Datasheet

LTM8025
14
8025fc
For more information www.linear.com/LTM8025
BIAS
AUX
V
OUT
V
IN
GND GND
8025 F03
GND
THERMAL VIAS TO GND
R
T
R
ADJ
PGOOD
C
IN
C
OUT
SYNC
SHDN
applicaTions inForMaTion
Figure 2. The Input Diode Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output. It Also Protects
the Circuit from a Reversed Input. The LTM8025 Runs Only When
the Input is Present.
1. Place the R
ADJ
and R
T
resistors as close as possible to
their respective pins.
2. Place the C
IN
capacitor as close as possible to the V
IN
and GND connection of the LTM8025.
3. Place the C
OUT
capacitor as close as possible to the
V
OUT
and GND connection of the LTM8025.
4. Place the C
IN
and C
OUT
capacitors such that their
ground current flow directly adjacent or underneath
the LTM8025.
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 LTM8025.
6. For good heatsinking, use vias to connect the GND cop
-
per area to the board’s internal ground planes. Liberally
distribute these GND vias to provide both a good ground
connection and thermal path to the internal planes of the
printed circuit board. Pay attention to the location and
density of the thermal vias in Figure 3. The LTM8025
can benefit from the heat-sinking afforded by vias that
connect to internal GND planes at these locations, due to
their proximity to internal power handling components.
The optimum number of thermal vias depends upon
the printed circuit board design. For example, a board
might use very small via holes. It should employ more
thermal vias than a board that uses larger holes.
Figure 3. Layout Showing Suggested External Components, GND
Plane and Thermal Vias.
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8025. However, these capacitors
can cause problems if the LTM8025 is plugged into a live
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 underdamped tank circuit, and the volt
-
age at the V
IN
pin of the LTM8025 can ring to more than
twice the nominal input voltage, possibly exceeding the
LTM8025’s rating and damaging the part. If the input
supply is poorly controlled or the user will be plugging
the LTM8025 into an energized supply, the input network
should be designed to prevent this overshoot. This can be
accomplished by installing a small resistor in series to V
IN
,
but the most popular method of controlling input voltage
overshoot is to add an electrolytic bulk capacitor to the
V
IN
net. This capacitor’s relatively high equivalent series
resistance damps the circuit and eliminates the voltage
overshoot. The extra capacitor improves low frequency
ripple filtering and can slightly improve the efficiency of
the circuit, though it is likely to be the largest component
in the circuit.
V
IN
RUN/SS
RT ADJ
V
OUT
GND
8025 F02
LTM8025
V
IN
V
OUT
AUX
BIAS
SYNC