Increase the output current and dissipate heat evenly. This voltage regulator is worth having.

This article explains how to parallel 3A LT3033 very low dropout regulators (VLDOs) to generate more than 3 A current and improve heat dissipation. The built-in output current monitoring function of the LT3033 can simplify the design of parallel circuits and achieve current sharing.

The LT3033 operates with an input voltage range of 1.14 V to 10 V, and can deliver an output voltage as low as 0.2 V at load currents up to 3 A. The dropout voltage is only 95 mV at full load. The quiescent current is 1.8 mA during operation, dropping to 22 μA when shut down. User-programmable current limiting and thermal protection give it the robustness necessary for high current, low voltage applications.

Figure 1 shows the LT3033 delivering a 0.9V, 3A output from a 1.2V input supply. Very low ESR ceramic capacitors of at least 10 μF are required at the IN and OUT pins for stability. Adding a feedforward capacitor (C _{FF ) between V OUT} and the ADJ pin improves transient response and reduces output voltage noise. A 10 nF bypass capacitor from the REF/BYP pin to GND typically reduces output voltage noise to 60 μV rms over a 10 Hz to 100 kHz bandwidth and soft-starts the reference. The minimum input voltage required for regulation is the regulated output voltage, V _OUT, plus the dropout voltage or 1.14 V, whichever is greater.

Figure 1. LT3033 typical application.

The customer can set the current limit by connecting a resistor from the ILIM pin to GND with an accuracy of ±12% over a wide temperature range. When the differential voltage between the input and output exceeds 5 V, the internal current limit with foldback function will replace the external current limit.

The LT3033 implements output current monitoring by measuring the resistor voltage from IMON to GND. The IMON pin is the collector of the internal PNP of the chip, which mirrors the current of the LT3033 output PNP at a ratio of 1:2650. When the resistor voltage is not higher than V _{O UT} – 400 mV, it is proportional to the output current.

This output current monitoring feature helps to achieve current sharing among multiple LT3033s.

Despite its small size, the LT3033 still integrates many protection features, including internal current limiting with foldback, thermal limiting, reverse current and reverse battery protection.

For applications that require more than 3 A, multiple LT3033s can be connected in parallel using their current monitoring capabilities. Figure 2 shows two LT3033s and two 2N3904 NPN devices connected in parallel to produce a 1.5 V, 6 A output. The IN pin and OUT pin of each device are connected separately. One LT3033 acts as a master device and controls the other LT3033 slave device.

The IMON pin is used in conjunction with an NPN current mirror to create a simple amplifier. This amplifier injects current into the feedback divider of the LT3033 slave, forcing the IMON current of each LT3033 to be equal. The 100 Ω resistor provides 113 mV of emitter degeneration at full load to ensure good current mirror matching. The output voltage of the LT3033 slave is set to 1.35 V, 10% below the circuit output, to ensure that the LT3033 master has control. The feedback resistor of the LT3033 slave is split into multiple sections to ensure sufficient headroom for the slave’s NPN. A 10 nF capacitor and 5.1 kΩ resistor combination is added to the slave’s IMON pin to frequency compensate the feedback loop.

Figure 2. Two LT3033s in parallel.

Although this circuit can provide 6 A of load current, the mismatch between the two NPN devices causes uneven heat distribution on the board, which limits the current sharing accuracy. Using matched monolithic transistors (for example, ADI's MAT14) to replace the two discrete NPN devices can achieve higher current sharing accuracy. The MAT14 is a four-channel monolithic NPN transistor with excellent parameter matching performance. Its maximum current gain matching is 4%.

Figure 3 compares the corresponding output current of each LDO regulator when using discrete and matched NPN devices. Compared to the 2N3904, the current mismatch of the MAT14 current mirror is reduced from 5.3% to 1.6%.

Figure 3. By using matched MAT14 single-chip quad transistors and parallel LDO regulators, the loss matching is reduced.

This parallel circuit architecture can be expanded as needed to use more LT3033s by extending the current mirror and adding LT3033 slave devices. Figure 4 shows a current sharing solution with four LT3033s in parallel using MAT14. The thermal performance is shown in Figure 5. The temperature of the four LT3033s ranged from 51°C to 58°C. Considering the voltage drop along the input trace of each device, the heat dissipation on the board is uniform, indicating that this solution achieves balanced current sharing. Figure 6 shows the transient response of a 12 A power supply running at 1.5 V output from a 1.8 V input.

Figure 4. Using the MAT14, four LT3033s are connected in parallel.

Figure 5. Thermal performance of four LT3033s in parallel.

Figure 6. Load transient response of four LT3033s in parallel.

The LT3033 is a 3A VLDO regulator in a 3 mm × 4 mm package. It has a built-in output current monitoring function, and multiple LT3033 VLDO regulators can be connected in parallel for high current applications. With a typical dropout voltage of only 95 mV under full load conditions, the LT3033 is well suited for high current applications from low input voltage to low output voltage, with electrical efficiency comparable to that of switching regulators. Other features include programmable current limit, power good flag, and thermal limiting to provide a reliable and stable solution. Battery-powered systems can benefit from low quiescent current and reverse battery protection.