LTC4244IGN datasheet(16/28 Pages) LINER

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LTC4244/LTC4244-1

42441f

immediately to its ILIMIT value. If that supply remains in

current limit for more than 25

µs, all of the supplies will be

latched off. The 25

µs prevents quick current spikes—for

example, from a fan turning on—from causing false trips

of the circuit breaker.

After power-up, the 5V and 3.3V supplies are protected

from short circuits by dual-level circuit breakers. In the

event that either supply’s current exceeds the nominal

current limit, an internal timer is started. If the supply is

still overcurrent after 25

µs, the circuit breaker trips and all

the supplies are turned off (Figure 5). An analog current

limit loop prevents the supply current from exceeding 3

the nominal current limit in the event of a short circuit

(Figure 6). The LTC4244 will stay in the latched off state

until the OFF/ON pin is cycled high then low or the 12VIN

power supply is cycled low then high.

The current limit and the foldback current level for the 5V

and 3.3V outputs are both a function of the external sense

resistor. As shown in Figure 1, a sense resistor is con-

nected between 5VIN and 5VSENSE for the 5V supply. For

the 3.3V supply, a sense resistor is connected between

3.3VIN and 3.3VSENSE. The typical current limit and the

foldback current levels are given by Equations 6 and 7:

LIMIT VOUT

SENSE VOUT

()

(6)

FOLDBACK VOUT

SENSE VOUT

()

(7)

where

nVOUT = 5VOUT or 3.3VOUT.

The current limit for the internal 12V switch is set at

850mA folding back to 360mA and the –12V switch at

610mA folding back to 225mA.

APPLICATIO S I FOR ATIO

GATE

20V/DIV

5VOUT

10V/DIV

FAULT

5V/DIV

TIMER

20V/DIV

5VIN – 5VSENSE

100mV/DIV

µs/DIV

4244 F05

Figure 5. Overcurrent Fault on 5V Output

GATE

20V/DIV

3.3VOUT

5V/DIV

FAULT

5V/DIV

TIMER

20V/DIV

3VIN – 3VSENSE

500mV/DIV

µs/DIV

4244 F06

Figure 6. Short-Circuit Fault on 3.3V Output

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