Shunting Linear Regulators Spreads Power and Heat

The basic 3-terminal regulator has been a staple in the designer's toolbox for over 30 years, and its basic architecture has not changed significantly. Using a fixed voltage reference, a resistor divider boosts the output voltage to the desired value. This type of regulator is very easy to use and therefore very popular, but this simple architecture has some inherent disadvantages.

One of the disadvantages of using traditional linear regulators is that the minimum output voltage is limited by the regulator reference voltage. Another disadvantage is that it is not easy to increase the available output current or spread the power consumption by paralleling devices. To distribute the load between multiple regulators, either large ballast resistors must be added, which will cause load regulation errors, or complex circuits consisting of input sense resistors and op amp loops must be used to balance the load, which will inevitably destroy the promise of simplicity that is intended to be achieved by using a seemingly simple linear regulator.

But what if we remove the voltage reference and replace it with a precision current source? The resulting circuit is incredibly simple, as shown in the block diagram of Figure 1. A precision current source is connected to the non-negative input of the amplifier, and the output drives a large NPN pass element, which is then connected to the negative input of the amplifier to provide unity gain. Introducing this small change to the age-old linear regulator device yields huge benefits in versatility and performance.

Figure 1: LT3083 block diagram

Now, in this new architecture, to parallel regulators, each SET pin is simply connected together, which provides a common reference point for all error amplifiers, thereby balancing any device-to-device offset variations with only milliohm ballast resistors. Suddenly, it is very easy to spread the power dissipation among no matter how many devices are needed, and similarly, the output current can be scaled up as needed. The beauty of this architecture is that only one resistor can provide a reference point for all regulators, whether one, 10, or 100 regulators are used. In addition, this architecture allows zero resistance to equal zero output, eliminating the need for a fixed reference voltage to limit the low end of the available output voltage range.

Benefits of the new architecture

The LT3080 1.1A linear regulator is the first linear regulator to utilize a precision current source architecture, making it possible to parallel any number of LT3080s to create a high current, surface mount power supply. The LT3083 is similar to the LT3080, with similar high performance specifications, but with an increased output current capability of 3A. This new architecture offers a number of performance advantages.

Frequency response and load regulation are fixed

With a traditional linear regulator, gain and bandwidth change as the output voltage is changed through a resistor divider. Bypassing the feedback pin of the regulator affects the loop response. Load regulation is not a fixed value, but it is a fixed percentage of the output as the resistor divider accumulates any voltage deviation. In addition, the resistor divider also causes accumulated reference voltage noise.

By using a current source and unity-gain buffer, these disadvantages are eliminated. Since the error amplifier is always in unity-gain, the frequency response does not change as a function of output voltage or by bypassing across the reference point. Load regulation is now a fixed value, regardless of output voltage. Since bypassing does not affect the loop response, two noise sources are eliminated: reference current noise and resistor scatter noise are suppressed by a single capacitor. Only the error amplifier noise remains at the output, and this noise remains fixed regardless of output voltage.

Top DC characteristics

The DC characteristics of the LT3083 are identical to the original LT3080. The LT3083 separates the collector of the NPN pass device to minimize power dissipation. Load regulation is typically less than 1mV for the error amplifier and nearly immeasurable at 50uA reference current. Voltage regulation is less than 0.0002%/V for the reference current and 2μV / V typical for the error amplifier offset. The temperature characteristics of the reference current are excellent, typically remaining within 0.2% over the entire operating junction temperature range, as shown in Figure 2.

Figure 2: Reference current temperature characteristics
SET PIN CURRENT: SET pin current
TEMPERATURE: Temperature

The LT3083 also offers the protection features for which Linear Technology devices are well known: current limiting with safe operating area protection protects the device from damage during short circuit conditions, while thermal limiting keeps the device in a safe state in the event of excessive power dissipation.

Top AC characteristics

Don’t think that the AC characteristics of the LTC3083 are sacrificed in the effort to achieve high DC performance. The transient response of the LTC3083 is excellent with output capacitance as low as 10uF. Small ceramic capacitors can be used without increasing ESR. Using a bypass capacitor across the reference resistor provides a slow start function; the output voltage follows the RC time constant set by the SET resistor and bypass capacitor. Paralleling devices also provides advantages in noise performance. Paralleling multiple LT3083 regulators reduces output noise, just as paralleling n op amps reduces noise by a factor of √n.

application

The LT3083's exceptionally simple architecture and high performance parameters make it a powerful building block that is not limited to basic linear regulators. The device can be paralleled very easily to increase output current and spread heat. Actively driving the SET pin is perfectly acceptable; the low offset and high output current allow for highly accurate reference supplies at high power levels. By driving the SET pin with a DAC, a digitally programmable supply can be implemented. Accurate current sources can be implemented without great difficulty. The device's uses are only as versatile as the user's creativity.

Connect regulators in parallel to increase current and spread heat

Figure 3 shows how multiple LT3083s can be paralleled to increase output current and spread the heat. Note that the ballast required to balance the load between the regulators is minimal. Simply by adding more LT3083s, it is possible to create a low noise and accurate high current surface mount power supply. Power dissipation is evenly distributed between the paralleled regulators, but thermal management is still necessary. Since the voltage drop across the regulators is as low as 0.5V, a 3A load equates to 1.5W of power dissipation, improving the thermal performance of the surface mount design.

Figure 3: Connecting multiple regulators in parallel to achieve higher current and spread the heat
 

High Current Reference Buffer

Very little work is required to build a high current reference buffer, as shown in Figure 4. In this circuit, the output of the LT1019-5 is connected to sink the regulator's 50uA reference current. The reference provides 0.2% accuracy over temperature, or 10mV. Since the LT3083 has a maximum offset voltage of 4mV, the output accuracy remains within 0.3%. The accuracy of the LT3083 reference current does not affect the output tolerance, and there are no resistors to introduce potential tolerance deviations.

Figure 4: High-current reference buffer
INPUT: input
OUTPUT: output
*MIN LOAD: minimum load current

Digitally set output

To digitally set the output voltage simply requires adding a DAC to drive the SET pin. Figure 4 highlights how the DAC sets the LT3083 output to within 1.5LSB from zero to over 16V. In this circuit, the LTC2641-12 drives the LT3083’s SET pin through the LT1991 (configured for a gain of 4) using a 4.096V reference.

It is again due to the tight specifications of the LT3083 that allow for such exceptional performance. Remember, when operating at the lowest output voltage, the minimum load current requirement must be met. When operating at very low input voltages, less than 500uA of load is required, which is much lower than the 5mA to 10mA required by traditional linear regulators.

Easy-to-use two-terminal current source

Current sources can be very difficult to implement in certain applications. Some must be referenced to ground, others to a positive rail, and the most difficult designs require floating two-terminal devices. The LT3083 is very easy to configure as a two-terminal current source simply by adjusting the ratio of external resistors and adding compensation, as shown in Figure 5. This current source can be referenced to ground, referenced to a positive rail, or completely floating without any concerns.

Figure 5: Digitally programmable power supply

Figure 6: Two-terminal current source

in conclusion

The architecture shown in the LT3083 block diagram may seem simple, but behind it is a high-performance and highly versatile breakthrough building block device. The LT3083 combines the LT3080's dramatic architectural improvements and superior AC and DC characteristics with increased current to easily solve problems that traditional three-terminal or low-dropout regulators cannot handle. The device can be used for power supplies operating as low as 0V, paralleled to provide larger output currents and dissipate heat, or driven dynamically. High-current linear power supplies can now be used on surface-mount circuit boards without sacrificing performance.