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HCPL-3760 Datasheet(PDF) 13 Page - Agilent(Hewlett-Packard) |
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HCPL-3760 Datasheet(HTML) 13 Page - Agilent(Hewlett-Packard) |
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13 / 13 page  1-360 Electrical Considerations The HCPL-3700/3760 optocoup- lers have internal temperature compensated, predictable voltage and current threshold points which allow selection of an external resistor, RX, to determine larger external threshold voltage levels. For a desired external threshold voltage, V± , a corre- sponding typical value of RX can be obtained from Figure 12. Specific calculation of RX can be obtained from Equation (1). Specification of both V+ and V- voltage threshold levels simul- taneously can be obtained by the use of RX and RP as shown in Figure 13 and determined by Equations (2) and (3). RX can provide over-current transient protection by limiting input current during a transient condition. For monitoring con- tacts of a relay or switch, the HCPL-3700/3760 in combination with RX and RP can be used to allow a specific current to be conducted through the contacts for cleaning purposes (wetting current). The choice of which input voltage clamp level to choose depends upon the application of this device (see Figure 1). It is recom- mended that the low clamp condition be used when possible. The low clamp condition in conjunction with the low input current feature will ensure extremely low input power dissipation. In applications where dVCM/dt may be extremely large (such as static discharge), a series resistor, RCC, should be connected in series with VCC and Pin 8 to pro- tect the detector IC from destruc- tively high surge currents. See Note 13 for determination of RCC. In addition, it is recommended that a ceramic disc bypass capacitor of 0.01 µF be placed between Pins 8 and 5 to reduce the effect of power supply noise. For interfacing ac signals to TTL systems, output low pass filtering can be performed with a pullup resistor of 1.5 k Ω and 20 µF capacitor. This application requires a Schmitt trigger gate to avoid slow rise time chatter problems. For ac input applica- tions, a filter capacitor can be placed across the dc input terminals for either signal or transient filtering. Either ac (Pins 1, 4) or dc (Pins 2, 3) input can be used to determine external threshold levels. V + - VTH+ (-) (-) R X = (1) I TH+ (-) For two specifically selected external threshold voltage levels, V+ and V-, the use of RX and RP will permit this selection via equations (2), (3) provided the following conditions are met. If the denominator of equation (2) is positive, then V+ VTH+ V+ - VTH+ ITH+ ≥ and < V- VTH- V- - VTH- ITH- Conversely, if the denominator of equation (2) is negative, then V+ VTH+ V+ - VTH+ ITH+ ≤ and > V- VTH- V- - VTH- ITH- VTH- (V+) - VTH+ (V-) RX = (2) ITH+ (VTH-) - ITH- (VTH+) VTH- (V+) - VTH+ (V-) RP = (3) ITH+(V--VTH-)+ITH-(VTH+-V+) Figure 13. External Threshold Voltage Level Selection. For one specifically selected external threshold voltage level V+ or V-, RX can be determined without use of RP via |
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