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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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5962-9685201KPC датащи(PDF) 8 Page - Agilent(Hewlett-Packard) |
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5962-9685201KPC датащи(HTML) 8 Page - Agilent(Hewlett-Packard) |
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8 / 16 page  8 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 14. The HCPL- 530X 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 the opto- coupler output pins and output ground as shown in Figure 15. 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 13), can achieve 10 kV/ µs CMR while minimizing compo- nent complexity. Note that a CMOS gate is recommended in Figure 13 to keep the LED off when the gate is in the high state. Another cause of CMR failure for a shielded optocoupler is direct coupling to the optocoupler output pins through CLEDO1 and CLEDO2 in Figure 15. Many factors influence the effect and magni- tude of the direct coupling including: the use of an internal or external output pull-up resistor, the position of the LED current setting resistor, the connection of the unused input package pins, and the value of the capacitor at the optocoupler output (CL). Techniques to keep the LED in the proper state and minimize the effect of the direct coupling are discussed in the next two sections. CMR With The LED On (CMRL) A high CMR LED drive circuit must keep the LED on during common mode transients. This is achieved by overdriving the LED current beyond the input threshold so that it is not pulled below the threshold during a transient. The recommended minimum LED current of 10 mA provides adequate margin over the maximum ITH of 5.0 mA (see Figure 1) to achieve 10 kV/ µs CMR. Capacitive coupling is higher when the internal load resistor is used (due to CLEDO2) and an IF = 16mA is required to obtain 10 kV/ µs CMR. The placement of the LED current setting resistor affects the ability of the drive circuit to keep the LED on during transients and interacts with the direct coupling to the optocoupler output. For example, the LED resistor in Figure 16 is connected to the anode. Figure 17 shows the AC equivalent circuit for Figure 16 during common mode transients. During a +dVCM/dt in Figure 17, the current available at the LED anode (ITOTAL) is limited by the series resistor. The LED current (IF) is reduced from its DC value by an amount equal to the current that flows through CLEDP and CLEDO1. The situation is made worse because the current through CLEDO1 has the effect of trying to pull the output high (toward a CMR failure) at the same time the LED current is being reduced. For this reason, the recommended LED drive circuit (Figure 13) places the current setting resistor in series with the LED cathode. Figure 18 is the AC equivalent circuit for Figure 13 during common mode transients. In this case, the LED current is not reduced during a +dVCM/dt transient because the current flowing through the package capacitance is supplied by the power supply. During a -dVCM/dt transient, however, the LED current is reduced by the amount of current flowing through CLEDN. But better CMR performance is achieved since the current flowing in CLEDO1 during a negative transient acts to keep the output low. Coupling to the LED and output pins is also affected by the connection of pins 1 and 4. If CMR is limited by perturbations in the LED on current, as it is for the recommended drive circuit (Figure 13), pins 1 and 4 should be connected to the input circuit common. However, if CMR performance is limited by direct coupling to the output when the LED is off, pins 1 and 4 should be left unconnected. CMR With The LED Off (CMRH) A high CMR LED drive circuit must keep the LED off (VF ≤ VF(OFF)) during common mode transients. For example, during a +dVCM/dt transient in Figure 18, the current flowing through CLEDN is supplied by the parallel combination of the LED and series resistor. As long as the voltage developed across the resistor is less than VF(OFF) the LED will remain off and no |
Аналогичный номер детали - 5962-9685201KPC |
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Аналогичное описание - 5962-9685201KPC |
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