Electronic Design

Constant-Current Source Source Simplifies Sensor Driving

Performance specs can be trumped, but elegance persists. Consider the most basic task in industrial control: driving sensors and bridges. It doesn’t matter if you’re dealing with a strain-gauge bridge in a weigh scale, or a temperature sensor, or some other kind of simple transducer. You need a driver, and an ideal driver would be a predictable, stable, constant-current source with minimal variation over temperature.

The usual driver circuits require a fair number of parts. There are several ways to produce a constantcurrent source using common components, such as a depletion-mode FET and a resistor, Zeners and transistors, or a pass transistor driven by an op amp. But they all have drawbacks, whether it’s a high temperature coefficient, bill of materials and assembly cost, or the nuisance of being tied to a ground reference when you really want something you can float.

SOPHISTICATED SIMPLICITY
In May, Linear Technology announced a basic building block with wide applicability and the design imprint of CTO Bob Dobkin (see “Monolithic Programmable Constant-Current Source Is A New Basic Building Block”). Its LT3092 is a 0.5- to 200-mA, two-terminal, low-temperature-coefficient, constantcurrent- source IC. Conceptually, it’s simple. But no company had previously been able to create a design like it, which required a certain amount of process-technology wizardry.

The LT3092 is a three-terminal bipolar device. It comes in several small-outline package variants. Inside, it has a precision 10-µA current source and a voltagefollower circuit (Fig. 1). The terminal designations are INPUT, SET, and OUT. The current source resides between INPUT and SET, and the SET node is at the input to the voltage follower. The voltage follower is set up so whatever voltage appears on the SET node also appears on the OUT pin. In the most basic configuration, the only external components required are two resistors.

To use it, apply a dc voltage (up to 40 V) on the INPUT pin and connect a resistor in series with the SET pin. When the other side of that resistor is connected, the chip will drive a highly regulated 10 µA through it. The second resistor goes between the OUT pin and the sensor that is to be driven. The first resistor is tied to the same point in the circuit. In operation, the voltage follower drives the voltage on OUT to the same value as the voltage on SET.

For example, if the resistor on the SET pin has a value of 20 k, that 10 µA will develop 200 mV across it. If the resistor on the OUT pin were 1 , the current being driven through it would be a constant 200 mA. If it were 10 , the current would be 20 mA, and if it were 100 , it would be 2 mA. Linear says the regulation is better than 10 ppm/V (Fig. 2).

“The resistor absolute values aren’t too critical,” Dobkin said. “Making the voltage caused by the precision current source and the SET resistor 200 mV leads to the error in the current from the current source and the offset of the op amp being about equal. If you make the voltage drop across the SET resistor bigger, the op-amp offset introduces less error.”

Fully floating, the LT3092 does not need to be ground-referenced. You can tie your loads on the top side or the bottom side. In almost every case, Dobkin said, the LT3092 does the job better, with fewer parts, for much less than $2.00, even at low-volume pricing.

The process-technology “secret sauce” shows up in the absence of a need for bypass capacitance. “We were developing another part with an internal regulator and we found it didn’t need much bypass capacitance,” Dobkin said. “And we kept on working on it until it didn’t need any across the full range of voltage, current, and temperature ranges. When we first started, I didn’t know we could get to zero capacitance, but we did.”

While the LT3092 doesn’t require input or output capacitors to frequency-compensate the circuit in many applications, that doesn’t mean you’ll never use a capacitor when designing with one. According to the datasheet, “Some current-source applications will use a capacitor connected in parallel with the SET pin resistor to lower the current source’s noise. This capacitor also provides a soft-start function for the current source.”

This opens the door to instability and leads to further compensation. The datasheet notes that in this case, “external compensation is usually required to maintain stability and compensate for the introduced pole.” Then it goes on to describe how to deal with situations in which the IC has to drive complex impedances. That’s not really a problem when driving simple sensors, but it’s obvious that Linear has anticipated broader applicability for the part.

How much noise are we talking about? The datasheet says that normally, the 10-µA reference current source generates noise current levels of 2.7 pA/vHz (0.7 nARMS over a 10-Hz to 100-kHz bandwidth), while the SET pin resistor generates Boltzmann noise that is RMS-summed with the 10-µA reference current source’s noise.

TIPS FROM THE TOP
“If you need more power, just parallel them. It’s like two Zeners, where, if you want more voltage, you put them in series,” Dobkin said. But how do you handle dissipation when you need a lot of current?

“This is a small package, but you can go up to 40 V on the input and 200 mA to your load. That’s 8 W. To bring the inpackage dissipation down to something it can handle, put a resistor across the package. That way, at high voltage, most of the current goes through the resistor, and at low voltage, most of the current goes through the devices in the package. In the middle, make the currents equal, so you wind up with four times less power dissipation in the IC,” he said.

“Let’s say you did have that 8 W, with 200 mA at 40 V. Now your worse-case point is at 20 V, and at 20 V, we have 200 mA split between the device and the resistor, so the dissipation is 2 W, and we can handle that with one IC,” Dobkin continued.

The datasheet shows more configurations, such as connecting the ICs in series to handle input voltages over 40 V. The datasheet also lists 0.5- to 200-mA output current with 1.0% initial current accuracy and 10-ppm/V current regulation from an input voltage range of 1.2 to 40 V dc.

Safety features include reverse-battery and reverse-current protection, overcurrent limiting, and thermal shutdown. Pricing starts at $1.65 each in 1000-unit lots, with pricing up to 99 units at $2.36.

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