LTC4413 datasheet(12/32 Pages) AD

12 / 32 page

LTC4421

Rev. 0

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The current limit amplifier (ILIM AMP) monitors the load

current using the difference between the SENSE and OUT

pin voltages. The amplifier and gate driver work together

to limit the current in the load by reducing the GATE-to-

SOURCE voltage in an active control loop. The SENSE-to-

OUT differential voltage is regulated to 25mV (ΔVSNS). An

external sense resistor placed between SENSE and OUT

sets the current limit value for each channel. Foldback

comparator (FOLDBACK CP) reduces the SENSE-to-

OUT differential voltage from 25mV (ΔVSNS) to 12.5mV

(ΔVSNS,FLD) to conserve power when the OUT1 voltage is

low. The foldback comparator’s rising and falling thresh-

old voltages are 460mV and 410mV, respectively. If the

SENSE–OUT voltage on a channel remains in current limit

for the time programmed by the TMR pin, the LTC4421

registers a current limit fault. Additionally, pulsed output

load currents exceeding current limit and occurring at

duty cycles of 25% or higher will integrate over time and

cause a current limit fault.

When a current limit fault occurs, the LTC4421 discon-

nects the channel and drives the FAULT pin low to indicate

that a current limit fault has occurred. After a current

fault occurs, driving DISABLE low initiates a cool down

period that is 1024 times longer than the time-out period.

Driving DISABLE back high terminates the cool-down

period, resets FAULT high and allows reconnection to the

output. Alternatively, if RETRY and DISABLE are both high

when a current limit fault occurs, the LTC4421 will try

to reconnect up to 6 additional times after the first fault,

with a cool-down period between attempted connections

that is 1024 times longer than the current limit fault time.

Driving DISABLE1 and DISABLE2 low disconnects V1 and

V2, respectively, from powering the output. The CASIN

OPERATION

and CASOUT pins of multiple LTC4421’s can be config-

ured to prioritize as many input supplies as desired. The

DISABLE1, DISABLE2 and CASIN inputs are connected

to comparators having 1V (VTH) threshold and 150mV

(VTH,HYST) hysteresis. See the Applications Information

section for circuits that use the DISABLE, FAULT2 and

VALID2 pins to redefine input supply priorities in real time

and to prevent the primary input from powering the output

until a valid back-up supply is available.

Driving SHDN low causes the device to turn off the exter-

nal N-Channel MOSFETs, enter a low current state and

invalidate V1 and V2. All circuitry is debiased except the

INTVCC rail generator and shutdown comparator. The total

internal bias current is reduced dramatically to 6µA to

conserve power. The INTVCC voltage is reduced to 3V and

is powered from the highest of the V1, V2, EXTVCC and

OUT1 voltages. The CASOUT pin is driven high to allow

a lower priority LTC4421 in a cascading application to

power VOUT.

When SHDN is driven high, the LTC4421 reactivates all

circuits. It may take several hundred milliseconds for a

valid input to connect to the output, because the external

charge pump capacitor CCPO must charge to 6.7V before

connection is allowed.

The LTC4421 includes its own internally generated low

voltage rail (INTVCC) that provides power to the low volt-

age sections of the device. Because most of the device’s

quiescent current is provided by INTVCC, the INTVCC

power source is prioritized to minimize current draw from

lower priority sources. The INTVCC rail is powered from

one of 4 prioritized sources. These sources in order of

priority are EXTVCC, V1 and V2. If none of these three

inputs is valid, INTVCC is powered by the highest of the

V1, V2, EXTVCC and OUT1 voltages.

LTC4413 Datasheet(PDF) 12 Page - Analog Devices

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