Chapter 8: Using MAX II Devices in Multi-Voltage Systems
Recommended Operating Condition for 5.0-V Compatibility
8–7
As shown above, R
1
= 5.0 V/135 mA.
The values usually shown in data sheets reflect typical operating conditions. Subtract
20% from the data sheet value for guard band. This subtraction applied to the above
example gives R
1
a value of 30.
Select R
2
so that the MAX II device’s I
OH
specification is not violated. For example, if
the above device has a maximum I
OH
of 8 mA, given the I/O clamp diode, V
IN
= V
CCIO
+ 0.7 V = 3.7 V. Given that the maximum supply load of a 5.0-V device (V
CC
) is 5.5 V,
the value of R
2
can be calculated as follows:
Equation 8–2.
(
5.5
V
–
3.7
V
)
–
(
8
mA
×
30
Ω
-
)
R
2
=
-------------------------------------------------------------------------------
=
194
Ω
8
mA
This analysis assumes worst-case conditions. If your system does not see a wide
variation in voltage-supply levels, you can adjust these calculations accordingly.
Because 5.0-V device tolerance in MAX II devices requires use of the I/O clamp, and
this clamp is activated only after power-up, 5.0-V signals may not be driven into the
device until it is configured. The I/O clamp diode is only supported in the EPM1270
and EPM2210 devices’ I/O Bank 3. An external protection diode is needed for other
I/O banks for EPM1270 and EPM2210 devices and all I/O pins in EPM240 and
EPM570 devices.
Recommended Operating Condition for 5.0-V Compatibility
As mentioned earlier, a 5.0-V tolerance can be supported with the I/O clamp diode
enabled with external series/pull-up resistance. To guarantee long term reliability of
the device’s I/O buffer, there are restrictions on the signal duty cycle that drive the
MAX II I/O, which is based on the maximum clamp current.
shows the
maximum signal duty cycle for 3.3-V V
CCIO
given a PCI clamp current-handling
capability.
Table 8–3.
Maximum Signal Duty Cycle
V
IN
(V)
4.0
4.1
4.2
4.3
4.4
4.5
4.6
Notes to
(1) V
IN
is the voltage at the package pin.
(2) The I
CH
is calculated with a 3.3-V V
CCIO
. A higher V
CCIO
value will have a lower I
CH
value
with the same V
IN
.
I
CH
(mA)
5.00
11.67
18.33
25.00
31.67
38.33
45.00
Max Duty Cycle (%)
100
90
50
30
17
10
5
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