Current Boost Regulator with Onsemi NCV4264 LDO

By Mike Donnelly - Principal Engineer at Siemens Digital Industries Software.

This 5V regulator circuit provides up to 3A load current capability, well above the 100mA current limit of the Onsemi NCV4264 LDO component itself. This is thanks to the load sharing role of the Onsemi 2N5195G PNP bypass transistor.

For the first 30ms of the simulation, the load switch is open so the total load current flows through the fixed 100 Ohm resistor (~50mA = 5V/100Ohm, see the yellow waveform on the right). After the switch closes and for the next 50ms, the load current jumps to approximately 2.55A (i.e. the additional 2.5A flows through the 2 Ohm switched resistor). Note that the output voltage is very close to 5V under both loading conditions (purple waveform), as desired for a regulated output!

To better understand how this works, note that most of the current supply to the LDO passes through the 15 Ohm bias resistor on the left. The corresponding voltage drop, as seen by comparing the brown and green waveforms on the left, is across the emitter-base junction of the PNP transistor. When that current rises above ~40mA, giving a sufficient (> 0.6V) forward bias to the transistor, it results in a large transistor "boost" current flow through the parallel path to supply the load. You can compare the "boost" current (magenta waveform) to the normal LDO current (blue waveform) under these two distinct operating load conditions. Very little "boost" current is used under light-load, but it supplies almost all of the current under heavy loading conditions.

The following RS components are used in the design: 

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Below we have the electro-thermal simulation version of this Current Boost Regulator with Onsemi NCV4264 LDO.

In this version of the design, the "junction to case" thermal resistance values from the datasheets for the NCV4264 LDO and the 2N5195G PNP were used in a dynamic thermal network. This network represents the heat transfer, heat capacitance and thermal coupling effects of an example PCB. The values for the external thermal elements depend on the specific board implementation. The user can change these and other parameters of the system, to better understand the sensitivity of the critical junction temperatures on these external mounting parameters, as well as the electrical operating conditions of the circuit.

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Do you have questions on the above? Ask in the comments section below and Mike is here to help!