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HCPL-2602 Datasheet(PDF) 14 Page - Agilent(Hewlett-Packard) |
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HCPL-2602 Datasheet(HTML) 14 Page - Agilent(Hewlett-Packard) |
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14 / 16 page  1-179 Figure 22. TTL Interface Circuit for the HCPL-261A/- 261N Families. 420 Ω (MAX) 1 3 2 4 2N3906 (ANY PNP) VCC 74L504 (ANY TTL/CMOS GATE) HCPL-261X LED anode/cathode and output ground (CLA and CLC). Also shown in Figure 21 on the input side is an AC-equivalent circuit. 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 rejec- tion (CMRL, 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), CMRL 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. CMRH, 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 consti- tutes a CMRH failure). By using the recommended circuit in Figure 20, good CMR can be achieved. (In the case of the -261N families, a minimum CMR of 15 kV/ µs is guaranteed using this circuit.) The balanced ILED-setting resistors help equalize ILP and ILN to reduce the amount by which ILED is modulated from transient coupling through CLA and CLC. CMR with Other Drive Circuits CMR performance with drive circuits other than that shown in Figure 20 may be enhanced by following these guidelines: 1. Use of drive circuits where current is shunted from the LED in the LED “off” state (as shown in Figures 22 and 23). This is beneficial for good CMRH. 2. Use of IFH > 3.5 mA. This is good for high CMRL. Using any one of the drive circuits in Figures 22-24 with IF = 10 mA will result in a typical CMR of 8 kV/ µs for the HCPL- 261N family, as long as the PC board layout practices are followed. Figure 22 shows a circuit which can be used with any totem-pole-output TTL/ LSTTL/HCMOS logic gate. The buffer PNP transistor allows the circuit to be used with logic devices which have low current- sinking capability. It also helps maintain the driving-gate power- supply current at a constant level to minimize ground shifting for other devices connected to the input-supply ground. When using an open-collector TTL or open-drain CMOS logic gate, the circuit in Figure 23 may be used. When using a CMOS gate to drive the optocoupler, the circuit shown in Figure 24 may be used. The diode in parallel with the RLED speeds the turn-off of the optocoupler LED. Figure 21. AC Equivalent Circuit for HCPL-261X. 350 Ω 1/2 RLED VCC+ 15 pF + VCM 8 7 6 1 3 SHIELD 5 2 4 CLA VO GND 0.01 µF 1/2 RLED CLC ILN ILP – |
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