LTC4413 datasheet(15/32 Pages) AD

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LTC4421

Rev. 0

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APPLICATIONS INFORMATION

Current Limit Regulation and Setting the Current Limit

The LTC4421 provides independently settable current limit

values for each input. On a given channel, the LTC4421

regulates the maximum voltage across the SENSE and

OUT pins to 25mV (ΔVSNS). Connect a sense resistor

RSENSE between SENSE and OUT to set the current limit

value ILIM, is given by Equation 6.

LIM =

25mV

SENSE

(6)

Ensure the input supply is capable of sourcing more cur-

rent than ILIM, so that the input supply does not drop out

and cause UV motorboating. Use standard 1% resistor

values and choose RSENSE to set ILIM at least 25% higher

than the maximum output load current ILOAD(MAX) to

account for tolerances in the current limit and to provide

sufficient charging current to the output capacitor when

charging the output.

ILIM and COUT set the rate at which the output voltage

will rise.

The minimum output rise rate is shown by Equation 7.

OUT

(min) =

LIM – ILOAD(MAX)

(

)

OUT

(7)

Equation 7 assumes that the output voltage is charging

under maximum output load current conditions, so that

only the difference between the programmed current limit

current and the maximum DC load current is available to

charge COUT. It is essential to set ILIM to ensure that

the output is fully charged before an overcurrent fault

timeout occurs.

The LTC4421 implements a stepped current limit foldback

feature. A foldback comparator monitors the voltage on the

OUT1 pin and reduces the current limit regulation voltage

from 25mV (ΔVSNS) to 12.5mV (ΔVSNS,FLD) for low OUT1

voltages, thereby cutting the current limit in half to reduce

power consumption. The comparator rising and falling

threshold voltages are 460mV (VFLD,TH) and 410mV, respec-

tively. When the OUT1 voltage is initially being powered up

from 0V, the current limit regulation voltage is 12.5mV until

OUT1 rises above 460mV, at which point it is increased

to 25mV. For output voltages below 460mV, ensure the

maximum output load current is lower than the foldback

current limit to ensure the output powers up. When the

OUT1 voltage is initially powered and then discharges, for

example due to an input or output short circuit, the current

limit regulation voltage will be 25mV until OUT1 drops below

410mV, at which point it is reduced to 12.5mV.

Use Kelvin connections from the RSENSE terminals to

the LTC4421’s SENSE and OUT pins for best accuracy.

Choose sense resistors having low inductance to mini-

mize the sense resistor’s impact on the current limit regu-

lation loop stability. A single sense resistor can be used if

the current limit is the same on both channels, as shown

in Figure 3.

Figure 3. Using a Single Sense Resistor RSENSE to Set the Same Current Limit for Both Channels

PSMN4R060YS

PSMN1R440YLD

PSMN4R060YS

PSMN1R440YLD

CCPO

1µF

2.5m

RSENSE

CG2

LTC4421 F03

CG1

47nF

CPO

CPOREF

VOUT

GATE1 SOURCE1

SENSE2 OUT1

LTC4421

V2 GATE2 SOURCE2

SENSE1

OUT2

LTC4413 Datasheet(PDF) 15 Page - Analog Devices

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