LTC4240IGN datasheet(17/28 Pages) LINER

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LTC4240

4240f

Note that capacitor C1 performs dual functions. In addi-

tion to controlling the ramp up rates of the 5V and 3.3V

outputs, it also compensates the current limit loop.

Current limit faults are ignored while the TIMER voltage is

less than 5.5V.

Once all four supplies are within tolerance, the PWRGD pin

(HEALTHY#) will be pulled low and LOCAL_PCI_RESET#

(RESETOUT) is free to follow PCI_RST#. Bit 3 of the I2C

command latch powers up low, thus not asserting

LOCAL_PCI_RST#.

Power-Down Sequence

When either BD_SEL# (OFF/ON) or Bit 2 of the command

latch (C2) is set high, a power-down sequence begins

(Figure 5).

The TIMER pin is immediately pulled low. The GATE pin

(Pin 19) is pulled down by a 200

µA current source to

prevent the load currents on the 3.3V and 5V supplies from

going to zero instantaneously and glitching the power

supply voltages. Internal switches are connected to each

of the output supply voltage pins to discharge the output

bulk capacitors to ground. When any one of the output

voltages drops below its PWRGD threshold, the HEALTHY#

signal pulls high, LOCAL_PCI_RST# (RESETOUT) is as-

serted low, and the external status LED turns on.

Once the power-down sequence is complete the status

LED will light up and the CPCI card may be removed from

the slot. During extraction, the precharge circuit will

continue to bias the bus I/O pins at 1V until the long

connector pin connections are broken.

Early Power

Early Power usage is restricted by the CompactPCI (CPCI)

specification. It is intended to power up the precharge

circuit and I/O cells. The CPCI specification allows any of

the long power pins (5V, 3.3V, V(I/O)) to be used for Early

Power. Since Early Power is not isolated, a resistor should

be placed in series with each CPCI connector pin. Note that

if any Early Power pin is shorted on the inserted card, the

current limiting resistor will dissipate the power.

In order to maximize the DC current available from the 5V

supply, all eight 5V connector pins should be tied together

on the inserted card. The same applies to the ten 3.3V CPCI

connector pins. Early Power should then be drawn from

either or both of the two V(I/O) long pins. If either or both

of 5V and 3.3V is used for Early Power, then the 5V and

3.3V sense resistor values must be chosen such that the

1A/pin CPCI rule is not violated.

Connecting VEEIN

To lessen the likelihood of faulting on power up, the VEEOUT

output pin should be bypassed with a capacitor that is only

as large as necessary. A value of 10

µF to 47µF is recom-

mended. If a large value bypass capacitor is used (e.g.

≥100µF) on VEEOUT, current limit faults may occur during

power-up or during recovery from power failures.

TIMER

10V/DIV

GATE

10V/DIV

12VOUT

10V/DIV

VEEOUT

10V/DIV

5VOUT

10V/DIV

3VOUT

10V/DIV

LCL_PCI_RST#

5V/DIV

BD_SEL#

5V/DIV

HEALTHY#

5V/DIV

10ms/DIV

4240 F05

Figure 5. Normal Power-Down Sequence

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