PCA85232DA datasheet(37/54 Pages) NXP

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PCA85232

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Product data sheet

Rev. 1 — 8 December 2010

37 of 54

NXP Semiconductors

PCA85232

LCD driver for low multiplex rates

12. Application information

12.1 Pull-up resistor sizing on I2C-bus

12.1.1 Max value of pull-up resistor

The bus capacitance (Cb) is the total capacitance of wire, connections, and pins. This

capacitance on pin SDA limits the maximum value of the pull-up resistor (RPU) due to the

specified rise time.

According to the I2C-bus specification the rise time (tr) is defined between the VDD related

input threshold of VIL =0.3VDD and VIH =0.7VDD. The value for tr(max) is 300 ns.

tr will be calculated with Equation 8:

(8)

whereas t1 and t2 are the time since the charging started. The values for t1 and t2 are

derivatives of the functions V(t1) and V(t2):

(9)

(10)

with the results of

(11)

(12)

(13)

RPU(max) is a function of the rise time (tr) and the bus capacitance (Cb) and will be

calculated with Equation 14:

(14)

12.1.2 Min value of pull-up resistor

The supply voltage limits the minimum value of resistor RPU due to the specified minimum

sink current (see value of IOL on pin SDAACK in Table 18). RPU(min) as a function of VDD is

calculated with Equation 15:

(15)

The designer now has the minimum and maximum value of RPU. The values for RPU(max)

and RPU(min) are shown in Figure 26 and Figure 27.

–

Vt1

()

0.3V

DD 1e

-t1 R

PU Cb

⁄

–

()

Vt2

()

0.7V

DD 1e

-t2 R

PU Cb

⁄

–

()

–

PUCb

ln 0.7

()

–

PUCb

ln 0.3

()

–

PUCb

ln 0.3

()

PUCb

ln 0.7

()

PU max

()

0.8473C

-----------------------

300

10 9

–

0.8473C

--------------------------

PU min

()

–

--------------------------

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