DS28CN01U+ MAXIM [Maxim Integrated Products], DS28CN01U+ Datasheet - Page 8

• Manufacturer Part Number: DS28CN01U+
• Manufacturer: MAXIM [Maxim Integrated Products]

Application Information
SDA and SCL Pullup Resistors
SDA is an open-drain output on the DS28CN01 that requires a pullup resistor (Figure 6) to realize high logic levels.
Because the DS28CN01 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 R
According to the I²C specification, a slave device must be able to sink at least 3mA at a V
specification requires a current sink capability of 4mA at 0.4V. The DS28CN01 can sink at least 4mA at 0.4V V
over its entire operating voltage range. This DC characteristic determines the minimum value of the pullup resistor:
R
1.275kΩ. The "Minimum R
(pullup) voltage.
Figure 6. Application Schematic
For I²C systems, the rise time and fall time are measured from 30% to 70% of the pullup voltage. The maximum
bus capacitance C
maximum resistor value at any given capacitance C
capacitance of 400pF the maximum pullup resistor would be 885Ω.
Since an 885Ω pullup resistor, as would be required to meet the rise time specification and 400pF bus capacitance,
is lower than R
first calculating the minimum pullup resistor at any given operating voltage ("Minimum R
the respective bus capacitance that yields a rise time of 300ns.
Only for pullup voltages of 4V and lower can the maximum permissible bus capacitance of 400pF be maintained. A
reduced bus capacitance of 300pF is acceptable for the entire operating voltage range. The corresponding pullup
resistor value at the voltage is indicated by the "Minimum R
Pmin
= (V
CC
- 0.4V)/4mA. With a maximum operating voltage of 5.5V, the minimum value for the pullup resistor is
Pmin
P
B
at 5.5V, a different approach is necessary. The "Max Load…" line in Figure 7 is generated by
Sizing
is 400pF. The maximum rise time must not exceed 300ns. Assuming maximum rise time, the
V
CC
P
GND
µC
V
" line in Figure 7 shows how the minimum pullup resistor changes with the operating
CC
SDA
SCL
R
P
B
8 of 9
is calculated as: R
P
" line.
R
P
SDA
SCL
AD1
AD0 GND
DS28CN01
V
CC
Pmax
= 300ns / (CB × ln(7/3)). For a bus
To additional
devices
P
" line) and then calculating
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of 0.4V. The SMBus
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