Datasheet

TCA4311A
SCPS226A –JANUARY 2011–REVISED JULY 2012 www.ti.com
Resistor Pull-Up Value Selection
The system pullup resistors must be strong enough to provide a positive slew rate of 1.25 V/μs on the SDA and
SCL pins, in order to activate the boost pullup currents during rising edges. Choose maximum resistor value R
using the formula:
R ≤ (V
CC(MIN)
– 0.6) (800,000) / C
where R is the pullup resistor value in ohms, V
CC(MIN)
is the minimum V
CC
voltage and C is the equivalent bus
capacitance in picofarads (pF).
In addition, regardless of the bus capacitance, always choose R ≤ 16 kΩ for V
CC
= 5.5 V maximum, R ≤ 24 kΩ
for V
CC
= 3.6 V maximum. The start-up circuitry requires logic high voltages on SDAOUT and SCLOUT to
connect the backplane to the card, and these pullup values are needed to overcome the precharge voltage.
Systems With Disparate Supply Voltages
In large 2-wire systems, the V
CC
voltages seen by devices at various points in the system can differ by a few
hundred millivolts or more. This situation is well modeled by a series resistor in the V
CC
line, as shown in
Figure 13. For proper operation of the TCA4311A, make sure that V
CC(BUS)
≥ V
CC(TCA4311A)
– 0.5 V.
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