Figure 3: Offset Voltage for the LT3081
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, offering nearly 1A of output current at 25V of differential.
devices such as the LT1086. The internal amplifier configuration, in conjunction with well-regulated internal bias supplies, allows the device to be stable with no external capacitors. If instability is found, external capacitors will ensure that the device is stable at all output currents. External capacitors also improve the transient response since it is no longer limited by the bandwidth of the internal amplifier. Paralleling devices, usually a forbidden application with previous regulators since they do not share current, is easy with these new current source reference regulators. Paralleling is useful for increasing output current or spreading the heat. Since it is set up as a voltage follower, tying all the set pins together makes the outputs the same voltage. If the outputs are at the same voltage, only a few milliohms of ballast are needed to ballast these devices and allow them to share current. Figure 3 is a distribution of the offset voltage for the LT3081. The distribution is all within 1mV so to ensure sharing to 10 per cent; 10m of ballast resistance is more than sufficient. The ballast resistor can be less than an inch of a trace on a PC board or a small piece of wire, and
Table 2 shows PC boards resistance
paralleled (if used) so one resistor sets the current limit. Figure 4 shows the LT3081 paralleled with a fixed regulator. This is useful when a system that has been designed has insufficient output current available as it provides a quick fix for higher output current. The output voltage of the fixed device is divided down by just a few millivolts by the divider. The SET pin of the LT3081 is tied about 4mV below the fixed output. This ensures no current flows from the LT3081 under a no-load condition. Then the 20m resistors provide sufficient ballast to overcome this offset and ensure current matching at higher output currents. With the 50µA current source used to generate the reference voltage, leakage paths to or from the SET pin can create
film potentiometers are not as stable as lower value wirewounds. Board leakage can also introduce instabilities in the output. Problems can be minimised by increasing the set current above the nominal 50µA. A solution can be using lower value set resistors. Here an increased current is generated through R2 and summed with the set current, giving a much larger current for adjusting the output. Set current flows through a 4k resistor, generating 200mV across R1. Then the current through R2 adds to the set current, giving a total of 1.05mA flowing through ISET
voltage less sensitive to leakage currents around the RSET
to ground. This makes the . Care should be taken to
Kelvin connect R2 directly to the output. Voltage drops from the output to R2 will
Figure 4 Figure 4: Increasing the output current of a fixed regulator
Even above 25V, the output current of 500mA is still usable. This allows the regulator to be used in applications where widely varying input voltages can be applied during operation. Wide operating safe area is obtained by using a large structure for the PNP pass device. Also, the LT3081 is protected (along with the load) for reverse input voltage. The LT3081, while not a low dropout
regulator, operates down to 1.2V across the device - slightly better than older
provides good current balance from parallel devices. Even at 1V output, this degrades the regulation by only about 1.5 per cent.
When two LT3081 devices are paralleled to obtain 3A output, the set resistor now has twice the set current flowing through it, so the output is 100µA times RSET and the 10m output resistors ensure ballasting at full current. Any number of devices can be paralleled for higher current. The ILIM
pins can be Figure 5: Using a lower value set resistor
errors in the reference and output voltages. Cleaning of all insulating surfaces to remove fluxes and other residues is required. Surface coating may be necessary to provide a moisture barrier in high humidity environments. Minimise board leakage by encircling the SET pin and circuitry with a guard ring tied to the OUT pin. Increasing the set current also decreases the effects of spurious leakages. The low 50µA SET current can cause problems in some applications. High value
affect the regulation. Another configuration uses an LT3092 as an external current source of 1mA. This provides increased set current and allows the output to be adjusted down to zero.
Conclusion New regulators provide an order of magnitude better regulation against load and line changes compared to prior devices. Regulation specs as well as transient response do not change with output. New functionality in these devices provides temperature and current monitoring, as well as adjustable current limiting. Paralleling no longer requires external current balance circuitry to prevent current hogging. Along with these improvements comes ruggedness. New applications are enabled. Paralleling is easy and line drops can be compensated. Current limit thresholds are now user defined and outputs are adjustable to zero. Safe operating area is increased for operation with wider input swings.
Table 1 shows the new regulators and main features
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Components in Electronics July/August 2016 11
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