LTM8023MPV datasheet(12/20 Pages) LINER

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LTM8023

8023fb

the peak start-up current can be reduced to the current

that is required to regulate the output, with no overshoot.

Choose the value of the resistor so that it can supply at

least 20μA when the RUN/SS pin reaches 2.5V.

Synchronization

The internal oscillator of the LTM8023 can be synchronized

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 LTM8023

oscillates 20% lower than the intended synchronization

frequency (see the Frequency Selection section).

The LTM8023 will not enter Burst Mode operation 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 LTM8023 is absent. This

may occur in battery charging applications or in battery

backup systems where a battery or some other supply is

diode OR-ed with the LTM8023’s output. If the VIN pin is

allowed to ﬂoat and the SHDN pin is held high (either by a

logic signal or because it is tied to VIN), then the LTM8023’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

SHDN pin, the SW pin current will drop to essentially zero.

However, if the VIN pin is grounded while the output is

held high, then parasitic diodes inside the LTM8023 can

pull large currents from the output through the VIN pin.

Figure 4 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 LTM8023. The LTM8023 is neverthe-

less a switching power supply, and care must be taken to

minimize EMI and ensure proper operation. Even with the

APPLICATIONS INFORMATION

Figure 4. 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 LTM8023 Runs Only When

the Input is Present.

Figure 5. Layout Showing Suggested External Components, GND

Plane and Thermal Vias

high level of integration, you may fail to achieve speciﬁed

operation with a haphazard or poor layout. See Figure 5

for a suggested layout.

Ensure that the grounding and heatsinking are acceptable.

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 LTM8023.

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

VOUT and GND connection of the LTM8023.

VOUT

VIN

RUN/SS

BIAS

ADJ

LTM8023

8023 F04

VOUT

GND

VIN

AUX

SYNC

VOUT

CIN

VIN

RADJ

COUT

GND

BIAS

AUX

RUN/SS

PGOOD

SYNC

8023 F05

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