SLLS612F June   2004  – February 2023 SN65HVD485E

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
    1.     6
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information
    5. 7.5  Electrical Characteristics: Driver
    6. 7.6  Electrical Characteristics: Receiver
    7. 7.7  Power Dissipation Characteristics
    8. 7.8  Supply Current
    9. 7.9  Switching Characteristics: Driver
    10. 7.10 Switching Characteristics: Receiver
    11. 7.11 Dissipation Ratings
    12. 7.12 Typical Characteristics
      1.      Parameter Measurement Information
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
    4. 8.4 Device Functional Modes
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
        1. 9.2.1.1 Data Rate and Bus Length
        2. 9.2.1.2 Stub Length
        3. 9.2.1.3 Bus Loading
        4. 9.2.1.4 Receiver Failsafe
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Power Usage in an RS-485 Transceiver
      3. 9.2.3 Application Curve
    3. 9.3 Power Supply Recommendations
    4. 9.4 Layout
      1. 9.4.1 Layout Guidelines
      2. 9.4.2 Layout Example
  10. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Device Nomenclature
        1. 10.1.1.1 Thermal Characteristics of IC Packages
    2. 10.2 Documentation Support
      1. 10.2.1 Related Documentation
    3. 10.3 Support Resources
    4. 10.4 Trademarks
    5. 10.5 Electrostatic Discharge Caution
    6. 10.6 Glossary
  11. 11Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

9.2.2.1 Power Usage in an RS-485 Transceiver

Power consumption is a concern in many applications. Power supply current is delivered to the bus load and to the transceiver circuitry. For a typical RS-485 bus configuration, the load that an active driver must drive consists of all of the receiving nodes plus the termination resistors at each end of the bus.

The load presented by the receiving nodes depends on the input impedance of the receiver. The TIA/EIA-485-A standard defines a unit load as allowing up to 1 mA. With up to 32 unit loads allowed on the bus, the total current supplied to all receivers can be as high as 32 mA. The SN65HVD485E device is rated as a ½ unit load device, so up to 64 can be connected on one bus.

The current in the termination resistors depends on the differential bus voltage. The standard requires active drivers to produce at least 1.5 V of differential signal. For a bus terminated with one standard 120-Ω resistor at each end, this sums to 25-mA differential output current whenever the bus is active. Typically, the SN65HVD485E device can drive more than 25 mA to a 60-Ω load, which results in a differential output voltage higher than the minimum required by the standard (see Figure 7-2).

Supply current increases with signaling rate primarily because of the totem pole outputs of the driver. When these outputs change state, there is a moment when both the high-side and low-side output transistors are conducting, which creates a short spike in the supply current. As the frequency of state changes increases, more power is used.