HCPL-3180-500 Просмотр технического описания (PDF) - HP => Agilent Technologies

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HCPL-3180-500

2 Amp Output Current, High Speed IGBT/MOSFET Gate Drive Optocoupler

HP => Agilent Technologies 

LED Drive Circuit Considerations for

Ultra High CMR Performance

Without a detector shield, the

dominant cause of optocoupler

CMR failure is capacitive

coupling from the input side of

the optocoupler, through the

package, to the detector IC as

shown in Figure 29. The HCPL-

3180 improves CMR

performance by using a detector

IC with an optically transparent

Faraday shield, which diverts

the capacitively coupled current

away from the sensitive IC

circuitry. However, this shield

does not eliminate the capacitive

coupling between the LED and

optocoupler pins 5-8 as shown

in Figure 30. This capacitive

coupling causes perturbations in

the LED current during common

mode transients and becomes

the major source of CMR failures

for a shielded optocoupler. The

main design objective of a high

CMR LED drive circuit becomes

keeping the LED in the proper

state (on or off ) during common

mode transients. For example,

the recommended application

circuit (Figure 25), can achieve

10 kV/us CMR while minimizing

component complexity.

Figure 29. Optocoupler Input to Output

Capacitance Model for Unshielded

Optocouplers.

Figure 31. Equivalent Circuit for Figure 25

During Common Mode Transient.

Techniques to keep the LED in

the proper state are discussed in

the next two sections.

Figure 30. Optocoupler Input to Output

Capacitance Model for Shielded

Optocouplers.

Vcc= 20V

Figure 32. Not Recommended Open Collector

Drive Circuit.

Figure 33. Recommended LED Drive Circuit for

Ultra-High CMR

16

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