New Linear Regulator Solve Old Problem

A voltage regulator regulates voltage, but it can do many other things. The architecture of linear regulators has remained virtually unchanged since the introduction of the three-terminal floating regulator in 1976. The regulator is either a floating architecture (LT317) or an amplifier loop with feedback from the output to the amplifier. Both architectures have limitations in versatility, regulation performance, and accuracy.

The feedback resistor sets the output voltage and attenuates the feedback signal going into the amplifier. Therefore, the regulation at the output is a percentage of the output voltage, so while the percentage may be the same, the regulation (in V) is worse at higher output voltages. Also, the bandwidth of the regulator changes with voltage. Since the loop gain decreases, the bandwidth is reduced at higher output voltages. This slows down the transient response and worsens the ripple performance as the output voltage increases.

Older regulators have a fixed current limit with no room for adjustment. It is built into the IC and different devices must be used for different output currents. Therefore, if the current limit must be matched to the application or an accurate current limit is required, an external circuit must be used. Figure 1a shows the basic architecture of an older regulator.

In 2007, a new architecture was introduced in the LT3080. This device uses a current source as the reference and a voltage follower as the output amplifier. Two advantages of this architecture are that multiple regulators can be paralleled to provide higher output currents, and the regulator can operate with output voltages as low as zero. Since the output amplifier always operates at unity gain, the bandwidth is constant and the regulation is constant. The transient response is independent of the output voltage, and the regulation can be specified in mV rather than a percentage of the output. Figure 1b shows the new regulator architecture.

Figure 1a: Old-style voltage regulator

Figure 1b: New architecture regulator

Table 1 lists the new regulators and their key features. Along with different output currents, these regulators are specifically designed to add features not previously available in existing regulators. Monitor outputs for temperature and current are included, as well as external control of current limit. One device (LT3086) also has external control of thermal shutdown. A new negative regulator provides monitoring functions and can act as a floating regulator or LDO. All of these new regulators can be paralleled for higher current, current sharing, and heat dissipation.

A new type of industrial voltage stabilizer

The LT3081 is an industrial regulator with a wide safe operating area. It provides 1.5A output current, has an adjustable output voltage to zero, is reverse protected, and has monitor outputs for temperature and output current. In addition, the current limit can be adjusted by connecting an external resistor to the device. Figure 2 shows the basic connections for the LT3081.

Figure 2: Basic regulator using the LT3081

The temperature and current monitor outputs are configured as current sources to operate from VOUT + 0.4V to VOUT – 40V. The temperature output is 1μA/°C, while the current monitor output is IOUT/5000. These current sources are measured by connecting a resistor in series with the current source to ground and reading the parameters across the resistor. The current source has a range of -40V to 0.4V (referenced to the output) and will continue to operate even if the output is shorted. The dynamic range of the monitor output is 400mV higher than the output, so when the output is shorted or set to zero, temperature and current can still be measured. Using a 1k resistor provides sufficient margin and ensures normal operation when the output is shorted. The output is set using a resistor connected between the SET pin and ground and a 50μA precision current source. The internal tracking amplifier forces the output voltage to be equal to the SET pin voltage. Unique to the LT3081 is that the output capacitor is optional. The regulator is stable with or without input and output capacitors. All internal operating currents flow through the output pins and require a minimum load to maintain regulation. Here, a 5mA load is required at all output voltages to keep the device in full regulation.

Setting resistors will add to the system temperature drift. Surface mount resistors are available with a wide range of temperature coefficients. Depending on the manufacturer, these temperature coefficients can be as low as 100ppm to over 500ppm. Although the resistors are not heated by the power dissipated in the regulator, their temperature coefficients can cause a 1% to 4% change in the output over a wide range of ambient temperatures. For high precision applications, thin film resistors with lower temperature coefficients can be used.

The benefit of using an internal true current source as the reference (rather than a bootstrapped reference as in previous regulators) is not immediately apparent. A true current source reference allows the regulator to have a gain and frequency response that is independent of the impedance at the positive input. With all previously introduced adjustable regulators (e.g., the LT1086), the loop gain and bandwidth change with changes in output voltage. If the adjust pin is bypassed to ground, the bandwidth also changes. With the LT3081, the loop gain does not change with changes in output voltage or bypassing of the adjust pin. Output regulation is not a fixed percentage of the output voltage, but a fixed mV value. Using a true current source allows all of the gain in the buffer amplifier to provide regulation, and none of the gain is needed to amplify the reference to a higher output voltage.

Industrial applications require a large safe operating area. The safe operating area reflects the ability to deliver high currents with a high input-output voltage differential. Figure 3 compares the safe operating areas of several regulators. The LT1086, introduced in the mid-1980s, is a 1.5A regulator whose output current drops to very low levels when the input/output voltage differential is above 20V. Only about 100mA of output current is available with an input/output voltage differential above 20V. This causes the output voltage to go unregulated if the load current is above 100mA, and transients on the input cause the high voltage current limit to be exceeded. The LT1963A is a low dropout regulator that also has a limited safe operating area. The LT3081 extends the safe operating area to deliver about 1A of output current with a voltage differential of 25V. Even with an input/output voltage differential above 25V, the available output current is still 500mA. This allows the regulator to be used in applications where a wide range of input voltages may be applied during operation. Using a large structure for the PNP pass device results in a wide safe operating area. In addition, the LT3081 (and the load) is protected against reverse input voltage.

Figure 3: Comparison of safe operating area performance

Figure 4 shows a block diagram of the LT3081. There are three current sources, two of which report the output current and temperature. The third current source is used to provide a 50μA reference current. Although not a low dropout regulator, the LT3081 can operate with voltages as low as 1.2V across the device, which is slightly better than older devices such as the LT1086. The internal amplifier configuration combined with a well-regulated internal bias supply makes the device stable without external capacitors. One caveat: it cannot be designed to tolerate all possible impedances in the input and load, so it is important to test stability in the system where it will be used. If instability is found, placing external capacitors will ensure that the device remains stable under all output current conditions. External capacitors also improve transient response because it is no longer limited by the bandwidth of the internal amplifier.

Figure 4: Block Diagram of the LT3081 For these new current source reference regulators, it is easy to parallel devices, which is not allowed for older regulators because they cannot share current. Parallel devices are suitable for increasing output current or heat dissipation. Since it is configured as a voltage follower, connecting all the SET pins together makes the outputs have the same voltage. If the outputs are at the same voltage, only a few mΩ ballasts are needed to ballast these devices and make them share current.

Figure 5 shows the offset voltage distribution of the LT3081. The distribution is all within 1mV to ensure 10% current sharing accuracy; a 10mΩ ballast resistor is more than adequate. The ballast resistor can be a trace less than an inch long on the PC board, or a short piece of wire with parallel devices providing excellent current matching. Even at 1V output, the resulting drop in regulation is only about 1.5%. Table 2 lists the PC board resistors.

Figure 5: Offset voltage

Trace resistance is expressed in mΩ/inch.

Figure 6 shows a schematic for paralleling two LT3081s to achieve a 3A output. Here, the set current flowing through the set resistor is increased by x2, so the output is 100μA x RSET, and the 10mΩ ballast resistor ensures ballast at full current. Higher currents can be provided by paralleling any number of devices. The ILIM pins can be paralleled (if used), so one resistor sets the current limit.

Figure 6: Connecting devices in parallel

Figure 7 shows the LT3081 in parallel with a fixed regulator. This is useful when the system is designed for insufficient output current. It provides an emergency method to increase the output current. The voltage divider only drops the output voltage of the fixed device by a few mV. The SET pin of the LT3081 is connected to a voltage about 4mV lower than the fixed output. This ensures that no current flows out of the LT3081 under no load conditions. In addition, the 20mΩ resistor provides sufficient ballast to overcome the offset and ensure current matching at higher output currents.

Figure 7: Increasing the output current of a fixed regulator

When a 50μA current source is placed to generate the reference voltage, the leakage path between the current source and the SET pin can produce errors in the reference and output voltages. All insulating surfaces need to be cleaned to remove flux and other residues. Surface coating may be necessary to provide a moisture barrier in high humidity environments. Board leakage can be minimized by surrounding the SET pin and circuitry with a guard ring connected to the OUT pin. In addition, increasing the SET current as shown can also reduce the effects of parasitic leakage.

In some applications, the low SET current of 50μA can cause problems. High-value film potentiometers are not as stable as lower-value wirewound potentiometers. Board leakage can also cause instability in the output. Problems can be minimized by increasing the SET current above the nominal 50μA. Figure 8 shows a solution using a lower-value set resistor. Here, an increased current is created through R2 and adds to the SET pin current to provide a much larger current for regulating the output. The SET current flows through a 4k resistor, creating 200mV across R1. The current through R2 then adds to the SET pin current to provide a total current of 1.05mA through ISET to ground. This reduces the sensitivity of the voltage to leakage current around REST. Care should be taken to connect R2 directly to the output in a Kelvin fashion. The voltage drop from the output to R2 will affect regulation. Another configuration uses the LT3092 as a 1mA external current source. This provides increased SET current and allows the output to be adjusted down to zero.

Figure 8: Using a lower value setting resistor

Figure 9 shows an LT3092 current source used to provide the current reference for the LT3081. The resulting 1mA reference current allows the adjustment setting resistor to be much lower in value while still adjusting the device's output down to zero.

Figure 9: Using an external reference circuit

As shown in Figure 10, the current monitor output can be used to compensate for line voltage drops. Feeding the current monitor through a portion of the setting resistor produces a voltage at the SET pin that is a function of the current. The compensation resistor value is R2 = 5000 • RCABLE(TOTAL) and VOUT = 50μA (RSET + RCOMP). Line voltage drops of several volts can be compensated in this way.

Figure 10: Using current monitor output to compensate for line voltage drops

in conclusion

The new regulators provide an order of magnitude better regulation for load and line variations than previous devices. Regulation specifications and transient response do not change with output. New features in these devices provide temperature and current monitoring, as well as adjustable current limit. Paralleling of multiple devices no longer requires external current balancing circuits to avoid current disturbances. Along with these improvements, the ruggedness of the devices has also been improved.

The new regulators enable new applications. Devices can be easily paralleled and line voltage drops can be compensated. The current limit threshold is now user-defined and the output can be adjusted to zero. The regulator's safe operating area has been expanded to operate with wider input swings.