LTC4244IGN datasheet(15/28 Pages) LINER

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LTC4244/LTC4244-1

42441f

Table 1. tTIMER vs CTIMER

CTIMER (±10%)

tTIMER(MIN)

tTIMER(MAX)

0.1

µF

35ms

74ms

0.22

µF

77ms

162ms

0.33

µF

115ms

243ms

0.47

µF

164ms

346ms

0.68

µF

238ms

500ms

0.82

µF

287ms

603ms

The TIMER pin is immediately pulled low when the BD_SEL#

pin signal goes high.

Thermal Shutdown

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

protected by a thermal shutdown circuit. When the junc-

tion temperature of the die reaches 150

°C, all switches will

be latched off and the FAULT pin will be pulled low.

Short-Circuit Protection

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

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

the pass transistors will be immediately turned off and

FAULT will be pulled low as shown in Figure 4.

In order to prevent excessive power dissipation in the pass

transistors and 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 guarantees that the supply current will be

kept at a safe level.

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

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

APPLICATIO S I FOR ATIO

TIMER

10V/DIV

GATE

10V/DIV

3.3VOUT

10V/DIV

BD_SEL#

10V/DIV

HEALTHY#

10V/DIV

FAULT

10V/DIV

LCL_PCI_RST#

10V/DIV

20ms/DIV

4244 F04

12VOUT

10V/DIV

VEEOUT

10V/DIV

5VOUT

10V/DIV

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

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