The LTC1485 is a low-power differential bus/line transceiver designed for standard RS485 applications for multipoint data transmission with an extended common model range (12V to -7V). At the same time meet the requirements of RS422. CMOS using Schottky's design offers significant power savings compared to bipolar CMOS without sacrificing robustness against overload or ESD damage.
Pin arrangement
A typical connection for the LTC1485 is shown below. Two twisted pairs connect up to 32 driver/receiver pairs for half duplex data transmission. There is no limit to where the chip can be connected to the wire, and there is no need to attach the chip at the end. However, the wire must only end at a resistance equal to its characteristic impedance, usually 120Ω. The input impedance of the receiver is usually 20k to GND, or 0.6 unit RS485 load, so in practice 50 to 60 transceivers can be connected to the same wire. Optional shielding around the twisted pair helps reduce unnecessary noise and connects to GND at one end.
The driver and receiver have three-state outputs, and the driver output maintains a high impedance throughout the common model perimeter. Excessive power consumption due to bus contention or failure is prevented by a thermal shutdown circuit that forces the driver output into a high impedance state.
Test circuit
I/O pins protect against multiple ESD strikes over ±10kV. The receiver has a fail-safe feature that guarantees a high output state when the input is turned on. Both AC and DC specifications are guaranteed in the supply voltage range of -40°C to 85°C and 4.75V to 5.25V
Features
● ESD Protection Over ±10kV
● Low Power: ICC = 1.8mA Typ
● 28ns Typical Driver Propagation Delays with 4ns Skew
● Designed for RS485 or RS422 Applications
● Single 5V Supply
● −7V to 12V Bus Common-Mode Range Permits ±7V Ground Difference Between Devices on the Bus
● Thermal Shutdown Protection
● Power-Up/Down Glitch-Free Driver Outputs
● Driver Maintains High Impedance in Three-State or with the Power Off
● Combined Impedance of a Driver Output and Receiver Allows up to 32 Transceivers on the Bus
● 60mV Typical Input Hysteresis
● Pin-Compatible with the SN75176A, DS75176A,and SN75LBC176
Applications
● Low Power RS485/RS422 Transceiver
● Level Translator
