LTC4240

23

4240f

APPLICATIO S I FOR ATIO

than 2

µA, then a 50K resistor can be connected directly

from the 1V bias voltage to the I/O line. However, many ICs

connected to the I/O lines can have leakage currents up to

10

µA. For these applications, a 10k resistor is used but

must be disconnected when the board has been seated as

determined by the state of the BD_SEL# signal. Figure 14

shows a precharge circuit that uses a bus switch to

GND

OFF/ON

LTC4240*

PRECHARGE

DRIVE

21

28

18

17

Z4: 1PMT5.0AT3

*ADDITIONAL DETAILS OMITTED FOR CLARITY

DATA BUS

I/O

4240 F14

Q3

MMBT2222A

10

R11

18

Ω, 5%

R18

1k, 5%

10

Ω

5%

10k

5%

10k

5%

PRECHARGE OUT

1V

±10%

I/O

10

Ω

5%

R22

2.74

Ω

R8

1k, 5%

R7

12

Ω, 5%

C3, 4.7nF

R9

24

Ω

PCI

BRIDGE

CHIP

MEDIUM 5V

LONG 5V

BD_SEL#

GROUND

I/O PIN 1

I/O PIN 128

Z4

UP TO 128 I/O LINES

0.1

µF

100

Ω

LONG

5V

Q4

MMBT3906

R26

51.1k, 5%

R27

75k

5%

BUS SWITCH

OE

OUT

OUT

IN

CompactPCI

BACKPLANE

CONNECTOR

(FEMALE)

CompactPCI

BACKPLANE

CONNECTOR

(MALE)

R17

1.2k

5%

Figure 14. Precharge Bus Switch Application Circuit for 3.3V and Universal Hot Swap Boards

Figure 13.Precharge Application Circuit for 5V Signaling Systems

GND

OFF/ON

LTC4240*

PRECHARGE

DRIVE

21

28

18

17

Z4: 1PMT5.0AT3

*ADDITIONAL DETAILS OMITTED FOR CLARITY

DATA BUS

I/O

4240 F13

PRECHARGE OUT

1V

±10%

10k

5%

R11

18

Ω, 5%

R18

1k, 5%

10

Ω

5%

I/O

10

Ω

5%

R17

1.2k

5%

R22

2.74

Ω

10k

5%

R8

1k, 5%

R7

12

Ω, 5%

C3, 4.7nF

R9

24

Ω

PCI

BRIDGE

CHIP

MEDIUM 5V

LONG 5V

BD_SEL#

GROUND

I/O PIN 1

I/O PIN 128

LONG

5V

Z4

UP TO 128 I/O LINES

Q3

MMBT2222A

CompactPCI

BACKPLANE

CONNECTOR

(FEMALE)

CompactPCI

BACKPLANE

CONNECTOR

(MALE)

connect the individual 10k precharge resistors to the

LTC4240 1V PRECHARGE pin. The electrical connection is

made (bus switches close) when the voltage on the

BD_SEL# pin of the plug-in card is above 4.4V, which

occurs just after the long pins have made contact. The bus

switches are subsequently electrically disconnected when

the board connector makes contact with the BD_SEL# pin

(bus switch OE pin is pulled high by Q4).