SLLA526C October   2020  – September 2023 ISO6720-Q1 , ISO6721 , ISO6721-Q1 , ISO6731 , ISO6740 , ISO6741 , ISO6741-Q1 , ISO6742 , ISO7021 , ISO7041 , ISO7131CC , ISO7140CC , ISO7140FCC , ISO7141CC , ISO7141FCC , ISO7142CC , ISO7142CC-Q1 , ISO721 , ISO721-Q1 , ISO721M , ISO721M-EP , ISO722 , ISO722-Q1 , ISO7220A , ISO7220A-Q1 , ISO7220B , ISO7220C , ISO7220M , ISO7221A , ISO7221A-Q1 , ISO7221B , ISO7221C , ISO7221C-HT , ISO7221C-Q1 , ISO7221M , ISO722M , ISO7230C , ISO7230M , ISO7231C , ISO7231C-Q1 , ISO7231M , ISO7240C , ISO7240CF , ISO7240CF-Q1 , ISO7240M , ISO7241A-EP , ISO7241C , ISO7241C-Q1 , ISO7241M , ISO7242C , ISO7242C-Q1 , ISO7242M , ISO7310-Q1 , ISO7310C , ISO7310FC , ISO7320-Q1 , ISO7320C , ISO7320FC , ISO7321-Q1 , ISO7321C , ISO7321FC , ISO7330-Q1 , ISO7330C , ISO7330FC , ISO7331-Q1 , ISO7331C , ISO7331FC , ISO7340-Q1 , ISO7340C , ISO7340FC , ISO7341-Q1 , ISO7341C , ISO7341FC , ISO7342-Q1 , ISO7342C , ISO7342FC , ISO7420 , ISO7420E , ISO7420FCC , ISO7420FE , ISO7420M , ISO7421 , ISO7421-EP , ISO7421A-Q1 , ISO7421E , ISO7421E-Q1 , ISO7421FE , ISO7520C , ISO7521C , ISO7631FC , ISO7631FM , ISO7640FM , ISO7641FC , ISO7641FM , ISO7710 , ISO7710-Q1 , ISO7720 , ISO7720-Q1 , ISO7721 , ISO7721-Q1 , ISO7730 , ISO7730-Q1 , ISO7731 , ISO7731-Q1 , ISO7740 , ISO7740-Q1 , ISO7741 , ISO7741-Q1 , ISO7741E-Q1 , ISO7742 , ISO7742-Q1 , ISO7760 , ISO7760-Q1 , ISO7761 , ISO7761-Q1 , ISO7762 , ISO7762-Q1 , ISO7763 , ISO7763-Q1 , ISO7810 , ISO7820 , ISO7821 , ISO7830 , ISO7831 , ISO7840 , ISO7841 , ISO7842 , ISOW7821 , ISOW7840 , ISOW7841 , ISOW7841A-Q1 , ISOW7842 , ISOW7843 , ISOW7844

 

  1.   1
  2.   Abstract
  3. 1Isolator Construction
  4. 2Switching Performance
  5. 3Isolator Lifetime Through TDDB Test
  6. 4Solution Size
  7. 5Aging and Reliability
  8. 6Common-Mode Transient Immunity (CMTI)
  9. 7Optocoupler Current Input vs Digital Isolator CMOS Voltage Input
  10. 8Conclusion
  11. 9References

Switching Performance

Isolators are extensively used in many industrial and automotive applications where isolation of data, control or status signals is needed. To enable processing of the isolated data, control, or status signals in a timely manner, it is critical for the isolator to have optimum switching characteristics, minimizing the impact on the overall system timing performance. Optocouplers fare very poorly when it comes to switching characteristics whereas digital isolators offer one of the best switching characteristics in the industry, enabling more systems to meet their performance requirements.

General purpose optocouplers usually do not have any supported data rates mentioned in their data sheets, making it difficult to know their suitability for a given application. Most of these optocouplers also have an open-collector output, due to which they are only characterized to a few select pullup or load resistor values. One of TI’s latest digital isolators, ISO6741, has its maximum supported data rate clearly specified in the data sheet as 50Mbps, which makes it easy to know its suitability for a given application. Unlike optocouplers, digital isolators do not require any external pullup resistors for operation and the maximum data rate is not heavily dependent on external components.

Table 2-1 compares timing specifications of a general purpose optocoupler with TI digital isolators. The information in the table also estimates the asynchronous and synchronous data rates that are achieved using the data sheet timing specifications. Table 2-1 shows that the data rate achieved using a general purpose optocoupler is much lower than what can be achieved using digital isolators. The two pullup resistor options listed with RL = 100 Ω and RL = 1.9 kΩ for optocoupler consume significantly higher current compared to digital isolators, making them unsuitable for many applications.

Table 2-1 Timing Specifications of General Purpose Optocoupler vs TI Digital Isolators
Part Number General Purpose Optocoupler ISO7741 ISO6741
Parameter RL = 100 Ω RL = 1.9 kΩ VCC = 5 V VCC = 5 V

Input forward current / ICC1 per channel (typ, mA)

2.0

16.0

2.2

1.8

On state collector current / ICC2 per channel (typ, mA)

50.0

2.6

4.5

3.2

Rise time, tr (typ, µs)

2.0

0.8(1)

0.002

0.005

Fall time, tf (typ, µs)

3.0

35.0(1)

0.002

0.005

Turn on time / propagation delay, tpHL (typ, µs)

3.0

0.5

0.011

0.011

Turn off time / propagation delay, tpLH (typ, µs)

3.0

40.0

0.011

0.011

Propagation delay skew, tsk (max, ns)

-

-

0.004

0.006

Max asynchronous data rate ((T = max(tr, tf) × 2/0.6 + tsk), typ, Mbps)

0.1

0.008

80.6

47.6

Max synchronous data rate ((T = max(tpHL, tpLH) × 4), typ, Mbps)

0.028

0.006

23.4

22.7

Estimated Values

High-speed optocouplers offer better switching characteristics compared to general-purpose optocouplers. Table 2-2 compares a typical high-speed optocoupler with TI digital isolators in which the asynchronous and synchronous data rates for the devices are estimated using the timing specifications given in their respective data sheets. As shown in the comparison table, digital isolators still support much higher data rate compared to the high-speed optocoupler.

Table 2-2 Timing Specifications of High-Speed Optocoupler vs TI Digital Isolators
Part Number High-Speed Optocoupler ISO7741 ISO6741
Parameter IF = 14 mA IF = 6 mA VCC = 5 V VCC = 5 V

Input forward current / ICC1 per channel (typ, mA)

14.0

6.0

2.2

1.8

Rise time, tr (typ, ns)

15.0

15.0

2.4

4.5

Fall time, tf (typ, ns)

15.0

15.0

2.4

4.5

Turn on time / propagation delay, tPHL (typ, ns)

33.0

40.0

10.7

11

Turn off time / propagation delay, tPLH (typ, ns)

27.0

30.0

10.7

11

Propagation delay skew, tsk (max, ns)

30.0

30.0

4.4

6

Max asynchronous data rate ((T = max(tr, tf) × 2/0.6 + tsk), typ, Mbps)

12.5

12.5

80.6

47.6

Max synchronous data rate ((T = max(tpHL, tpLH) × 4), typ, Mbps)

7.6

6.3

23.4

22.7