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Distributed linear regulators increase output current and spread the heat

PARALLEL-BOARD LAYOUT AND HEAT SPREADING OFFER DESIGNERS NEW WAYS OF USING LINEAR REGULATORS IN ALL-SURFACE-MOUNT APPROACHES. THESE TECHNIQUES WORK WELL IN TODAY’S HIGH-PERFORMANCE, HIGH-DENSITY PCBs.

Modern system boards include big, current hungry digital ICs along with a broad mix of other lower-current digital and analog circuitry. Such a system board can require as many as five or six power supplies, depending on the number and types of ICs you use. Moreover, some circuits, such as audio or high-speed serial links, are noise-sensitive and generally require designers to use inherently quieter linear regulators. Heat problems accompany the use of multiple power supplies; therefore, designers must add heat sinks along with various types of regulators. But the heat sinks make the board more cumbersome and harder to assemble than an all-surface-mount approach. An ingenious designer can turn to techniques such as using parallel regulators and using the PCB (printed-circuit board) itself for cooling to develop an all-surface-mount design.

When you employ surface-mounted regulators, thermal conduction and air cooling limit the amount of power that each chip can internally dissipate. With a typical board, allowing a maximum ambient temperature of 60 to 70°C, a surfacemounted linear regulator can dissipate approximately 1 to 2W. The total dissipation depends on the heat spreading of the board and the airflow across the board. If a design must dissipate more power, the engineer generally mounts the regulator on a heat sink to achieve these higher power levels with no thermal problems. Paralleling regulators on the board spreads the heat, provides greater maximum output current than does a singleregulator design, and helps maintain low peak temperatures.

Figure 1 shows a parallel-capable adjustable regulator. A precision 0 TC (temperature coefficient)—less than 1% over temperature—of 10-µA internal-current source connects to the noninverting input of a power operational amplifier. The amplifier provides a low-impedance, buffered output, which the voltage on the noninverting input controls. The control and input pins connect, input and output capacitors add stability, and a resistor from the adjust pin sets the output voltage. A 180-kΩ resistor from the noninverting input to ground provides the 1.8V output. A short or a 0Ω resistor would set the output to 0V. Designers can control the output with a resistor or with a DAC, adjusting the output from 0V to the maximum that the input power supply defines. The design requires a minimum load current of 1 mA because it has no ground pin. An optional 0.1-µF capacitor reduces noise.

HOW MUCH POWER?

In considering such a power source, a designer would need to determine how much power the circuit can deliver relative to the heat dissipated. As a rule of thumb for temperature rise on PCBs, you can expect approximately 40°C/1W rise. With a 60°C ambient temperature, 1W would raise the temperature to 100°C. The temperature rise at 2W puts the 140°C peak temperature above the safe operating temperature on most semiconductors. The designer would need either a high-thermalconductivity board or airflow to keep the peak temperature down. A designer can implement a 1.1A supply regulating from 2.5 to 1.8V or 1.8 to 1.2V directly on a PCB. For the same peak operating temperatures, the power dissipation can limit the output current for the case of regulating 5V down to 3.3V, or 3.3V down to 1.5V, depending on ambient temperature and airflow.

Figure 2 shows the maximum output current at different I/O differentials for a regulator when the power dissipation is 1 or 2W. To remain generally useful in many circuits, a regulator must support 2W. Paralleling regulators or using circuit tricks to spread the heat allows higher power dissipation and higher output currents. You can spread the power dissipation among several devices so that no hot spots result in the system board. If your application requires more current than a single regulator can supply, you can add a second regulator.

Designers can also use other tricks to dissipate the heat. For instance, you can move some of the power dissipation—that is, heat—from the regulator to an external resistor, thereby reducing the peak temperature of the regulator. Instead of having one point with a 80°C rise, you can spread the heat to two points, each with a 40°C rise. You can place the resistor directly in series with the regulator as long as the input voltage at the regulator at maximum current does not place the regulator in dropout. This technique is most useful in multiple-output power systems in which the input is regulated and the regulator is generating an additional voltage. Some regulator designs separately expose the power and the control circuitry. In such cases, the dropout on the power pin is only 100 to 300 mV, allowing connection to a lower voltage supply to reduce dissipation. If you use the resistor only in series with the power input, the design can transfer more power to the resistor without dropout.

Figure 3 shows the collector of the output transistor connected to split the power dissipation between the internal power transistor and an external resistor. In this example, a designer can place a maximum resistor of 2.9Ω in series with the input, and the regulator won’t enter a dropout state. At full load, the design drops approximately 2.9V across the external resistor, and the external resistor dissipates about 3W. To minimize peak temperatures on a PCB, you can break this resistance value into several 1Ω resistors across the board. The power dissipation in the regulator peaks at approximately 750 mW when the power dissipation in the resistors and the power dissipation in the transistor are equal. The copper planes in the PCB easily handle this power. In actual systems, you should ensure that less than 2.9V drops across the resistor to allow for tolerance in the 5V input supply.

If you need higher output current, consider using regulators in parallel to share the power demand. In such a design, you should tie together the adjust, output, and input pins. You should connect the input pins when you use an external series resistor or connect directly to an input-voltage supply. Although you should connect the outputs, you must also ensure current sharing using a ballast on each output. The size of the ballast depends on the voltage mismatch between the regulators. The ballast must drop enough voltage to absorb the output difference. Without the ballast, one regulator supplies all the current until it reaches its limit before the next regulator turns on—an unreliable situation.

In regulators that use op amps as the output-power stage, the offset voltage determines the mismatch between regulators, which is in the millivolt range. This situation requires 10- to 20-mΩ ballast resistors. But you need not add actual resistors. Instead, you can rely on a small piece of PCB trace. A ballast drop of only 10 mV still allows good regulation—1% at 1V output. For this level of ballast, you must ensure that regulator-dc errors are less than the ballast drop. For a 1-oz board, a 10-m ballast requires a trace length of 0.18 in. (4.5 mm) with 0.010-in.- wide traces (0.25 mm) or 0.37 in. (9 mm) with a trace width of 0.020 in/ (0.5 mm). For a 2-oz board, the corresponding figures are 0.37 and 0.74 in. (9 and 18 mm).

Figure 4 shows a design that uses parallel regulators. The two devices have a 10-mΩ ballast resistor. At full output current, this design gives better than 80% equalized sharing of the current. The external resistance of 10 mΩ (5 mΩ for the two devices in parallel) adds only approximately 10 mV of outputregulation drop at an output of 2A. With a 3.3V output, this drop adds only 0.3% error to the regulation. You can use more than two regulators in parallel for even higher output current. Spread the regulators on the PCB to spread the heat. You can also use input resistors to further spread the heat if the I/O difference is high.

THERMAL PERFORMANCE

Consider the thermal performance of a parallel-regulator design. For example, assume QFN devices mounted on a doublesided PCB: the design locates the regulators approximately 1.5 in. apart. Also assume that you will vertically mount the board for convection cooling. Two tests on such a board measure the peak temperature and current sharing of these devices. In the first test, the circuit operates with an approximately 0.7V input-to-output differential, and each regulator produces 1A. This configuration produces 700-mW dissipation in each device and a cumulative 2A output current. The test yielded a temperature rise above ambient of approximately 28°C, and both devices were within ±1°C. The test demonstrated excellent thermal- and electrical-sharing characteristics. Figure 5 shows the temperature distribution between the regulators and the PCB, and the peak temperature reaches ambient temperature within approximately 0.5 in. of the devices.

The test then increased the power demand with a 1.7V differential across each device. This second test resulted in 1.7W dissipation in each device and a device temperature of approximately 90°C—about 65°C above ambient temperature (figures 6 and figure 7). The test revealed that temperature matching between the regulators is within 2°. The board temperature decreased to approximately 40°C within approximately 0.75 in. of each device. Although 95°C is an acceptable operating temperature for the tested regulators, the rise in these tests was in a 25°C ambient environment. For higher ambient temperatures, designers must control the temperature to prevent device temperatures from exceeding 125°C. A 3m/sec airflow across the devices decreases the device temperature by approximately 20°C, providing margin for higher operating ambient temperatures. In addition, this test relied on a two-layer board. A fourlayer board would provide better power dissipation.

Parallel-board layout and heat spreading provide designers with new ways of using linear regulators in all-surface-mount approaches. These techniques work well in today’s high-performance, high-density PCBs. To achieve good sharing performance, your design must carefully control the dc characteristics in the regulator. Once designers understand the control characteristics, they can routinely parallel regulators and spread heat to achieve all-surface-mount systems.

AUTHOR’S BIOGRAPHY

Robert Dobkin is a founder and chief technical officer of Linear Technology. Before founding Linear Technology in 1981, Dobkin was director of advanced circuit development at National Semiconductor.