Power supply, low voltage, high output implementation needs to increase
ADI 3A 'LT3033' VLDO regulator in parallel
Increase output current to 3A or more, and heat dissipation is also possible.


Today's computing systems require more total power and lower supply voltage than in the past, so power supply designers must implement high output current in a small space.

At low output voltages and high power densities, heat dissipation is a top design consideration, especially for linear regulators in low-noise applications. If LDO regulators are connected in parallel, the supply current capability can be increased and heat dissipation can be reduced, reducing the temperature rise of specific components and reducing the size and number of cooling devices required.

Paralleling ADI's 3A 'LT3033' ultra-low voltage dropout regulator (VLDO) can drive applications requiring more than 3A of current while dissipating heat. The LT3033 also has built-in output current monitoring, making it easy to balance current.

The LT3033 converts input power from 1.14V to 10V to outputs up to 3A of load current and down to 0.2V. The voltage dropout is only 95mV at full load. Quiescent current is 1.8mA during operation, dropping to 22µA in shutdown. Programmable current limit and thermal protection provide the robustness needed for high-current, low-voltage applications.

◇ 3A, Single VLDO Application Example

The LT3033, which supplies 3A at 0.9V from a 1.2V input, requires a minimum 10µF ESR ceramic capacitor on the IN/OUT pins for safety. Adding a forward-biased capacitor (CFF) between the VOUT and ADJ pins can improve transient response and reduce output voltage noise.

Using a 10nF bypass capacitor between the REF/BYP pin and GND reduces the output voltage noise to typically 60µVrms over a 10Hz to 100kHz bandwidth and provides a soft-start reference. The minimum input voltage required for regulation is the greater of the regulated output voltage VOUT plus the dropout voltage or 1.14V.

The current limit is programmable by connecting a single resistor from the ILIM pin to GND and has an accuracy of ±12% over a wide temperature range. The external current limit can be overridden by the foldback internal current limit when the differential voltage between the input and output exceeds 5 V.

The LT3033 provides an output current monitor by driving the IMON pin to GND through a resistor. The IMON pin serves as the collector of a PNP, mirroring the LT3033 output PNP at a ratio of 1:2650. The resistor voltage is proportional to the output current as long as it is not higher than VOUT-400mV.
This output current monitor allows multiple LT3033 devices to share current. Despite its small size, the LT3033 includes several useful protection features, including foldback internal current limiting, thermal limiting, reverse current and reverse battery protection.

◇ 2 LT3033s in parallel for 6A applications

Applications requiring more than 3A can be supported by paralleling multiple LT3033s.

The above figure shows how to connect two 2N3904 NPN devices and two LT3033s in parallel to produce a 1.5V, 6A output. The individual IN and OUT pins are connected together. One master LT3033 controls the slave LT3033s.

The IMON pin is coupled with an NPN current mirror to create an amplifier. This amplifier injects current into the feedback divider of the slave LT3033, equalizing the IMON current of each LT3033.

The 100Ω resistor provides 113mV emitter degeneration at full load, ensuring excellent current mirror matching. The output voltage of the slave LT3033 is set to 1.35V, 10% lower than the circuit output, to ensure that the master LT3033 remains in control.

The feedback resistor of the slave LT3033 is sectioned to ensure adequate headroom for the slave NPN. A combination of a 10nF, 5.1kΩ capacitor and resistor added to the IMON pin of the slave device frequency compensates the feedback loop.

This circuit can supply 6A of load current, but the current sharing accuracy is limited by the mismatch between the two NPN devices. This mismatch causes uneven heat distribution on the board. Replacing the two discrete NPN devices with a matched monolithic transistor, such as ADI's 'MAT14', allows for more accurate current sharing.

The MAT14 is a quad monolithic NPN transistor that provides high parametric matching, with a maximum current gain matching of 4%. Compared to the 2N3904, the MAT14 current mirror reduces the current mismatch from 5.3% to 1.6%. The above compares the output current of an LDO regulator using discrete and matched NPN devices.

◇ Parallel connection of 4 LT3033 using matching components

This parallel circuit architecture can be expanded by adding as many current mirrors and slave LT3033 devices as needed.

The above is an example of 4 LT3033s connected in parallel using MAT14 for current sharing.

Above is the thermal performance. The temperature of four LT3033s is around 51 to 58°C. Considering the voltage drop along the input traces for each component, the solution appears to have even current sharing, as heat is spread evenly across the board.

Above is the transient response of a 1.5V output, 12A power supply operating from 1.8V input.

◇ LT3033, similar electrical efficiency to switching regulators

The LT3033 is a 3A VLDO regulator in a 3 × 4mm package with built-in output current monitoring that allows paralleling for high current applications. With a voltage drop of only 95mV at full load, it is suitable for low input/output voltage, high current applications, and achieves electrical efficiency comparable to switching regulators.

Other features include programmable current limiting, power good indication, and thermal limiting for a reliable and robust solution. Engineers developing battery-powered systems can achieve low standby current and reverse battery protection.



This article is a summary of the article titled, “Paralleling Very Low Dropout Linear Regulators for Increased Output Current and Even Heat Distribution,” co-authored by Molly Zhu, Senior Applications Engineer, and Fei Guo, Field Applications Engineer, Analog Devices (ADI).