LTC4218CDHC datasheet(7/16 Pages) LINER

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LTC4218

4218fb

PIN FUNCTIONS

Exposed Pad: Exposed pad may be left open or connected

to device ground.

FB: Foldback and Power Good Comparator Input. Connect

this pin to an external resistive divider from SOURCE for

the LTC4218 (adjustable version). The LTC4218-12 ver-

sion uses a ﬁxed internal divider with optional external

adjustment. Open the pin if the LTC4218-12 thresholds

for 12V operation are desired. If the voltage falls below

0.6V, the output power is considered bad and the current

limit is reduced. If the voltage falls below 1.21V the PG

pin will pull low to indicate the power is bad.

FLT: Overcurrent Fault Indicator. Open drain output pulls

low when an overcurrent fault has occurred and the circuit

breaker trips. For overcurrent auto-retry tie to UV pin (see

Applications Information for details).

GATE: Gate Drive for External N-Channel FET. An internal

24μA current source charges the gate of the external

N-channel MOSFET. A resistor and capacitor network

from this pin to ground sets the turn-on rate. During an

undervoltage or overvoltage generated turn-off a 250μA

pull-down current turns the MOSFET off. During a short

circuit or undervoltage lockout, a 170mA pull-down current

source between GATE and SOURCE is activated.

GND: Device Ground.

IMON: Current Monitor Output. The current sourced from

this pin is deﬁned as the current sense voltage (between

the SENSE+ and SENSE– pins) multiplied by 6.67μA/mV.

Placing a 20k resistor from this pin to GND creates a 0V to

2V voltage swing when the current sense voltage ranges

from 0mV to 15mV.

INTVCC: Internal 3V Supply Decoupling Output. This pin

must have a 0.1μF or larger capacitor.

ISET: Current Limit Adjustment Pin. For 15mV current limit

threshold, open this pin. This pin is driven by a 20k resis-

tor in series with a voltage source. The pin voltage is used

to generate the current limit threshold. The internal 20k

resistor and an external resistor between ISET and ground

create an attenuator that lowers the current limit value.

NC: No Connection

OV: Overvoltage Comparator Input. Connect this pin to

an external resistive divider from VDD for the LTC4218

(adjustable version). The LTC4218-12 version uses a ﬁxed

internal divider with optional external adjustment for 12V

operation. Open the pin if the LTC4218-12 thresholds are

desired. If the voltage at this pin rises above 1.235V, an

overvoltage is detected and the switch turns off. Tie to

GND if unused.

PG: Power Good Indicator. Open drain output pulls low

when the FB pin drops below 1.21V indicating the power

is bad.

SENSE–: Current Sense Minus Input. Connect this pin to

the opposite of VDD current sense resistor side. The cur-

rent limit circuit controls the GATE pin to limit the sense

voltage between the SENSE+ and SENSE– pins to 15mV

or less depending on the voltage at the FB pin.

SENSE+: Current Sense Plus Input. Connect this pin to

the VDD side of the current sense resistor.

SOURCE: N-Channel MOSFET Source Connection. Connect

this pin to the source of the external N-channel MOSFET

switch. This pin provides a return for the gate pull-down

circuit. In the LTC4218-12 version, the powergood com-

parator monitors an internal resistive divider between the

SOURCE pin and GND.

TIMER: Timer Input. Connect a capacitor between this pin

and ground to set a 12ms/μF duration for current limit

before the switch is turned off. If the UV pin is toggled

low while the MOSFET switch is off, the switch will turn on

again following a cool down time of 518ms/μF duration.

UV: Undervoltage Comparator Input. Tie high if unused.

Connect this pin to an external resistive divider from VDD

for the LTC4218 (adjustable version). The LTC4218-12

version drives the UV pin with an internal resistive divider

from VDD. Open the pin if the preset LTC4218-12 thresh-

olds for 12V operation are desired. If the UV pin voltage

falls below 1.15V, an undervoltage is detected and the

switch turns off. Pulling this pin below 0.62V resets the

overcurrent fault and allows the switch to turn back on

(see Applications Information for details). If overcurrent

auto-retry is desired then tie this pin to the FLT pin.

VDD: Supply Voltage. This pin has an undervoltage lockout

threshold of 2.73V.

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