ISO721D 3.3/5V High-Speed Digital Isolators

Texas Instruments' optocoupler replacement for harsh environments

Texas Instruments ISO721D 3.3/5V High-Speed Digital IsolatorsThe design of LED lighting systems for extreme environments must ensure the projected lifetime of the product. This is particularly the case when the power converter is sealed and installed on a streetlight where the cost of replacement or maintenance becomes significant. One of the weak links in an isolated converter is the degradation through time and temperature of the characteristics of the analog optocoupler. Texas Instruments' innovative family of capacitive digital isolators is leading the industry with the best speed and life expectancy.

The ISO721D is a digital isolator with a logic input and output buffer separated by a silicon dioxide (SiO2) insulation barrier. This barrier provides galvanic isolation of up to 4000V. Used in conjunction with isolated power supplies, these devices prevent noise currents on a data bus or other circuits from entering the local ground and interfering with or damaging sensitive circuitry.   ISO721D Datasheet

  • Overview
  • Features & Applications
  • Diagram
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A binary input signal is conditioned, translated to a balanced signal, and then differentiated by the capacitive isolation barrier. Across the isolation barrier, a differential comparator receives the logic transition information, and then sets or resets a flip-flop and the output circuit accordingly. A periodic update pulse is sent across the barrier to ensure the proper DC level of the output. If this DC-refresh pulse is not received for more than 4µs, the input is assumed to be un-powered or not being actively driven and the failsafe circuit drives the output to a logic-high state.

  • 4000V(peak) isolation, 560Vpeak VIORM
    • UL 1577, IEC 60747-5-2 (VDE 0884, Rev. 2),
      IEC 61010-1, IEC 60950-1, and CSA approved
    • 50kV/µs transient immunity, typical
  • Signaling rate 0Mbps to 150Mbps
    • Low propagation delay
    • Low pulse skew (pulse-width distortion)
  • Low-power sleep mode
  • High electromagnetic immunity
  • Low input-current requirement
  • Failsafe output
  • Drop-in replacement for most opto and magnetic isolators

  • Industrial fieldbus
    • Modbus
    • Profibus
    • DeviceNet™ data buses
    • Smart distributed systems (SDS™)
  • Computer peripheral interface
  • Servo control interface
  • Data acquisition

RoHS Compliant


ISO721D GraphU1B is configured as a high frequency sawtooth generator. The sawtooth signal amplitude and slope defines the system input dynamic range and gain. U1A compares the input analog feedback signal with the sawtooth generated by the U1B.

The output of U1A is a PWM signal. The duty cycle is proportional to the analog input amplitude. In this figure the analog input is 1/2Vcc resulting in a 50% duty cycle at the input of the isolator (bottom trace).

The result is a system with a gain of two and little phase lag. The system can be easily incorporated in an isolated power supply as an analog optocoupler replacement.

ISO721D GraphThe result is consistent with the Phase-Gain Bode Plots traces.

ISO721D GraphTI’s TINA simulation software confirmed that the phase lag (loss) at 40kHz is consistent with the phase lag of the output RC filter. Simulation to 100kHz of the dual RC output filter.

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Texas Instruments
Part Number
Part Number
Voltage -
Data Rate
3 V ~ 5.5 V
100 Mbps
17 ns
(0.154", 3.90 mm width)