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P82B715DR Datasheet(PDF) 7 Page - Texas Instruments |
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P82B715DR Datasheet(HTML) 7 Page - Texas Instruments |
7 / 17 page www.ti.com APPLICATION INFORMATION V CC Special Buffered Bus Special Buffered Bus Standard I C Bus 2 Standard I C Bus 2 SDA SCL I C Device 2 SDA SCL P82B715 P82B715 LDA LCL Long Cable ½ ½ ½ ½ I 2C Systems Pullup Resistance Calculation P82B715 I2C BUS EXTENDER SCPS145A – DECEMBER 2007 – REVISED FEBRUARY 2008 By using two (or more) P82B715 devices, a subsystem can be built that retains the interface characteristics of a normal I2C device so that the subsystem may be included in, or added to, any I2C or related system. The subsystem features a low-impedance or buffered bus capable of driving large wiring capacitance (see Figure 3). Figure 3. Minimum Subsystem Diagram The P82B715 can operate with a supply voltage from 3 V to 12.5 V, but the logic-signal levels at Sx/Lx are independent of the supply voltage. They remain at the levels presented to the chip by the attached devices. The maximum static I2C bus sink current, 3 mA, flowing in either direction in the internal current sense resistor, causes a difference less than 100 mV in the bus logic low levels at Sx and Lx. This makes P82B715 fully compatible with all logic signal drivers, including TTL. The P82B715 cannot modify the bus logic signal voltage levels, but it contains internal diodes connected between Lx/Sx and VCC that conduct and limit the logic signal swing if the applied logic levels would have exceeded the supply voltage by more than 0.7 V. In normal applications, external pullup resistors pull the connected buses up to the desired voltage high level. Usually this is the supply voltage, VCC, but for very low logic voltages, it is necessary to use a VCC of at least 3.3 V and preferably higher. Note that full performance over temperature is ensured only from 4.5 V. Specification deratings apply when its supply voltage is reduced below 4.5 V. The absolute minimum VCC is 3 V. As in standard I2C systems, pullup resistors are required to provide the logic high levels on the buffered bus, as the standard open-collector configuration is retained. The size and number of pullup resistors depends on the system. If P82B715 devices are to be permanently connected into a system, the circuit may be configured with only one pullup resistor on the buffered bus and none on the I2C buses, but the system design is simplified, and performance is improved by fitting separate pullups on each section of the bus. When a subsystem using P82B715 may be optionally connected to an existing I2C system that already has a pullup, the effects of the subsystem pullups acting in parallel with the existing I2C bus pullup must be considered. When calculating the pullup resistance values, the gain of the buffer introduces scaling factors that must be applied to the system components. In practical systems, the pullup resistance value is calculated to meet the rise time limit for I2C systems. As an approximation, this limit is satisfied in a 100-kHz system if the time constant of the total system (product of the net resistance and net capacitance) is set to 1 µs or less. In systems using the P82B715, it is convenient to set the total system time constant by considering each bus node separately (i.e., the I2C nodes and the buffered bus node) and selecting a separate pullup resistor for each node to provide time constants of less than 1 µs. If each node complies then the system requirement is also met. Copyright © 2007–2008, Texas Instruments Incorporated Submit Documentation Feedback 7 Product Folder Link(s): P82B715 |
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