6

FN6303.2

October 14, 2011

Functional Description

DDC Overview

DDC is a 2-wire serial communication standard based on the

clock (SCL) and one data (SDA) line. These are both

bidirectional.

Each signal is connected to a positive supply voltage via a

current-source or pull-up resistor (see “System Diagram” on

page 3). When the bus is free, both lines are HIGH. The

output stages of all devices connected to the bus must have

an open-drain or open-collector to perform the wired-AND

function.

Simple pull-up resistors on the clock and data lines work well

unless there are long signal lines. The combined

capacitance of long cables increases the rise time on the

signal to such an extent that the communication becomes

unreliable or fails to meet the bus timing specifications.

Smaller value resistors can sometimes compensate for the

extra capacitance, but this increases the current

consumption when the signal lines are pulled LOW.

ISL54103 Operation

To improve the operation of the DDC where larger bus

capacitance exists, the ISL54103 provides active pull-up

using switched current sources. When the bus is idle and

both lines are HIGH, a standby pull-up current of 100µA is

used to maintain the signal level while minimizing power

consumption. When either of the two signals is pulled LOW,

noise margin.

FIGURE 7. BOOST PULL-UP CURRENT vs DDC VOLTAGE

0.50

0.55

0.60

0.65

0.70

0.45

0.40

0.35

0.30

-40

-20

0

204060

80

100

-60

TEMPERATURE (°C)

VDD = 5.5V

VDD = 2.7V

75

80

85

90

95

70

65

60

55

-40

-20

0

204060

80

100

-60

TEMPERATURE (°C)

VDD = 5.5V

VDD = 2.7V

2.0

2.5

3.0

3.5

1.5

1.0

0.5

0

12

34

5

0

DDC VOLTAGE (V)

6

VDD = 5.0V

VDD = 2.7V

ISL54103