Linear regulators provide a simple, low noise solution for dc-dc regulation. However, at higher VIN-VOUT differentials the low efficiency and high power dissipation of linear regulators limits the amount of output current that can realistically be delivered. Connecting multiple linear regulators in parallel spreads the load (and the heat) over several ICs, increasing the useful range of output currents a solution can deliver. However, connecting linear regulators in parallel is not always straightforward.
Paralleling Voltage Reference-Based Linear Regulators
Current sharing with linear regulators is traditionally not as simple as connecting the parts in parallel. Two voltage reference-based linear regulators set to the same output voltage and with the outputs tied together will not share current equally. An LDO's output voltage is determined by the reference voltage multiplied by a gain factor based on the feedback resistors. Due to tolerance errors in the voltage reference and feedback resistors, the output voltages will be mismatched. With unmatched outputs, the LDOs will not share current; one LDO will provide the majority of the current until it hits current limit, thermal limiting or its output droops low enough for the other LDO to begin supplementing its current. These three situations present circuit operation challenges and can pose reliability concerns, leading to possible premature failure of the overstressed LDO.
Let's look at the LT1763-3.3, a popular 3.3V output, 500mA rugged PNP linear regulator that operates from 1.8V to 20V. It has a 1% maximum output voltage error at room temperature and 2.5% over temperature.
At full load and over temperature, The LT1763's output voltage ranges from 3.22V to 3.38V, which is a 16mV span. When paralleling devices, If one LDO output is at the upper value and the other is at the lower value, the paralleled LDOs will not share current; the one with the higher output voltage dominates over the entire load current range.
To improve current sharing capability, identical balancing resistors can be added at the output of each regulator as shown in the figure below, but for tight matching (i.e. on the order of 90%), the resistor values need to be fairly large so that the difference in regulator output voltages can be offset by a small change in output current.
For example, when paralleling two 3.3V output, 1A LDOs with a 3% tolerance, the worst case output voltage scenario is when one LDO (#1) has a 3.4V output and the other (#2) is at 3.2V. With a 2Ω balancing resistor, it only takes an additional 100mA of output current through LDO #1's balancing resistor to balance the two voltages (the higher voltage output LDO supplies more current). Once it supplies an additional 100mA, the additional drop across the balancing resistor causes the two output voltages to match and the LDOs will share current. This provides tight current sharing (only a 10% difference at maximum load current). However, the voltage drop across the current balancing resistors is much too large at full load (1.1A* 2Ω = 2.2V drop ).
One can add current sensing circuitry (current sense resistors and an amplifier) at the input or output (or on current limiting pins if the amplifier has this feature) to balance the currents and maintain proper output voltage, but the external circuitry adds cost and requires additional board space.
Another method involves using LDOs that have an adjustable current limit as shown in the LT3065 example below (the LT3065 is a 1.8V to 45V input, 500mA output, 25µVRMS linear regulator with a programmable 10% accurate precision current limit). A feedback loop is used to match the two current limits by adjusting the output voltage of one of the amplifiers. As with the previous example, an external amplifier and current setting resistors are required for operation.
Paralleling Current Source-Based Reference Linear Regulators
The LT3081 is an example of a linear regulator solves this problem in a very simple, unique way. The LT3081 is a 1.5A output LDO that is part of a unique family of positive and negative linear regulators with a current source reference. The devices are easy to parallel and share current very well. They positive output regulators have an input voltage range up to 40V and provide output currents from 0.2A to 3A.
A simplified diagram of the LT3081 is shown below. Instead of a voltage reference, the LT3081 uses a current source-based reference. This current is driven through an external RSET resistor to set the reference voltage value. Based on the resistor selected the reference value can be set down to zero volts; no additional feedback resistors are required.
Another key is the tight distribution of the SET pin current as shown below. This results in a very low ±1.5mV maximum offset voltage specification from the VSET pin to the output pin at room temperature.
Higher output current is obtained by paralleling multiple LT3081s together. Tie the individual SET pins together and tie the individual IN pins together. Connect the outputs in common using small pieces of PC trace as ballast resistors to promote equal current sharing. PC trace resistance in milliohms/inch is shown in Table 1. Ballasting requires only a tiny area on the PCB.
The worst-case room temperature offset, only ±1.5mV between the SET pin and the OUT pin, allows the use of very small ballast resistors. As shown in the Figure below, each LT3081 uses a small 10mΩ ballast resistor, which at full output current gives better than 80% equalized sharing of the current. The external resistance of 10mΩ (5mΩ for the two devices in parallel) only adds about 15mV of output regulation drop at an output of 3A. Even with an output voltage as low as 1V, this only adds 1.5% to the regulation. Of course, paralleling more than two LT3081s yields even higher output current. Spreading the devices on the PC board also spreads the heat. Series input resistors can further spread the heat if the input-to-output voltage differential is large.
For convenience, the 1.1A LT3080 is available in an LT3080-1 version that integrates the ballast resistor. Linear Technology's family of current sense amplifiers offer a plethora of other useful features between the devices, including output current monitoring, die junction temperature monitoring, shutdown, reverse current and reverse battery protection and others. All devices make the problems associated with paralleling LDOs problems of the past.