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Solving Current Source Design Challenges

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Solving Current Source Design Challenges Current Sources Solving Current Source Design Challenges Simple, high performance two-terminal current source Compared to other analog circuitry, current source design appears relatively easy on the surface, but in reality it is more complicated than meets the eye. While high quality voltage sources are commonplace, current sources, as components, have remained elusive. Furthermore, two-terminal current sources generate a new set of problems, especially if high accuracy and stability over temperature are desired. By Robert Dobkin, Vice President, Engineering & Chief Technical Offi cer, Linear Technology Corporation current source must operate ear Technology, overcomes the prob- techniques have been used for stable over a wide voltage range, lems of earlier two-terminal current operation without a supply bypass ca- A have high DC and AC imped- sources. It has better than 1% initial pacitor, allowing it to provide high AC ance when connected in series with accuracy and a very low temperature impedance as well as high DC imped- unknown reactance and exhibit good coeffi cient. Output currents can be ance. Transient and start-up times are regulation and a low temperature coef- set from 0.5mA to 200mA, and current about 20µS. fi cient. For optimal two-terminal solu- regulation is typically 10ppm per volt. tions, no power supply bypass capaci- The LT3092 operates down to 1.5V or Figure 1 shows a basic diagram of tor should be used since it degrades up to 40V. This gives an impedance Linear Technology’s LT3092 current AC impedance. of 100MOhms at 1mA or 1MOhm at regulator. The architecture is similar to 100mA. Unlike almost any other ana- Linear’s LT3080 voltage regulator, but A new device, the LT3092 from Lin- log integrated circuit, special design it uses a PNP transistor as the output Figure 1: Two-terminal current source. Figure 2: Reduced dissipation. www.powersystemsdesign.com 35 NA_TF16.indd 35 2009-4-5 10:12:36 Current Sources device instead of an NPN. Internal However, the regulation of the circuitry is differential and buff- loop is so good that in all but the ered, with a regulator to isolate it most extreme cases, 100mV to from power supply changes. This 200mV across the set resistor will isolation allows stable operation be fine. without bypass capacitors. Ad- ditionally, for environments that Reducing power may have power supply reversal, dissipation the LT3092 is immune to damage For high set currents and high from reverse power supply voltage voltage, there is considerable and does not conduct current, so power dissipation in the LT3092. it protects the load. For example, 30 volts and 100 milliamps equals 3 watts of dissi- The internal current source pation, which can result in signifi- and the offset of the amplifier are cant temperature rise depending designed to reject power supply on the thermal resistance of the changes by 100dB or better, so PC board. An external resistor the regulation is very good. Set- can shift a portion of the power to ting the RSet down to zero allows the resistor and reduce the power the output to be adjusted down to dissipation in the LT3092. Figure zero. 2 shows the basic current source with a resistor RX from the input A small voltage is impressed to the output of the device. As upon an external set resistor, 20k long as the total current is more in this case, to generate a 200mV than the current through RX, reference. That forces 200mV regulation is not impaired and the across a current-determining current source impendence does resistor R, and the total current is not change. Figure 3: Stacked current sources for higher then equal to 0.2V divided by R operating voltage. (plus 10µA). The current regulator Current through RX is within works from about 1.5V across it the feedback loop and gets up to 36V, and the current regula- compensated as the voltage from tion and temperature stability is input to output changes. The extremely good. As a two-terminal current flows through the internal current source, the load can be PNP transistor or the external either in the positive leg or in the resistor while the feedback loop ground leg of the circuit. keeps the total current constant. The 200mV generated reference For good regulation and to is chosen to equalize the errors ensure reasonable margin, the due to changes in the internal current through RX should not current source and in the offset of be any larger then 90% of the de- the amplifier with supply voltage. sired current for the device at the With supply changes, the internal maximum voltage. The formulas current sources change approxi- in the illustration show how to mately 50pA per volt. The offset of choose RX so that the current the internal op amp changes less through RX always leaves at than 5µV per volt. Assuming worst least 10% of the current flowing case for both the current source through the LT3092. This drops and the offset of the amplifier, Figure 4: Intrinsically safe regulator—no the maximum internal power a 200mV reference contributes capacitors needed. down by shifting some power to equal error from both the amplifier the external resistor. The result is and the internal current source. If the internal op amp offset declines. This a significant reduction in device dis- 200mV is increased to 500mV using improves the regulation of the cur- sipation, as well as a reduced rise in a 50k resistor, the contribution of the rent source against supply changes. temperature. There is negligible effect 36 Power Systems Design North America March/April 2009 NA_TF16.indd 36 2009-4-5 10:12:38 Current Sources on the performance of the circuit by current control goes from one current voltage at the output pin as the Set including this external resistor. source to the other, there is a small pin. The load is connected from the discontinuity in the output current output pin to ground. Increasing voltage compliance equal to the error between the two For higher voltages, current sources current sources. Typically this is less Conclusion can be stacked to operate at a higher than 1% and again no bypass capaci- As a new IC, the LT3092 solves cur- total voltage. Figure 3 shows stacked tors are needed to make the device rent source design, which is a more current sources. work. difficult challenge than a voltage regu- lator design. Current source values of Two current sources are set up for Intrinsically safe as a voltage 1mA up to high currents are achiev- the same currents and a voltage- regulator able using a single unit or paralleled limiting Zener is placed across each The LT3092 will act as a voltage units. Regulation against line, load and of the current sources. At low voltage, regulator that needs no output capaci- temperature is excellent and unique whichever current source has the in- tor. “Intrinsically safe” applications are design techniques have made the de- crementally higher current will saturate usually designed with low current, and vice operable without bypass capaci- and the current will be controlled by small or no capacitors. Figure 4 shows tors even though there is a complex the other current source. As the volt- the LT3092 as a 200mA regulator. As feedback circuit internally. This device age increases, at some point the Zener a voltage regulator, the 10µA cur- adds a versatile component to a de- breaks down and starts to conduct. rent that’s generated internally flows signer’s toolbox. Then the voltage across the saturated through an external RSet resistor. This current source starts to increase and 10µA times the RSet value impresses www.linear.com it regulates the current as the volt- a voltage on the Set pin. The internal age continues to increase. When the voltage follower provides the same A Powerful Combination PowerPack and ePowerPack is a new advertising program in print and online designed to promote your company’s new products, seminars, and announcements while driving traffi c to your company’s website. Advertisers receive a 100 word listing, plus product photo and url link in print to 20, 357+ subscribers of Power Systems Design’s magazine and online through ePowerPack e-newsletter which is delivered every month to an audience of 24,000 power elec- tronic engineers in Pan Europe and 32,000 power design engineers in North America. To participate contact: [email protected] PowerPack Power Systems Design PowerPack Power Systems Design PowefulComboAd.indd 1 3/13/08 5:58:47 PM www.powersystemsdesign.com 37 NA_TF16.indd 37 2009-4-5 10:12:39.
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