LTC4380 datasheet(16/24 Pages) AD

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LTC4381

Rev. 0

For more information www.analog.com

Ignoring the supply current (ICC), the VCC potential at the

end of cold crank is given by Equation 12.

VCC = (VIN(NOM) – VIN(LOW))•

–t

R1•C1 + VIN(LOW) (12)

where VIN(NOM) is the input voltage before the cold crank

starts, VIN(LOW) is the lowest input voltage during cold

crank, and t is the duration of the cold crank.

With the combination of R1 (10kΩ) and C1 (6.8µF), VCC

drops to 8V after the input voltage drops from 12V to 3V

for 40ms. During this time GATE stays high, keeping the

MOSFET on to continue providing current to the output.

Shutdown

The LTC4381 can be shut down to a lower current mode

by pulling the ON pin below the shutdown threshold of

1.05V. The quiescent current drops down to 5µA. An

external Zener diode from the input supply to the ON pin

can be used to implement undervoltage lockout, as illus-

trated in Figure 7. The UV threshold is the Zener voltage

plus 1.05V.

Figure 5. Recommended PCB Layout

4381 F05

SRC

OUT

The ON pin can be pulled up to 80V or below GND by up

to 60V without damage. Leaving the pin open allows an

internal resistor to pull it up and turn on the part. The leak-

age current at the pin should be limited to no more than

1µA if no pull-up device is used to help turn on the part.

Layout Considerations

To achieve accurate current sensing, use Kelvin connec-

tions to the current sense resistor (RSNS in Figure 5).

The minimum trace width for 1oz copper foil is 0.02" per

amp to ensure the trace stays at a reasonable tempera-

ture. 0.03" per amp or wider is recommended. Note that

1oz copper exhibits a sheet resistance of about 530µΩ/

square. Small resistances can cause large errors in high

current applications. During an overvoltage event, the

LTC4381 clamps the gate of the pass MOSFET to limit

the output voltage at an acceptable level. The load circuitry

may continue operating throughout this interval, but only

at the expense of dissipation in the MOSFET pass device.

The power dissipated in the MOSFET could be as high as

140W. To remove this heat, solder the IN exposed pad to

a copper trace that contains vias underneath the pad. The

SRC pins also conduct substantial heat from the MOSFET.

Connect all the SRC pins to a plane of 1oz or 2oz copper.

APPLICATIONS INFORMATION

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