Datasheet

TMDS361B
SLLS988A –SEPTEMBER 2009–REVISED JULY 2011 www.ti.com
Table 4. Value of k for Different Input Threshold Voltages
V
th–
\V
th+
0.7 V
DD
0.65 V
DD
0.6 V
DD
0.55 V
DD
0.5 V
DD
0.45 V
DD
0.4 V
DD
0.35 V
DD
0.3 V
DD
0.1 V
DD
1.0986 0.9445 0.8109 0.6931 0.5878 0.4925 0.4055 0.3254 0.2513
0.15 V
DD
1.0415 0.8873 0.7538 0.6360 0.5306 0.4353 0.3483 0.2683 0.1942
0.2 V
DD
0.9808 0.8267 0.6931 0.5754 0.4700 0.3747 0.2877 0.2076 0.1335
0.25 V
DD
0.9163 0.7621 0.6286 0.5108 0.4055 0.3102 0.2231 0.1431 0.0690
0.3 V
DD
0.8473 0.6931 0.5596 0.4418 0.3365 0.2412 0.1542 0.0741 —
From Equation 1, R
up(min)
= 5.5 V/3 mA = 1.83 kΩ to operate the bus under a 5-V pullup voltage and provide less
than 3 mA when the I
2
C device is driving the bus to a low state. If a higher sink current, for example 4 mA, is
allowed, R
up(min)
can be as low as 1.375 kΩ.
Given a 5-V I
2
C device with input low- and high-threshold voltages at 0.3 V
dd
and 0.7 V
dd
, respectively, the value
of k is 0.8473 from Table 4. Taking into account the 1.83-kΩ pullup resistor, the maximum total load capacitance
is C
(total-5V)
= 645 pF. C
cable(max)
should be restricted to be less than 545 pF if C
source
and C
i
can be as high as 50
pF. Here the C
i
is treated as C
sink
, the load capacitance of a sink device.
Fixing the maximum transition time from Table 4, T = 1 μs, and using the k values from Table 4, the
recommended maximum total resistance of the pullup resistors on an I
2
C bus can be calculated for different
system setups.
To support the maximum load capacitance specified in the HDMI spec, C
cable(max)
= 700 pF/C
source
= 50 pF/C
i
=
50 pF, R
(max)
can be calculated as shown in Table 5.
Table 5. Pullup Resistor for Different Threshold Voltages and 800-pF Load
V
th–
\V
th+
0.7 V
DD
0.65 V
DD
0.6 V
DD
0.55 V
DD
0.5 V
DD
0.45 V
DD
0.4 V
DD
0.35 V
DD
0.3 V
DD
UNIT
0.1 V
DD
1.14 1.32 1.54 1.80 2.13 2.54 3.08 3.84 4.97 kΩ
0.15 V
DD
1.20 1.41 1.66 1.97 2.36 2.87 3.59 4.66 6.44 kΩ
0.2 V
DD
1.27 1.51 1.80 2.17 2.66 3.34 4.35 6.02 9.36 kΩ
0.25 V
DD
1.36 1.64 1.99 2.45 3.08 4.03 5.60 8.74 18.12 kΩ
0.3 V
DD
1.48 1.80 2.23 2.83 3.72 5.18 8.11 16.87 — kΩ
Or, limiting the maximum load capacitance of each cable to 400 pF to accommodate with I
2
C specification
version 2.1, C
cable(max)
= 400 pF/C
source
= 50 pF/C
i
= 50 pF, the maximum values of R
(max)
are calculated as
shown in Table 6.
Table 6. Pullup Resistor for Different Threshold Voltages and 500-pF Load
V
th–
\V
th+
0.7 V
DD
0.65 V
DD
0.6 V
DD
0.55 V
DD
0.5 V
DD
0.45 V
DD
0.4 V
DD
0.35 V
DD
0.3 V
DD
UNIT
0.1 V
DD
1.82 2.12 2.47 2.89 3.40 4.06 4.93 6.15 7.96 kΩ
0.15 V
DD
1.92 2.25 2.65 3.14 3.77 4.59 5.74 7.46 10.30 kΩ
0.2 V
DD
2.04 2.42 2.89 3.48 4.26 5.34 6.95 9.63 14.98 kΩ
0.25 V
DD
2.18 2.62 3.18 3.92 4.93 6.45 8.96 13.98 28.99 kΩ
0.3 V
DD
2.36 2.89 3.57 4.53 5.94 8.29 12.97 26.99 — kΩ
Obviously, to accommodate the 3-mA drive-current specification, a narrower threshold voltage range is required
to support a maximum 800-pF load capacitance for a standard-mode I
2
C bus.
When the input low- and high-level threshold voltages, V
th–
and V
th+
, are 0.7 V and 1.9 V, respectively, which is
0.15 V
DD
and 0.4 V
DD
, approximately, then with V
DD
= 5 V from Table 5, the maximum pullup resistor is 3.59 kΩ.
The allowable pullup resistor is in the range of 1.83 kΩ and 3.59 kΩ.
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