TCA4311A Datasheet, PDF (8/22 Pages) Texas Instruments – HOT SWAPPABLE 2-WIRE BUS BUFFERS

TCA4311A
SCPS226 – JANUARY 2011
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/ms 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 15. For proper operation of the TCA4311A, make sure that V_CC(BUS) ≥ V_CC(TCA4311A) – 0.5 V.
8
Submit Documentation Feedback
Product Folder Link(s): TCA4311A
Copyright © 2011, Texas Instruments Incorporated