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Isolated Power

Timing specifications are important in digital isolator applications to ensure proper and consistent system operation

Operating Power

At a minimum, optocouplers require current to bias the LED and some form of bias on the output side. The total input plus output current varies widely, depending on the type of optocoupler.

When forward biased, the optocoupler LED is low-impedance, and device power consumption increases with LED forward current, which can range from 1 mA to over 15 mA. In some cases, the LED may require an external driver, further decreasing system efficiency while increasing BOM complexity and cost. The optocoupler output impedance can be low or high depending on its architecture. Most low-cost optocouplers have a simple transistor output that is highimpedance when LED forward current is at zero and relatively lower impedance when LED forward current is in its specified operating range. Other (usually higher speed) optocouplers have an active photo coupler and output driver that requires an external bias voltage. Such devices have low output impedance but at the expense of increased total operating current, which can range from 15 mA to over 40 mA.

Compared to optocouplers, ISOpro isolators offer significantly higher operating efficiency, consuming approximately 1.7 mA per channel at 10 Mbps at VDD = 5.0 V with a 15 pF load. Its high-impedance input buffer consumes only microamps of leakage current while its 50 CMOS output driver can source or sink 4 mA. The bulk of the ISOpro isolator's power savings results from the use of an RF carrier instead of light, eliminating the power-hungry LED. Losses in the isolation path are minimized by the isolation capacitor structures, which are optimized for robust data transfer and minimum power loss. ISOpro isolators' power dissipation remains relatively flat and substantially less than that of the optocoupler. The only noteworthy contributor to increased supply current is increased data rate.

Timing Characteristics

Timing specifications are important in digital isolator applications to ensure proper and consistent system operation. Propagation delays were measured with different LED currents, both with and without a "peaking" capacitor.

Note: The peaking capacitor in this case is 20 pF in parallel with the LED current limit resistor.

This capacitor momentarily increases LED current during turn-on and turn-off for faster optocoupler response.

A 0.5 mA decrease in LED current results in a 50% increase in propagation time (80 ns to 120 ns) at 20 °C, demonstrating the large timing variations that result from changes in LED current and/or wear-out. Propagation delay is not symmetrical; curve A shows a high-to-low transition fastest propagation time of roughly 35 ns at 20 °C, but the low-to-high transition is twice the propagation delay time. Therefore, a system using this component must take into account these asymmetric delays and provide additional timing margin. While this example demonstrates the changes in propagation time, it is important to note that other optocoupler timing parameters, such as pulse width distortion, channel-to-channel matching, rise and fall time, etc., will follow the same trend because all timing behavior, including LOP effects, is related to LED emission.

Unlike the optocoupler, the ISOpro isolator's timing parameters are a function of internal precision timing ic circuits and fixed propagation delays within its signal path. All timing parameters vary only slightly with changes in VDD, and all remain flat over temperature. For example, rise and fall times vary by only 1 ns across temperature and supply voltage, and worst-case propagation time is approximately 9 ns at 120 °C.

About the Author

Silicon Labs - 8051 Microcontroller and Optocoupler. Industry leader in the innovation of high-performance, analog-intensive, mixed-signal ICs. Developed by a world-class engineering team with unsurpassed expertise in mixed-signal design.