LTC1646IGN datasheet(12/20 Pages) LINER

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LTC1646

1646fa

APPLICATIO S I FOR ATIO

Figure 4. Power-Up into a Short on 3.3V Output

GATE

5V/DIV

5VOUT

3VOUT

2V/DIV

TIMER

1V/DIV

BD_SEL#

5V/DIV

LCL_PCI_RST#

5V/DIV

HEALTHY#

5V/DIV

FAULT

5V/DIV

10ms/DIV

1646 F04

Table 2. tTIMER vs CTIMER

CTIMER

tTIMER

CTIMER

tTIMER

0.01

µF

2.5ms

0.22

µF

55ms

0.022

µF

5.5ms

0.33

µF

82.5ms

0.033

µF

8.25ms

0.47

µF

118ms

0.047

µF

11.8ms

0.68

µF

170ms

0.068

µF

17ms

0.82

µF

205ms

0.082

µF

20.5ms

µF

250ms

0.1

µF

25ms

The TIMER pin is immediately pulled low when BD_SEL#

goes high.

Short-Circuit Protection

During a normal power-up sequence, if the TIMER pin is

done ramping and a supply is still in current limit, all of the

pass transistors will be immediately turned off and FAULT

(Pin 3) 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 huge

currents can flow before the breaker trips, the current

foldback feature assures that the supply current will be

kept at a safe level and prevents voltage glitches at the

input supply when powering up into a short circuit.

After power-up (TIMER pin voltage >1.25V), the 5V and

3.3V supplies are protected from overcurrent and short-

circuit conditions by dual-level circuit breakers. If the

sense resistor voltage of either supply current exceeds

56mV but is less than 150mV, an internal timer is started.

If the supply is still overcurrent after 21

µs, the circuit

breaker trips and both supplies are turned off (Figure 5).

5VIN –5VSENSE

50mV/DIV

GATE

10V/DIV

FAULT

5V/DIV

µs/DIV

1646 F05

Figure 5. Overcurrent Fault on 5V

If a short-circuit occurs and the sense resistor voltage of

either supply current exceeds 150mV, the circuit breakers

trip without delay and the chip latches off (Figure 6). The

chip will stay in the latched-off state until OFF/ON (Pin 15)

is cycled high then low, or the 5VIN (Pin 12) power supply

is cycled.

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

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