LTC1644IGN datasheet(14/24 Pages) LINER

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LTC1644

1644f

APPLICATIO S I FOR ATIO

Table 2. tTIMER vs CTIMER

CTIMER

tTIMER

CTIMER

tTIMER

0.01

µF

5.24ms

0.22

µF

115ms

0.022

µF

11.5ms

0.33

µF

173ms

0.033

µF

17.3ms

0.47

µF

246ms

0.047

µF

24.6ms

0.68

µF

356ms

0.068

µF

35.6ms

0.82

µF

430ms

0.082

µF

43.0ms

µF

524ms

0.1

µF

52.4ms

The TIMER pin is immediately pulled low when the BD_SEL#

signal goes high.

Thermal Shutdown

The internal switches for the 12V and –12V supplies are

protected by an internal current limit and a thermal shut-

down circuit. When the temperature of the chip reaches

130

°C, all switches will be latched off and the FAULT pin

(Pin 6) will be pulled low.

Short-Circuit Protection

During a normal power-up sequence, if the TIMER (Pin 4)

is done ramping and any supply is still in current limit, all

of the pass transistors will be immediately turned off and

FAULT (Pin 6) will be pulled low as shown in Figure 4.

In order to prevent excessive power dissipation in the pass

transistors and to prevent voltage spikes on the supplies

during short-circuit conditions, the current limit on each

supply is designed to be a function of the output voltage.

As the output voltage drops, the current limit decreases.

Unlike a traditional circuit breaker function where large

currents can flow before the breaker trips, the current

foldback feature assures that the supply current will be

kept at a safe level. In addition, current foldback prevents

voltage glitches when powering up into a short.

If either the 12V or –12V supply exceeds current limit after

power up, the shorted supply’s current will drop immedi-

ately to its ILIMIT value. If that supply remains in current

limit for more that 45

µs, all of the supplies will be latched

off. The 45

µs delay 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 overcurrent and short-circuit conditions by dual-

level circuit breakers. In the event that either supply

current exceeds the nominal limit but is less than 3 times

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

is still overcurrent after 45

µs, the circuit breaker trips and

all the supplies are turned off (Figure 5). If a short-circuit

occurs and the supply current exceeds 3 times the set

limit, the circuit breakers trip without any delay and the

chip latches off (Figure 6). The chip will stay in the latched

off state until OFF/ON (Pin 5) is cycled high then low or the

12VIN (Pin 1) power supply is cycled off then on.

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 Figure 1). As

shown in Figure 1, a sense resistor is connected between

5VIN (Pin 13) and 5VSENSE (Pin 12) for the 5V supply. For

the 3V supply, a sense resistor is connected between 3VIN

(Pin 9) and 3VSENSE (Pin 10). The current limit and the

foldback current level are given by Equations 4 and 5:

LIMIT XVOUT

SENSE XVOUT

()

(4)

FOLDBACK XVOUT

SENSE XVOUT

()

(5)

where XVOUT = 5VOUT or 3VOUT.

As a design aid, the current limit and foldback level for

commonly used values for RSENSE is shown in Table 3.

Table 3. ILIMIT(XVOUT) and IFOLDBACK(XVOUT) vs RSENSE

RSENSE (Ω)ILIMIT(XVOUT)

IFOLDBACK(XVOUT)

0.005

10.2A

2.4A

0.006

8.5A

2.0A

0.007

7.3A

1.7A

0.008

6.4A

1.5A

0.009

5.7A

1.3A

0.01

5.1A

1.2A

where XVOUT = 3VOUT or 5VOUT.

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

840mA folding back to 360mA and the –12V switch at

320mA folding back to 100mA.

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