SMH4042
CompactPCI Applications Aid
I/O Buffers
Depending upon the application requirements there are a number of silicon solutions that employ low on-resistance
CMOS switches. Figure 6 shows one implementation using a QuickSwitch® from Quality Semiconductor. This
particular device exhibits very Flat RON characteristics from 0 to 5V. The only drawback is the extra space required
for the external pull-up resistors.
early power
SMH4042
-
1Vref
+
LMV321
SGNL_VLD
OE1 OE2 OE3 OE4
B0
A0
A1
B1
B2
A28
A29
A30
A31
B29
B30
B31
QS34XVH245
2037 ILL27.0
Figure 6. Bus buffers with external pull-ups
Figure 7 shows another implementation, but the pull-up resistor structure is incorporated in the switch. The circuit also
automatically switches the bias voltage out of the circuit as the CMOS switches are enabled. A potential advantage
istheabilitytoplacetheinterfaceclosertotheedgeofthecard. Theboarddesignershouldevaluatetheirrequirements
and design goals and determine their best solution. The bus switches are available from both Texas Instruments and
Pericom Semiconductor.
BIASV
ON
-
1Vref
A1
+
B1
Equivalent Internal
LMV321
SGNL_VLD
Circuit
BIASV
ON
B0
A0
A1
B1
B2
A8
B9
B10
A9
A10
SN74CBT6800
or PI5C6800
2037 ILL28.0
Figure 7. Bus buffers with integrated pull-ups
2037-07 9/23/99
18
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
