COVER STORY
Industrial strength regulators
Bob Dobkin, vice president, engineering and chief technical officer, Linear Technology Corporation, talks about the changes in industrial strength regulators and the differences between them
R
egulators regulate but are capable of doing much more. The architecture of linear regulators has remained virtually unchanged since the introduction of the three terminal floating voltage regulator in 1976. Regulators were either a floating architecture (LT317) or an amplifier loop with feedback from the output to the amplifier. Both of these architectures suffer from limitations on versatility, regulation and accuracy. The feedback resistors set the output voltage and attenuate the feedback signal into the amplifier. Therefore, the regulation at the output is a percentage of the output voltage, so higher output voltages have worse regulation in “Volts” while the percentage may be the same. Also, the bandwidth of the regulator changes with voltage. Since the loop gain is decreased, the bandwidth is decreased as well at higher output voltages. This makes transient response slower and ripple worse as output voltage goes up. The older regulator fixes current limiting and it has no adjustment. It is built into the IC and different devices must be used for different output currents. So, if the current limit needs to be matched to the application or accurate current limit is needed, an external circuit must be used. A new architecture was introduced in 2007 in the LT3080. It used a current source for the reference and a voltage follower for the output amplifier. Two advantages of this architecture are the
ability to parallel the regulators for more output current and the ability for the regulator to operate down to zero output voltage. Since the output amplifier always operates at unity gain, bandwidth is constant and regulation is constant as well. Transient response is independent of output voltage and regulation can be specified in millivolts rather than a per cent of output.
Along with different output current variations, these new regulators were specifically designed to add functional features not previously available in existing regulators. There are monitor outputs for temperature, current and external control of current limit. One device (LT3086) also has external control of thermal shutdown. A new negative regulator provides monitoring and can operate as a floating regulator or an LDO. All of these new regulators can be paralleled for higher current, current sharing, and heat spreading.
A new industrial regulator The LT3081 is a wide safe operating area industrial regulator. It provides 1.5A of output current, is adjustable to zero output voltage, 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. Temperature and current monitor outputs are current sources configured to
Figure 1b: New architecture regulator
operate from 0.4V above VOUT below VOUT
to 40V . Temperature output is 1µA/°C
per degree and the current monitor is IOUT
/5,000. These current sources are
measured by tying a resistor to ground in series with the current source and reading across the resistor. The current source has a range of –40V to 0.4V referred to the output and it continues to work even if the output is shorted. The dynamic range for the monitor outputs is 400mV above the output so, with the output shorted or set to zero, temperature and current can still be measured. Using a 1k resistor provides sufficient margin and ensures operation when the output is shorted. The output is set with a resistor from set pin to ground and a 50µA precision current source set to the output. The internal follower amplifier forces the output voltage to be the same voltage as the SET pin. Unique to the LT3081, an output capacitor is optional. The regulator is stable with or without input and output capacitors. All the internal operating current flows through the output pin and minimum load is required to maintain regulation. Here, a 5mA load is required at all output voltages to maintain the device in full regulation.
Figure 1a Figure 1a: Basic architecture of older regulators 10 July/August 2016 Components in Electronics
The benefit of using an internal true current source as the reference, rather than a bootstrapped reference, as in prior regulators, is not so obvious. A true reference current source allows the regulator to have gain and frequency response independent of the impedance on the positive input. With all previous adjustable regulators, such as the LT1086, loop gain and bandwidth change with output voltage changes. If the adjustment pin is bypassed to ground,
bandwidth also changes. For the LT3081, the loop gain is unchanged with output voltage or bypassing. Output regulation is not a fixed percentage of output voltage, but is a fixed number of millivolts. Use of a true current source allows all of the gain in the buffer amplifier to provide regulation, and none of that gain is needed to amplify up the reference to a higher output voltage. Industrial applications require large safe operating area. Safe operating area is the ability to carry large currents at high input-output differentials. The LT1086, introduced in the mid-1980s, is a 1.5A regulator in which output current drops very low above 20V input/output differential. Above 20V only about 100mA of output current is available. This causes output voltage to go
Figure 1b
Figure 2
Figure 2: Comparative safe operating area performance
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