
R
ADJ
R
T
AUX
BIAS
V
OUT
GND
OPTIONAL
FIN
CAPACITOR