LTM8031MPV#PBF datasheet(12/22 Pages) LINER

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LTM8031

8031fb

Synchronization

The internal oscillator of the LTM8031 can be synchro-

nized by applying an external 250kHz to 2MHz clock to

the SYNC pin. Do not leave this pin floating. The resistor

tied from the RT pin to ground should be chosen such

that the LTM8031 oscillates 20% lower than the intended

synchronization frequency (see the Frequency Selection

section).TheLTM8031willnotenterBurstModeoperation

while synchronized to an external clock, but will instead

skip pulses to maintain regulation.

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

VIN pin is allowed to float and the RUN/SS pin is held high

(either by a logic signal or because it is tied to VIN), then

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

will drop to essentially zero. However, if the VIN pin is

grounded while the output is held high, then parasitic

diodes inside the LTM8031 can pull large currents from

the output through the VIN pin, potentially damaging the

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

VOUT

VIN

RUN/SS

BIAS

ADJ

LTM8031

8031 F02

VOUT

GND

VIN

AUX

SYNC

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

operation with a haphazard or poor layout. See Figure 3

for a suggested layout.

Ensurethatthegroundingandheatsinkingareacceptable.

A few rules to keep in mind are:

1. Place the RADJ and RT resistors as close as possible to

their respective pins.

2. Place the CIN capacitor as close as possible to the VIN

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

connection is as close as possible to that of the CIN

capacitor.

3. Place the COUT capacitor as close as possible to the

VOUT and GND connection of the LTM8031.

8031 F01

0.5A/DIV

VRUN/SS

2V/DIV

VOUT

2V/DIV

RUN/SS

GND

0.22µF

RUN

15k

2ms/DIV

Figure 1. To Soft-Start the LTM8031, Add a Resistor

and Capacitor to the RUN/SS Pin

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