ACPL-M61L-560E 데이터 시트보기 (PDF) - Avago Technologies

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ACPL-M61L-560E

Ultra Low Power 10 MBd Digital CMOS Optocouplers

Avago Technologies 

Split LED Bias Resistor for Optimum CMR

Figure 11 shows the recommended drive circuit for the

ACPL-x6xL that gives optimum common-mode rejection.

The two current setting resistors balance the common

mode impedances at the LED’s anode and cathode.

Common-mode transients can capacitively couple from

the LED anode (or cathode) to the output-side ground

causing current to be shunted away from the LED (which

is not wanted when the LED should be on) or conversely

cause current to be injected into the LED (which is not

wanted when the LED should be off ).

Figure 12 shows the parasitic capacitances (CLA and

CLC) between the LED’s anode and cathode, and output

ground. Also shown in Figure 12 on the input side is an

AC-equivalent circuit.

Table 1 shows the directions of ILP and ILN depend on the

polarity of the common-mode transient. For transients

occurring when the LED is on, common-mode rejection

(CML, since the output is at "low" state) depends on

LED current (IF). For conditions where IF is close to the

switching threshold (ITH), CML also depends on the extent

to which ILP and ILN balance each other. In other words,

any condition where a common-mode transient causes a

momentary decrease in IF (i.e. when dVCM/dt > 0 and |IFP|

> |IFN|, referring to Table 1). will cause a common-mode

failure for transients which are fast enough.

Likewise for a common-mode transient that occurs when

the LED is off (i.e. CMH, since the output is at "high" state),

if an imbalance between ILP and ILN results in a transient

IF equal to or greater than the switching threshold of the

optocoupler, the transient “signal” may cause the output

to spike below 2 V, which constitutes a CMH failure.

The resistors recommended in Figure 11 include both

the output impedance of the logic driver circuit and

the external limiting resistor. The balanced ILED-setting

resistors help equalize the common mode voltage change

at the anode and cathode. This reduces ILED changes

caused by transient coupling through the parasitic capaci-

tors CLA and CLC shown in Figure 12.

For ACPL-M61L/W61L:

VDD = 3.3 V: R1 = 510 Ω ± 1%, R2 = 360 Ω ± 1%

VDD = 5.0 V: R1 = 1000 Ω ± 1%, R2 = 680 Ω ± 1%

RT = R1 + R2 R1/R2 ≈ 1.5

For ACPL-064L/K64L:

VDD = 3.3 V: R1 = 430 Ω ± 1%, R2 = 430 Ω ± 1%

VDD = 5.0 V: R1 = 845 Ω ± 1%, R2 = 845 Ω ± 1%

RT = R1 + R2 R1/R2 ≈ 1

VDD

74LS04 or any

totem-pole output

logic gate

GND1

R1

R2

Shield

VDD2

VO

0.1µF

GND2

Figure 11. Recommended high-CMR drive circuit for the ACPL-x6xL.

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