參數(shù)資料
型號： TMDS442PNPRG4
廠商： TEXAS INSTRUMENTS INC
元件分類： 消費(fèi)家電
英文描述： SPECIALTY CONSUMER CIRCUIT, PQFP128
封裝： PLASTIC, TQFP-128
文件頁數(shù)： 30/45頁
文件大?。?/td> 1977K
代理商： TMDS442PNPRG4
www.ti.com
T + k
RC
(2)
V(t)
+ V
DD
(1
* e*t RC)
(3)
SLLS757A – AUGUST 2006 – REVISED MARCH 2007
value, and C is the total load capacitance. The parameter, k, can be calculated from equation 3 by solving for
t, the times at which certain voltage thresholds are reached. Different input threshold combinations introduce
different values of t. Table 6 summarizes the possible values of k under different threshold combinations.
Table 6. Value K Upon Different Input Threshold Voltages
Vth-\Vth+
0.7VDD
0.65VDD
0.6VDD
0.55VDD
0.5VDD
0.45VDD
0.4VDD
0.35VDD
0.3VDD
0.1VDD
1.0986
0.9445
0.8109
0.6931
0.5878
0.4925
0.4055
0.3254
0.2513
0.15VDD
1.0415
0.8873
0.7538
0.6360
0.5306
0.4353
0.3483
0.2683
0.1942
0.2VDD
0.9808
0.8267
0.6931
0.5754
0.4700
0.3747
0.2877
0.2076
0.1335
0.25VDD
0.9163
0.7621
0.6286
0.5108
0.4055
0.3102
0.2231
0.1431
0.0690
0.3VDD
0.8473
0.6931
0.5596
0.4418
0.3365
0.2412
0.1542
0.0741
-
From equation 1, Rup(min) = 5.5V/3mA = 1.83 k to operate the bus under a 5-V pull-up voltage and provide less
than 3 mA when the I2C device is driving the bus to a low state. If a higher sink current, for example 4 mA, is
allowed, Rup(min) can be as low as 1.375 k.
Given a 5-V I2C device with input low and high threshold voltages at 0.3 Vdd and 0.7 Vdd, the valued of k is
0.8473 from Table 6. Taking into account the 1.83-k
pull-up resistor, the maximum total load capacitance is
C(total-5V) = 645 pF. Ccable(max) should be restricted to be less than 545 pF if Csource and Ci can be as heavy as 50
pF. Here the Ci is treated as Csink, the load capacitance of a sink device.
Fixing the maximum transition time from Table 6, T = 1
μs, and using the k values from Table 6, the
recommended maximum total resistance of the pull-up resistors on an I2C bus can be calculated for different
system setups.
To support the maximum load capacitance specified in the HDMI spec, Ccable(max) = 700pF/Csource = 50pF/Ci =
50pF, R(max) can be calculated as shown in Table 7.
Table 7. Pull-Up Resistor Upon Different Threshold Voltages and 800-pF Loads
Vth-\Vth+
0.7VDD
0.65VDD
0.6VDD
0.55VDD
0.5VDD
0.45VDD
0.4VDD
0.35VDD
0.3VDD
UNIT
0.1VDD
1.14
1.32
1.54
1.80
2.13
2.54
3.08
3.84
4.97
k
0.15VDD
1.20
1.41
1.66
1.97
2.36
2.87
3.59
4.66
6.44
k
0.2VDD
1.27
1.51
1.80
2.17
2.66
3.34
4.35
6.02
9.36
k
0.25VDD
1.36
1.64
1.99
2.45
3.08
4.03
5.60
8.74
18.12
k
0.3VDD
1.48
1.80
2.23
2.83
3.72
5.18
8.11
16.87
-
k
Or, limiting the maximum load capacitance of each cable to be 400 pF to accommodate with I2C spec version
2.1. Ccable(max) = 400pF/Csource=50pF/Ci = 50pF, the maximum values of R are calculated as shown in Table 8.
Table 8. Pull-Up Resistor Upon Different Threshold Voltages and 500-pF Loads
Vth-\Vth+
0.7VDD
0.65VDD
0.6VDD
0.55VDD
0.5VDD
0.45VDD
0.4VDD
0.35VDD
0.3VDD
UNIT
0.1VDD
1.82
2.12
2.47
2.89
3.40
4.06
4.93
6.15
7.96
k
0.15VDD
1.92
2.25
2.65
3.14
3.77
4.59
5.74
7.46
10.30
k
0.2VDD
2.04
2.42
2.89
3.48
4.26
5.34
6.95
9.63
14.98
k
0.25VDD
2.18
2.62
3.18
3.92
4.93
6.45
8.96
13.98
28.99
k
0.3VDD
2.36
2.89
3.57
4.53
5.94
8.29
12.97
26.99
-
k
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 I2C bus.
When the input low and high level threshold voltages, Vth- and Vth+, are 0.7 V and 1.9 V, which is 0.15 VDD and
0.4 VDD approximately with VDD = 5 V, from Table 7, the maximum pull-up resistor is 3.59 k. The allowable
pull-up resistor is in the range of 1.83 k
and 3.59 k.
36
Copyright 2006–2007, Texas Instruments Incorporated
Product Folder Link(s): TMDS442
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