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LTC1644

1644f

Output Voltage Monitor

The status of all four output voltages is monitored by the

power good function. In addition, the PCI_RST# signal is

logically combined on-chip with the HEALTHY# signal to

create LOCAL_PCI_RST# (see Table 5). As a result,

LOCAL_PCI_RST# will be pulled low whenever HEALTHY#

is pulled high independent of the state of the PCI_RST#

signal.

Table 5. LOCAL_PCI_RST# Truth Table

PCI_RST#

HEALTHY#

LOCAL_PCI_RST#

If any of the output voltages drop below the power good

threshold for more than 10

µs, the PWRGD pin will be

pulled high and the LOCAL_PCI_RST# signal will be

asserted low.

Precharge

The PRECHARGE input and DRIVE output pins are in-

tended for use in generating the 1V precharge voltage that

is used to bias the bus I/O connector pins during board

insertion. The LTC1644 is also capable of generating

precharge voltages other than 1V. Figure 8 shows a circuit

that can be used in applications requiring a precharge

voltage less than 1V. The circuit in Figure 9 can be used for

applications that need precharge voltages greater than 1V.

Table 6 lists suggested resistor values for R10A and R10B

vs precharge voltage for the application circuits shown in

Figures 8 and 9.

Table 6. R10A and R10B Resistor Values vs Precharge Voltage

VPRECHARGE

R10A

R10B

VPRECHARGE

R10A

R10B

1.5V

Ω

9.09

Ω

0.9V

16.2

Ω

1.78

Ω

1.4V

Ω

7.15

Ω

0.8V

14.7

Ω

3.65

Ω

1.3V

Ω

5.36

Ω

0.7V

12.1

Ω

5.11

Ω

1.2V

Ω

3.65

Ω

0.6V

Ω

7.15

Ω

1.1V

Ω

1.78

Ω

0.5V

9.09

Ω

9.09

Ω

Due to leakage current constraints, precharge resistor

values of less than 50k are often required. In these

precharge applications, it may also be necessary to dis-

connect the individual resistors from the LTC1644’s

PRECHARGE pin when the plug-in board is completely

seated in the board slot. The circuit in Figure 10 uses a bus

switch to connect the individual precharge resistors to the

LTC1644’s PRECHARGE pin while the BD_SEL# pin volt-

age is pulled up to 5VIN, i.e., when the BD_SEL# short

connector pin is still unconnected. After the plug-in board

is completely seated, the BD_SEL# pin voltage will drop to

approximately 3.8V (assuming BD_SEL# isn’t asserted

low), and the bus switch OE pin is pulled high by Q2. When

the plug-in card is removed from the connector, the

BD_SEL# connection is broken first and the BD_SEL# pin

voltage pulls up to 5V. This causes Q2 to turn off, which re-

enables the bus switch and the precharge resistors are

reconnected to the LTC1644’s PRECHARGE pin for the

remainder of the board extraction process.

Other CompactPCI Applications

The LTC1644 can be easily configured for applications

where no VEE supply is present by simply connecting the

VEEIN pin to GND and floating the VEEOUT pin (Figure 11).

For CPCI applications where no 5V supply input is re-

quired, short both the 5VIN and 5VSENSE pins to the 3VIN

pin and short the 5VOUT pin to the 3VOUT pin (Figure 12).

If no 3.3V supply input is required, Figure 13 illustrates

how the LTC1644 should be configured. First, 3VSENSE

(Pin 16) is connected to 3VIN (Pin 17), 3VOUT (Pin 18) is

connected to 5VOUT (Pin 3) and the LTC1644’s 3VIN pin is

connected through a pair of signal diodes (BAV99) to 5VIN.

For applications where the BD_SEL# connector pin is

typically grounded on the backplane, the circuit in

Figure 14 allows the LTC1644 to be reset simply by

pressing a pushbutton switch on the CPCI plugin board.

This arrangement eliminates the requirement to extract

and reinsert the CPCI board in order to reset the LTC1644’s

circuit breakers.

Power MOSFET Selection Criteria

Three device parameters are key in selecting the optimal

power MOSFET for Hot Swap applications. The three

parameters are: (1) device power dissipation (PD); (2)

device drain-source channel ON resistance, RDS(ON); and

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