DS2482X-101-U View Datasheet(PDF) - Maxim Integrated

Part Name

Description

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DS2482X-101-U

Single-Channel 1-Wire Master with Sleep Mode

Maxim Integrated 

Single-Channel 1-Wire Master with Sleep Mode

Applications Information

SDA and SCL Pullup Resistors

SDA is an open-drain output on the DS2482-101 that

requires a pullup resistor to realize high-logic levels.

Because the DS2482-101 uses SCL only as input (no

clock stretching), the master can drive SCL either

through an open-drain/-collector output with a pullup

resistor or a push-pull output.

Pullup Resistor RP Sizing

According to the I2C specification, a slave device must

be able to sink at least 3mA at a VOL of 0.4V. This DC

condition determines the minimum value of the pullup

resistor as:

RP(MIN) = (VCC - 0.4V)/3mA

With an operating voltage of 5.5V, the minimum value

for the pullup resistor is 1.7kΩ. The “MINIMUM RP” line

in Figure 11 shows how the minimum pullup resistor

changes with the operating voltage.

For I2C systems, the rise time and fall time are mea-

sured from 30% to 70% of the pullup voltage. The maxi-

mum bus capacitance, CB, is 400pF. The maximum rise

time must not exceed 1000ns at standard speed and

300ns at fast speed. Assuming maximum rise time, the

maximum resistor value at any given capacitance CB is

calculated as:

RPMAXS = 1000ns/[CB x ln(7/3)] (standard speed)

RPMAXF = 300ns/[CB x ln(7/3)] (fast speed)

For a bus capacitance of 400pF, the maximum pullup

resistor values are 2.95kΩ at standard speed and 885Ω

at fast speed. A value between 1.7kΩ and 2.95kΩ

meets all requirements at standard speed.

Because an 885Ω pullup resistor, as would be required

to meet the rise time specification at fast speed and

400pF bus capacitance, is lower than RP(MIN) at 5.5V, a

different approach is necessary. The “MAX LOAD AT

MIN RP FAST MODE” line in Figure 11 is generated by

first calculating the minimum pullup resistor at any

given operating voltage (“MINIMUM RP” line) and then

calculating the respective bus capacitance that yields a

300ns rise time.

Only for pullup voltages of 3V and lower can the maximum

permissible bus capacitance of 400pF be maintained. A

reduced bus capacitance of 300pF is acceptable for

pullup voltages of 4V and lower. For fast speed operation

at any pullup voltage, the bus capacitance must not

exceed 200pF. The corresponding pullup resistor value

at the voltage is indicated by the “MINIMUM RP” line.

2000

500

1600

400

1200

MINIMUM RP

800

400

300

200

MAX LOAD AT MIN RP FAST MODE

100

0

0

1

2

3

4

5

PULLUP VOLTAGE (V)

Figure 11. I2C Fast Mode Pullup Resistor Selection Chart

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