LTM8025

14

8025f

BIAS

AUX

GND

GND

8025 F03

GND

THERMAL VIAS TO GND

PGOOD

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.

high level of integration, you may fail to achieve specified

operation with a haphazard or poor layout. See Figure 3

for a suggested layout. Ensure that the grounding and

heatsinking are acceptable.

their respective pins.

and GND connection of the LTM8025.

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

Figure 3. Layout Showing Suggested External Components, GND

Plane and Thermal Vias.

might use very small via holes. It should employ more

thermal vias than a board that uses larger holes.

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-

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

but the most popular method of controlling input voltage

overshoot is to add an electrolytic bulk capacitor to the

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.

RUN/SS

RT

ADJ

GND

8025 F02

LTM8025

AUX

BIAS

SYNC