LTC4240IGN datasheet(19/28 Pages) LINER

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LTC4240

4240f

Once the TIMER voltage has reached 5.5V, all of the

supplies will be latched off if any supply enters current

limit for at least 35

µs. The 35µs delay prevents quick

current spikes—for example, from a fan turning on—

from causing false trips of the circuit breaker.

During normal operation, the 5V and 3.3V supplies are

protected from overcurrent and short-circuit conditions

by dual-level circuit breakers. In the event that either

supply current exceeds the nominal limit, an internal timer

is started. If the supply is still overcurrent after 35

µs, the

circuit breaker trips and all the supplies are turned off

(Figure 7). If a short-circuit occurs on 5VOUT or 3VOUT and

the supply current exceeds three times the set limit, an

analog loop will limit the current to 3 times the value set

by RSENSE and 55mV. If the short persists for more than

µs, the LTC4240 latches off (Figure 8). It will stay in the

latched off state until it is reset using the OFF/ON pin or by

using the I2C interface. The LTC4240 can also be reset by

cycling any of the power supplies.

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

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

resistor (R1 for 3VOUT and R2 for 5VOUT, see front page).

A sense resistor is connected between 5VIN (Pin 21) and

5VSENSE (Pin 20) for the 5V supply. For the 3.3V supply, a

sense resistor is connected between 3VIN (Pin 22) and

3VSENSE (Pin23).Thecurrentlimitandthecurrentfoldback

current level are given by Equations 4 and 5:

LIMIT XVOUT

SENSE XVOUT

FOLDBACK XVOUT

SENSE XVOUT

()

where XVOUT = 5VOUT or 3VOUT.

Equation 4 is the current limit for XVOUT ≈ XVIN. Equation

5 shows the ILIMIT for shorted outputs. Both equations

assume voltage on TIMER pin is greater than 5.5V.

XVOUT = 3VOUT or 5VOUT. Note that since there are only 8

pins connecting 5VIN, RSENSE ≥ 0.007Ω for 5VIN.

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

–1200mA folding back to –350mA and the –12V switch at

500mA folding back to 250mA.

Selecting RSENSE

An equivalent circuit for the 5V and 3.3V circuit breakers

is shown in Figure 9. The sense resistor and the circuit

breaker threshold voltage determine the fault current that

turns off the external FETs. Sense resistors with a 1%

tolerance are recommended. Due to part to part and

temperature variations for both the sense resistor value

and the circuit breaker threshold voltage, the actual cur-

rent limit threshold will exhibit some variation. To calcu-

late the smallest value of current that will trip the fault

comparator, use the largest value of the sense resistor and

the smallest value of the threshold voltage. A 0.005

Ω 1%

sense resistor (on the 3.3V supply, for example) with

typical temperature coefficients would increase to ap-

proximately 0.0051

Ω (nominal value multiplied by the 1%

tolerance and the TC at 70

°C). Since the minimum value of

the threshold voltage is 50mV, this implies a current limit

of 9.8A. To arrive at the largest value of the current limit

that will turn off the external FETs, the nominal value of the

APPLICATIO S I FOR ATIO

Figure 7. Overcurrent Fault on 5V

Figure 8. Short-Circuit Fault on 5V

GATE

5V/DIV

5VIN–5VSENSE

100mV/DIV

FAULT

5V/DIV

µs/DIV

4240 F07

GATE

5V/DIV

5VIN–5VSENSE

100mV/DIV

FAULT

5V/DIV

µs/DIV

4240 F08

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