ADG3257
Rev. E | Page 10 of 12
APPLICATIONS INFORMATION
MIXED VOLTAGE OPERATION, LEVEL TRANSLATION
Bus switches can be used to provide a solution for mixed voltage
systems where interfacing bidirectionally between 5 V and 3.3 V
devices is required. To interface between 5 V and 3.3 V buses,
an external diode is placed in series with the 5 V power supply
as shown in Figure 15.
BE
5V
3.3V 3.3V
CC
= 5
5V MEMORY
5V I/O
3.3V CPU/DSP/
MICROPROCESSOR/
MEMORY
4-015
3.3V
0291
Figure 15. Level Translation Between 5 V and 3.3 V Devices
The diode drops the internal gate voltage down to 4.3 V. The
bus switch limits the voltage present on the output to
V
CC
− External Diode Drop = V
TH
Therefore, assuming a diode drop of 0.7 V and a V
TH
of 1 V, the
output voltage is limited to 3.3 V with a logic high.
3.3V
0V
5V
OUT
5V SUPPLY
ADG3257
2.5V
2.5V
SWITCH
OUTPUT
SWITCH
INPUT
V
IN
02914-016
Figure 16. Input Voltage to Output Voltage
Similarly, the device could be used to translate bidirectionally
between 3.3 V to 2.5 V systems. In this case, there is no need for
an external diode. The internal V
TH
drop is 1 V, so with a
V
CC
= 3.3 V the bus switch limits the output voltage to
V
CC
− 1 V = 2.3 V
.3
.5
3.3V
2.5V
0V
3.3V
OUT
SWITCH
OUTPUT
SWITCH
INPUT
V
IN
3.3V SUPPLY
02914-017
Figure 17. 3.3 V to 2.5 V Level Translation Using the ADG3257 Bus Switch
MEMORY SWITCHING
This quad bus switch may be used to allow switching between
different memory banks, thus allowing additional memory and
decreasing capacitive loading. Figure 18 illustrates the
ADG3257 in such an application.
SDRAM NO. 1
SDRAM NO. 2
SDRAM NO. 7
SDRAM NO. 8
BE
S
LOGIC
02914-018
Figure 18. Allows Additional Memory Modules Without Added Drive or Delay
