Datasheet NCV5230 (ON Semiconductor) - 9

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NCV5230 REMOTE TRANSDUCER WITH CURRENT
Where VCC min is the worst−case power supply voltage
TRANSMISSION
(approximately 1.8 V) that will still keep the part There are many ways to transmit information along two operational. As an example, when using a 15 V remote wires, but current transmission is the most beneficial when power supply, a current sensing resistor of 1.0 W, and an the sensing of remote signals is the aim. It is further input voltage (VIN) of 20 mV, the output current (IL) is enhanced in the form of 4.0 to 20 mA information which is 20 mA. Furthermore, a load resistance of zero to used in many control−type systems. This method of approximately 650 W can be inserted in the loop without any transmission provides immunity from line voltage drops, change in current when the bias current−control pin is tied large load resistance variations, and voltage noise pickup. to the negative supply pin. The voltage drop across the load The zero reference of 4mA not only can show if there is a and line resistance will not affect the NCV5230 because it break in the line when no current is flowing, but also can will operate down to 1.8 V. With a 15 V remote supply, the power the transducer at the remote location. Usually the voltage available at the amplifier is still enough to power it transducer itself is not equipped to provide for the current with the maximum 20 mA output into the 650 W load. transmission. The unique features of the NCV5230 can provide high output current capability coupled with low VCC I power consumption. It can be remotely connected to the OUT 3 7 REMOTE + + V transducer to create a current loop with minimal external POWER 6 − SUPPLY components. The circuits for this are shown in Figures 6 NCV5230 and 7. Here, the part is configured as a voltage−to−current, 2 5 − or transconductance amplifier. This is a novel circuit that 4 VEE takes advantage of the NCV5230’s large open−loop gain. In T R RL AC applications, the load current will decrease as the A N S open−loop gain rolls off in magnitude. The low offset D U VIN voltage and current sinking capabilities of the NCV5230 C RC E must also be considered in this application. R The NCV5230 circuit shown in Figure 6 is a pseudo transistor configuration. The inverting input is equivalent to the “base,” the point where VEE and the non−inverting input NOTES: meet is the “emitter,” and the connection after the output 1. IOUT = VIN/RC diode meets the V V For R CC pin is the collector. The output diode 2. R REMOTE * 1.8V * VINMAX C = 1.0 W L MAX ≈ is essential to keep the output from saturating in this I I V OUT OUT IN configuration. From here it can be seen that the base and 4mA 4mV 20mA 20mV emitter form a voltage−follower and the voltage present at R
Figure 6. The NCV5230 as a Remote Transducer
C must equal the input voltage present at the inverting input. Also, the emitter and collector form a
Transconductance Amp with 4.0 − 20 mA Current
current−follower and the current flowing through R
Transmission Output Capability
C is equivalent to the current through RL and the amplifier. This sets up the current loop. Therefore, the following equation can be formulated for the working current transmission line. + The load current is: RC V VIN I IN L + (eq. 2) RC VCC − 3 7 + + V and proportional to the input voltage for a set R CC C. Also, the 6 − current is constant no matter what load resistance is used NCV5230 while within the operating bandwidth range of the op amp. 2 5 − 4 When the NCV5230’s supply voltage falls past a certain VEE + I point, the current cannot remain constant. This is the OUT RL “voltage compliance” and is very good for this application because of the near rail output voltage. The equation that determines the voltage compliance as well as the largest
Figure 7. The Same Type of Circuit as Figure 6, but
possible load resistor for the NCV5230 is as follows:
for Sourcing Current to the Load
ƪVremote supply * VCC min* VIN maxƫ RLmax + IL (eq. 3)
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