LTM8031

12

8031fa

Shorted Input Protection

Care needs to be taken in systems where the output will

be held high when the input to the LTM8031 is absent.

This may occur in battery charging applications or in

battery back-up systems where a battery or some other

supply is diode ORed with the LTM8031’s output. If the

the LTM8031’s internal circuitry will pull its quiescent

current through its internal power switch. This is fine if

your system can tolerate a few milliamps in this state. If

you ground the RUN/SS pin, the internal switch current

grounded while the output is held high, then parasitic

diodes inside the LTM8031 can pull large currents from

device. Figure 2 shows a circuit that will run only when

the input voltage is present and that protects against a

shorted or reversed input.

PCB Layout

Most of the headaches associated with PCB layout have

been alleviated or even eliminated by the high level of

integration of the LTM8031. The LTM8031 is neverthe-

less a switching power supply and care must be taken to

minimize EMI and ensure proper operation. Even with the

high level of integration, you may fail to achieve specified

APPLICATIONS INFORMATION

Figure 2. The Input Diode Prevents a Shorted Input from Discharging

a Back-Up Battery Tied to the Output. It Also Protects the Circuit from

a Reversed Input. The LTM8031 Runs Only When the Input is Present

RUN/SS

BIAS

ADJ

LTM8031

8031 F02

GND

AUX

SYNC

operation with a haphazard or poor layout. See Figure 3

for a suggested layout.

Ensure that the grounding and heat sinking are acceptable.

A few rules to keep in mind are:

their respective pins.

and GND connection of the LTM8031. If a capacitor

is connected to the FIN terminals, place it as close

as possible to the FIN terminals, such that its ground

capacitor.

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.