Current Source Design
243
Todd Owen
Convert temperature to current at high linearity with current source
current source and error amplifier, together with the ratio of two external resistors, to program a constant output current at any level between 0.5mA and 200mA. The flat tempera- ture coefficient of the internal reference current (highlighted in Figure 243.2) is as good as many voltage references. Low TC resistors do not need to be used; the temperature coef- ficients of the external resistors need only match one another for optimum results.
No frequency compensation or supply bypass capaci- tors are needed. Frequency compensation is internal and the internal reference circuitry is buffered to protect it from line changes.
No input-to-output capacitors are required. While exten- sive testing has been done to ensure stable operation under the widest possible set of conditions, complex load imped- ance conditions could provoke instability. As such, testing in situ with final component values is highly recommended. If stability issues occur, they can be resolved with small capaci- tors or series RC combinations placed on the input, output, or from input to output.
The LT3092 offers all the protection features expected from a high performance product: thermal shutdown, over- current protection, reverse-voltage and reverse-current pro- tection. Because a simple resistor ratio sets the current, a wide variety of techniques can be utilized to adjust the
Electronics 101
One of the first lessons in a basic electronics course covers the symbols for resistors, capacitors, inductors, voltage sources and current sources. Although each symbol represents a functional component of a real-world circuit, only some of the symbols have direct physical counterparts. For instance, the three dis- crete passive devices—resistors, capacitors, inductors—can be picked off a shelf and placed on a real board much as their symbolic analogs appear in a basic schematic. Likewise, while voltage sources have no direct 2-terminal analog, a voltage source can be easily built with an off-the-shelf linear regulator.
The black sheep of basic electronics symbols has long been the 2-terminal current source. The symbol shows up in every basic electronics course, but Electronics 101 instructors must take time to explain away the lack of a real-world equivalent.
The symbol presents a simple electronics concept, but building a current source has, until now, been a complex undertaking.
A real 2-terminal current source
With the introduction of the LT3092, it is now as easy to pro- duce a 2-terminal current source as it is to create a voltage source. Figure 243.1 shows how the LT3092 uses an internal
Figure 243.1 • 2-Terminal Current Source Requires Only Two
Resistors to Program Figure 243.2 • SET Pin Current vs Temperature
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00243-X
current on the fly. The LT3092 can also be configured as a linear regulator without output capacitors for use in intrinsic safety environments.
The LT3092 as a T-to-I converter
Omega’s 44200 series linear thermistor kits1 include thermis- tors and resistors that together create a linear response to temperature when appropriately configured. These kits gener- ate either a voltage or resistance proportional to temperature with high accuracy; the #44201 kit is listed for the 0°C to 100°C temperature range with 0.15°C accuracy.
Obviously, these kits easily satisfy the needs of a wide vari- ety of applications, but problems arise when the thermistor must be placed at the end of a long wire—application infor- mation from Omega suggests no more than 100 feet of #22 wire for thermistor kit #44201. Wire impedance interferes with the thermistor resistance and defeats the accuracy inher- ent in the kit.
By adding the LT3092 to the thermistor kit along with three 0.1% accuracy resistors and one final trim, a very accu- rate 2-terminal temperature-to-current converter can be built.
This circuit measures 700μA operating current at 0°C, drop- ping by 2μA every degree until 100°C, at which point the cur- rent measures 500μA. The obvious advantage to this T-to-I converter over a T-to-V converter is that current remains con- stant regardless of the wire length—as long as there is suffi- cient voltage to meet the compliance of the LT3092 circuit
while not exceeding its absolute maximum. Electronics 101:
Kirchoff ’s laws dictate conservation of current in the wire runs as long as there are no nodes for current to leak along the run.
Figure 243.3 shows the schematic for linear thermistor kit
#44201 from Omega with the LT3092 and the additional resistor values. The formulas under the figure allow for sub- stitution of other thermistor kit values and determination of appropriate complementary resistors to fit the application.
Once the initial circuit is built, any initial tolerance, vari- ations, and offsets are easily trimmed out by connecting a voltmeter from node A to node B and trimming the poten- tiometer to measure 302mV (for this design). This voltage remains constant regardless of temperature.
Now, one wire runs out and back for temperature sensing at significant distances. By providing input voltage above the compliance level of the LT3092 (less than 2V for this circuit and resistor combination) and sensing the resultant current (use a 1k resistor and DVM) one can measure temperature.
Figure 243.4 shows the current output from the circuit across temperature and the difference between measured and calcu- lated response.
Conclusion
The LT3092 requires only two external resistors to produce a 2-terminal current source that references to input or ground, or sits in series with signal lines.
A 2-terminal current source enables a number of applica- tions, especially those involving long wire runs, as Kirchoff ’s laws dictate the conservation of current over long wire dis- tances—distances where a voltage signal would be corrupted.
The example presented here uses the LT3092 and a linear thermistor kit to convert temperature to current, creating a 2-terminal current output thermometer. Placing this in series with long distances of wire maintains accuracy despite the dis- tance of wire used.
244
David Ng
Versatile current source safely and quickly charges everything from large capacitors to batteries
Simple strobe capacitor charger
Figure 244.1 shows a LT3750 circuit that charges a 400μF strobe capacitor to 300V. This capacitor and voltage combi- nation is typical of professional photoflash systems, security devices and automotive light strobes. The target voltage is set by the two resistors R2 and R3, which together monitor the MOSFET drain voltage. This voltage, when referenced to the input rail, is directly proportional to the output potential while power is being transferred to the output capacitor. The LT3750 compares this to an internal reference and terminates the charge cycle when the output has reached the desired target voltage, after which the LT3750 sets the DONE bit to signal the system microcontroller that the charge cycle is complete.
Introduction
The LT3750 is a current mode flyback controller optimized to easily and efficiently provide a controlled current to charge just about any capacitive energy storage device. The LT3750’s simple but flexible feature set allows it to handle a wide vari- ety of charging needs. These include large high voltage capaci- tors for professional photoflash equipment and emergency beacons, small capacitors that are charged and discharged thousands of times a second, and batteries for long term energy needs.
Safe, small and flexible
All of the control and feedback functions of the LT3750 are referred to the charger’s input. The target voltage is set by just two resistors in a simple, low voltage network that monitors the flyback voltage of the transformer. When charging a capacitor to a high voltage, there is no need to con- nect any components to the hazardously high output poten- tial. The charging current is a triangle wave whose amplitude is set by an external sense resistor and the flyback transformer turns ratio.
The LT3750 operates in boundary mode, at the edge of continuous and discontinuous conduction, which significantly reduces switching losses. This in turn allows for high fre- quency operation, and a correspondingly small flyback trans- former size. The LT3750 is itself tiny, available in a 10-lead MSOP package.
The LT3750 is also compatible with a wide range of control circuitry. It is equipped with a simple interface consisting of a CHARGE command input bit and an open-drain DONE status flag. Both of these signals are compatible with most digital systems, yet tolerate voltages as high as 24V. The LT3750 operates from 3V to 24V DC.
Figure 244.1 • LT3750 Circuit Charges 400μF Capacitor to 300V.
Danger High Voltage—Operation by High Voltage Trained Personnel Only
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00244-1
As shown in Figure 244.2, the LT3750 charges the 400μF to 300V in about 0.92 seconds when the circuit is powered from a 12V source. Note that the output current amplitude is constant throughout the charge cycle.
Charge small capacitors fast
Many devices need to provide energy to a transducer mul- tiple times per second, such as diagnostic equipment and device testers. Figure 244.3 shows that, for the same circuit
as in Figure 244.1, the LT3750 is capable of charging a 0.1μF capacitor to 300V in just 180μs. The only change in the cir- cuit is the replacement of the 400μF output cap with one that is much smaller. The performance of the circuit is essen- tially the same, other than the charge time. As far as the output device is concerned, the LT3750 circuit is a current source.
Charge batteries too
Another type of system that needs a controlled current source is a fast charger for a lead-acid battery. A fast charger for a lead-acid battery differs from the capacitor charging applica- tions in that it needs to charge at high current, but at a much lower voltage. Figure 244.4 shows a circuit that charges at 6A until the lead-acid battery potential reaches the 14V float voltage. Again, the circuit is remarkably similar to the previ- ous two designs—the transformer turns ratio is now 1:1 and the R2 set resistor has been changed to set the target float voltage to 14V. Other float voltages may be accommodated by simply changing R2 to the appropriate value.
When the battery voltage reaches 14V, the LT3750 sets the DONE bit. This can then be used to signal the system micro- controller, which can then enter a “trickle-charge” mode by setting the CHARGE bit at a fixed, low frequency interval.
Conclusion
The LT3750 is an easy-to-use controller that is ideal for appli- cations where there is a need to charge an energy storage device to a predetermined target voltage. Its unique architec- ture allows it to be used in just about any application where a controlled current source is needed, with almost no limitation on the output voltage.
Figure 244.2 • LT3750 Charges 400μF to 300V in 0.92 Seconds
Figure 244.3 • LT3750 Charges 0.1μF to 300V in 180μs