POWER DEVICES
full load to guarantee good current mirror matching. The output voltage of the slave LT3033 is set at 1.35V, 10% lower than the circuit output to ensure the master LT3033 stays in control. The feedback resistors of the slave LT3033 are split into sections to ensure adequate headroom for the slave NPN. A 10nF, 5.1kohmcapacitor and resistor combination added to the IMON pin of the slave device frequency compensates the feedback loop. Though this circuit can supply 6A load current, current sharing
accuracy is limited by the mismatch between two NPN devices - mismatches result in uneven heat distribution on the board. Higher current sharing accuracy can be achieved by using a matched monolithic transistor, such as the MAT14 from Analog Devices, to replace the two discrete NPN devices. The MAT14 is a quad monolithic NPN transistor that offers excellent parametric matching. Its maximum current gain matching is 4%. Figure 4 compares the output current of each LDO regulator using
Figure 3: Paralleling two LT3033s This output current monitor enables current sharing for multiple
LT3033s. Despite its tiny size, the LT3033 also includes a number of protection features, including internal current limit with foldback, thermal limit, reverse current, and reverse-battery protections.
Using two LT3033s in parallel for a 6A application For applications that require more than 3A, multiple LT3033s can be paralleled by taking advantage of its current monitor feature. Figure 3 illustrates two LT3033s in parallel together with two 2N3904 NPN devices to generate a 1.5V, 6A output. The individual IN pins and OUT pins are tied together, respectively. One LT3033 acts as master, controlling the LT3033 slave device.
discrete and matched NPN devices, respectively. Compared to the 2N3904, the MAT14 current mirror reduces the current mismatch from 5.3% to 1.6%.
Figure 6: Thermal performance of four LT3033s in parallel
Paralleling four LT3033s using matched components The paralleling circuit architecture is scalable to as many LT3033s as needed by extending the current mirror and adding slave LT3033 devices. Figure 5 shows a solution with four LT3033s in parallel using the MAT14 for current sharing. The thermal performance is shown in Figure 6. The four LT3033s reach temperatures ranging from 51°C to 58°C. Considering the voltage drop along the input trace for each part, the heat spreads evenly on the board, indicating balanced current sharing for this solution. Figure 7 shows the transient response of a 1.5V output, 12A power supply operating from a 1.8V input.
Figure 4: Current sharing mismatch is reduced by using the MAT14 matched monolithic quad transistor with parallel LDO regulators
The IMON pins combined with an NPN current mirror create a simple
amplifier. This amplifier injects current into the feedback divider of the slave LT3033 to force the IMON currents from each LT3033 to be equal. The 100ohmresistors provide 113mV emitter degeneration at
Figure 7: Load transient response of four LT3033s in parallel The LT3033 is a 3A VLDO regulator in a 3mm × 4mm package.
Multiple LT3033 VLDO regulators can be paralleled for high current applications because of their built-in output current monitor feature. With only 95mV typical voltage drop at full load, LT3033 is ideal for low input voltage to low output voltage, high current applications, yielding comparable electrical efficiency to a switching regulator. Other features include programmable current limit, power good flag, and thermal limiting for reliable and robust solutions. Battery- powered systems benefit from low quiescent current and reverse-battery protection.
Figure 5: Four LT3033s in parallel using the MAT14
Analog Devices
www.analog.com
DECEMBER/JANUARY 2022 | ELECTRONICS TODAY 25
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