HCNR200 HCNR201数据手册(4)

As a fi nal example of circuit design fl exibility, the simpli-fi ed schematics in Figure 15 illus trate how to implement 4-20 mA analog current-loop transmitter and receiver circuits using the HCNR200/201 optocoupler. An impor-tant feature of these circuits is that the loop side of the circuit is powered entirely by the loop current, eliminat-ing the need for an isolated power supply.

The input and output circuits in Figure 15a are the same as the negative input and positive output circuits shown in Figures 13c and 13b, except for the addition of R3 and zener diode D1 on the input side of the circuit. D1 regu-lates the supply voltage for the input amplifi er, while R3 forms a current divider with R1 to scale the loop current down from 20 mA to an appropriate level for the input circuit (<50 μA).

As in the simpler circuits, the input amplifi er adjusts the LED current so that both of its input terminals are at the same voltage. The loop current is then divided

between R1 and R3. Iis given by the following equation:PD1 is equal to the current in R1 and

IPD1 = ILOOP*R3/(R1+R3).

Combining the above equation with the equations used for Figure 12a yields an overall expression relating the output voltage to the loop current,

VOUT/ILOOP = K*(R2*R3)/(R1+R3).

Again, you can see that the relationship is constant, lin-ear, and independent of the charac teristics of the LED. The 4-20 mA transmitter circuit in Figure 15b is a little dif-ferent from the previous circuits, partic ularly the output circuit. The output circuit does not directly generate an output voltage which is sensed by R2, it instead uses Q1 to generate an output current which fl ows through R3. This output current generates a voltage across R3, which is then sensed by R2. An analysis similar to the one above yields the following expression relating output current to input voltage:

ILOOP/VIN = K*(R2+R3)/(R1*R3).

模拟光耦

The preceding circuits were pre sented to illustrate the fl exibility in designing analog isolation circuits using the HCNR200/201. The next section presents several com-plete schematics to illustrate practical applications of the HCNR200/201.

Example Application Circuits

The circuit shown in Figure 16 is a high-speed low-cost circuit designed for use in the feedback path of switch-mode power supplies. This application requires good bandwidth, low cost and stable gain, but does not re-quire very high accuracy. This circuit is a good example of how a designer can trade off accuracy to achieve improve ments in bandwidth and cost. The circuit has a bandwidth of about 1.5 MHz with stable gain character-istics and requires few external components.

Although it may not appear so at fi rst glance, the circuit in Figure 16 is essentially the same as the circuit in Fig-ure 12a. Amplifi er A1 is comprised of Q1, Q2, R3 and R4, while amplifi er A2 is comprised of Q3, Q4, R5, R6 and R7. The circuit operates in the same manner as well; the only diff erence is the performance of amplifi ers A1 and A2. The lower gains, higher input currents and higher off set voltages aff ect the accuracy of the circuit, but not the way it operates. Because the basic circuit operation has not changed, the circuit still has good gain stability. The use of discrete transistors instead of op-amps allowed the design to trade off accuracy to achieve good band-width and gain stability at low cost.

To get into a little more detail about the circuit, R1 is se-lected to achieve an LED current of about 7-10 mA at the nominal input operating voltage according to the fol-lowing equation:

IF = (VIN/R1)/K1,

where K1 (i.e., IPD1/IF) of the optocoupler is typically about

0.5%. R2 is then selected to achieve the desired output volt age according to the equation,

VOUT/VIN = R2/R1.

The purpose of R4 and R6 is to improve the dynamic re-sponse (i.e., stability) of the input and output circuits by lowering the local loop gains. R3 and R5 are selected to provide enough current to drive the bases of Q2 and Q4. And R7 is selected so that Q4 operates at about the same collector current as Q2.

The next circuit, shown in Figure 17, is designed to achieve the highest possible accuracy at a reasonable cost. The high accuracy and wide dynamic range of the circuit is achieved by using low-cost precision op-amps with very low input bias currents and off set voltages and is limited by the performance of the opto coupler. The circuit is de-signed to operate with input and output voltages from 1 mV to 10 V.

The circuit operates in the same way as the others. The only major diff erences are the two compensa tion capaci-tors and additional LED drive circuitry. In the high-speed circuit discussed above, the input and output circuits are stabilized by reducing the local loop gains of the input and output circuits. Because reducing the loop gains would decrease the accuracy of the circuit, two compen-sation capacitors, C1 and C2, are instead used to improve circuit stability. These capacitors also limit the bandwidth of the circuit to about 10 kHz and can be used to reduce the output noise of the circuit by reducing its bandwidth even further.

The additional LED drive circuitry (Q1 and R3 through R6) helps to maintain the accuracy and band width of the circuit over the entire range of input voltages. Without these components, the transcon duc t ance of the LED driver would decrease at low input voltages and LED currents. This would reduce the loop gain of the input circuit, reducing circuit accuracy and bandwidth. D1 pre-vents excessive reverse voltage from being applied to the LED when the LED turns off completely.No off set adjustment of the circuit is necessary; the gain can be adjusted to unity by simply adjusting the 50 kohm poten tiometer that is part of R2. Any OP-97 type of op-amp can be used in the circuit, such as the LT1097 f …… 此处隐藏：6537字，全部文档内容请下载后查看。喜欢就下载吧 ……