TMDS361 datasheet(26/42 Pages) TI

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TMDS361

SLLS919 – DECEMBER 2008............................................................................................................................................................................................ www.ti.com

Table 4. Value of k for Different Input Threshold Voltages

Vth–\Vth+

0.7 VDD

0.65 VDD

0.6 VDD

0.55 VDD

0.5 VDD

0.45 VDD

0.4 VDD

0.35 VDD

0.3 VDD

0.1 VDD

1.0986

0.9445

0.8109

0.6931

0.5878

0.4925

0.4055

0.3254

0.2513

0.15 VDD

1.0415

0.8873

0.7538

0.6360

0.5306

0.4353

0.3483

0.2683

0.1942

0.2 VDD

0.9808

0.8267

0.6931

0.5754

0.4700

0.3747

0.2877

0.2076

0.1335

0.25 VDD

0.9163

0.7621

0.6286

0.5108

0.4055

0.3102

0.2231

0.1431

0.0690

0.3 VDD

0.8473

0.6931

0.5596

0.4418

0.3365

0.2412

0.1542

0.0741

—

From Equation 1, Rup(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 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 kΩ.

Given a 5-V I2C device with input low and high threshold voltages at 0.3 Vdd and 0.7 Vdd, respectively, the value

of k is 0.8473 from Table 4. Taking into account the 1.83-k

Ω pullup 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 high as 50

pF. Here the Ci is treated as Csink, 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 I2C bus can be calculated for different

system setups.

To support the maximum load capacitance specified in the HDMI spec, Ccable(max) = 700 pF/Csource = 50 pF/Ci =

50 pF, R(max) can be calculated as shown in Table 5.

Table 5. Pullup Resistor for Different Threshold Voltages and 800-pF Load

Vth–\Vth+

0.7 VDD

0.65 VDD

0.6 VDD

0.55 VDD

0.5 VDD

0.45 VDD

0.4 VDD

0.35 VDD

0.3 VDD

UNIT

0.1 VDD

1.14

1.32

1.54

1.80

2.13

2.54

3.08

3.84

4.97

Ω

0.15 VDD

1.20

1.41

1.66

1.97

2.36

2.87

3.59

4.66

6.44

Ω

0.2 VDD

1.27

1.51

1.80

2.17

2.66

3.34

4.35

6.02

9.36

Ω

0.25 VDD

1.36

1.64

1.99

2.45

3.08

4.03

5.60

8.74

18.12

Ω

0.3 VDD

1.48

1.80

2.23

2.83

3.72

5.18

8.11

16.87

—

Ω

Or, limiting the maximum load capacitance of each cable to 400 pF to accommodate with I2C spec version 2.1.

Ccable(max) = 400 pF/Csource = 50 pF/Ci = 50 pF, the maximum values of R(max) are calculated as shown in Table 6.

Table 6. Pullup Resistor Upon Different Threshold Voltages and 500-pF Loads

Vth–\Vth+

0.7 VDD

0.65 VDD

0.6 VDD

0.55 VDD

0.5 VDD

0.45 VDD

0.4 VDD

0.35 VDD

0.3 VDD

UNIT

0.1 VDD

1.82

2.12

2.47

2.89

3.40

4.06

4.93

6.15

7.96

Ω

0.15 VDD

1.92

2.25

2.65

3.14

3.77

4.59

5.74

7.46

10.30

Ω

0.2 VDD

2.04

2.42

2.89

3.48

4.26

5.34

6.95

9.63

14.98

Ω

0.25 VDD

2.18

2.62

3.18

3.92

4.93

6.45

8.96

13.98

28.99

Ω

0.3 VDD

2.36

2.89

3.57

4.53

5.94

8.29

12.97

26.99

—

Ω

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, respectively, which is

0.15 VDD and 0.4 VDD, approximately. With VDD = 5 V from Table 5, the maximum pullup resistor is 3.59 kΩ. The

allowable pullup resistor is in the range of 1.83 k

Ω and 3.59 kΩ.

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