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ACNW261L-500E Datasheet(PDF) 16 Page - AVAGO TECHNOLOGIES LIMITED |
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ACNW261L-500E Datasheet(HTML) 16 Page - AVAGO TECHNOLOGIES LIMITED |
16 / 17 page 16 Table 1 indicates the directions of ILP and ILN flow depending on the direction of the common-mode transient. For transients occurring when the LED is on, common-mode rejection (CML, since the output is in the “low” state) depends upon the amount of LED current drive (IF). For conditions where IF is close to the switching threshold (ITH), CML also depends on the extent which ILP and ILN balance each other. In other words, any condition where common-mode transients cause a momentary decrease in IF (i.e. when dVCM/dt>0 and |IFP| > |IFN|, referring to Table 1) will cause common-mode failure for transients which are fast enough. Likewise for common-mode transients which occur when the LED is off (i.e. CMH, since the output is “high”), 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 balanced ILED-setting resistors help equalize the common mode voltage change at anode and cathode to reduce the amount by which ILED is modulated from transient coupling through CLA and CLC. Figure 12. Recommended drive circuit for ACPL-061L/ACPL-C61L/ACNW261L for high-CMR Figure 13. AC equivalent of ACPL-061L/ACPL-C61L/ACNW261L This CMR optimization method fundamentally makes use of the differential input capability of the LED input. By ensuring the common mode impedance on both the cathode and anode of the LED are balanced, it effectively nullifies the effect of a CMR transient on the LED. This is most easily achieved by splitting the input bias resistor into two (as shown in Figure 9). Split resistor configuration for ACPL-061L/ACPL-C61L/ ACNW261L Figure 12 shows the recommended drive circuit for the ACPL-061L/ACPL-C61L/ACNW261L for optimal common- mode rejection performance. Two LED-current setting resistors are used to balance the common mode impe- dance at LED anode and cathode. Common-mode tran- sients can capacitively couple from the LED anode (or cathode) to the output-side ground causing current to be shunted away from the LED (which can be bad if the LED is on) or conversely cause current to be injected into the LED (bad if the LED is meant to be off). Figure12 shows the parasitic capacitances which exists between LED anode/ cathode and output ground (CLA and CLC). 8 5 6 Anode VDD VO Shield Cathode 7 VE 0.1 µF R1 R2 CLA CLC 2 3 1 4 8 5 6 Shield 7 0.1 µF R1 R2 GND2 RT = R1 + R2, R1/R2 ≈ 1.5 2 3 1 4 Anode Cathode VI GND1 VDD VO VE |
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Similar Description - ACNW261L-500E |
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