How to choose the right power chip

    LDO is a linear regulator. A linear regulator uses transistors or FETs operating in its linear region to subtract excess voltage from the applied input voltage to produce a regulated output voltage. The so-called drop voltage refers to the minimum value of the difference between the input voltage and the output voltage required by the regulator to maintain the output voltage within 100 mV above or below its rated value.

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    LDO (low voltage drop) regulators with positive output voltages usually use power transistors (also known as transfer devices) as PNPs. This transistor allows saturation, so the regulator can have a very low voltage drop, usually about 200mV; In contrast, the voltage drop of a conventional linear regulator using NPN composite power transistors is about 2V. The negative output LDO uses NPN as its transfer device, and its operation mode is similar to the PNP device of the positive output LDO.

    Newer developments use CMOS power transistors, which can provide the lowest voltage drop. With CMOS, the only voltage drop through the regulator is caused by the on resistance of the load current of the power supply device. If the load is small, the voltage drop generated in this way is only several tens of millivolts.

    DC / DC means DC to DC (conversion of different DC power values). As long as this definition is met, it can be called DC / DC converter, including LDO. But generally speaking, the device that converts DC to DC by switching is called DC / DC.

    LDO means low-voltage drop, which is explained by a paragraph: low-voltage drop (LDO) linear regulator has low cost, low noise and small static current, which are its outstanding advantages. It also requires few external components, usually only one or two bypass capacitors. The new LDO linear regulator can achieve the following indicators: output noise 30 μ 5. PSRR is 60dB, static current is 6 μ A. The voltage drop is only 100 mV.

    The performance of LDO linear regulator can reach this level mainly because the regulator is a p-channel MOSFET, while the common linear regulator is a PNP transistor. The p-channel MOSFET is voltage driven and does not need current, so the current consumed by the device itself is greatly reduced; On the other hand, in the circuit using the PNP transistor, in order to prevent the PNP transistor from entering the saturation state and reducing the output capability, the voltage drop between the input and the output may not be too low; And the voltage drop on the p-channel MOSFET is approximately equal to the product of the output current and the on resistance. Since the on resistance of the MOSFET is very small, the voltage drop on it is very low.

    If the input voltage and the output voltage are very close, it is best to choose LDO voltage regulator to achieve high efficiency. Therefore, LDO voltage regulator is mostly used in the application of converting the lithium-ion battery voltage to 3V output voltage. Although 10% of the battery‘s energy is not used in the end, the LDO regulator can still ensure that the battery‘s working time is long and the noise is low.

    If the input voltage and the output voltage are not very close, it is necessary to consider switching type DCDC, because from the above principle, the input current of LDO is basically equal to the output current. If the voltage drop is too large, the energy consumed on LDO is too large and the efficiency is not high.

    The DC-DC converter includes a step-up, a step-down. The DC-DC converter has the advantages of high efficiency, large output current and small static current. With the improvement of integration, many new DC-DC converters need only a few external inductors and filter capacitors. However, the output pulsation and switching noise of this kind of power controller are large and the cost is relatively high.

    In recent years, with the development of semiconductor technology, the cost of surface mounted inductors, capacitors, and highly integrated power control chips has been continuously reduced and the volume has become smaller and smaller. Since MOSFETs with small on resistance can output large power, there is no need for external high-power FETs. For example, for an input voltage of 3V, an output of 5V / 2A can be obtained by using the nFET on the chip. Second, for small and medium power applications, a low-cost compact package can be used. In addition, if the switching frequency is increased to 1 MHz, it is also possible to reduce the cost and use inductors and capacitors with smaller sizes. Some new devices also add many new functions, such as soft start, current limiting, PFM or PWM mode selection.

    In general, DCDC must be selected for boosting, and DC-DC or LDO should be selected for boosting. Cost, efficiency, noise and performance should be compared.

    LDO has small volume and small interference. When the difference between input and output voltage is large, the conversion efficiency is low.

    The advantages of DC-DC are high conversion efficiency and large current, but the output interference is large and the volume is relatively large.

    LDO generally refers to linear voltage regulator - lowdropout, while DC / DC is the general term of linear and switching voltage regulators.

    If your output current is not large (such as within 3a), and the input-output voltage difference is not large (such as 3.3V to 2.5V, etc.), you can use LDO voltage regulator (the advantage is that the ripple of the output voltage is very small). Otherwise, it is better to use a switch type regulator. If it is boosting, only a switch type regulator can be used (if the ripple control is not good, it will easily affect the system operation).

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