“DC-DC converters include boost, buck, boost/buck, and inverse equalization circuits. The advantages of DC-DC converters are high efficiency, high output current, and low quiescent current. With increased integration, many new DC-DC converters require only a few external inductors and filter capacitors. However, the output ripple and switching noise of this type of power controller are relatively high, and the cost is relatively high.

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I. Introduction

What is a power chip? What does it do? When choosing a power chip, what should be considered? The input voltage linear adjustment rate, the relative influence of the input voltage on the output voltage when the input voltage changes linearly? Here are a few conceptual questions:

1. Output voltage load regulation: relative change of output voltage when the load current changes
2. Output voltage accuracy: the error range of the output voltage of the device
3. Load transient response: When the load current changes rapidly from a small value to the maximum current, the output voltage fluctuates.
4. Should the power chip choose DC/DC or LDO?

This depends on your application. For example, in boosting applications, of course, only DC/DC can be used, because LDO is a voltage drop type and cannot be boosted.

Also take a look at their main features:

DC/DC: high efficiency, high noise;
LDO: low noise, small quiescent current;

Therefore, if it is used in the case of a relatively large voltage drop, choose DC/DC because of its high efficiency, and LDO will lose a large part of its efficiency due to the large voltage drop;

If the voltage drop is relatively small, choose LDO because of its low noise, clean power supply, simple peripheral circuit and low cost.

LDO is lowdropoutregulator, which means low dropout linear regulator, which is relative to traditional linear regulators. Traditional linear regulators, such as the 78xx series of chips, require the input voltage to be higher than the output voltage by more than 2v~3V, otherwise it will not work properly. But in some cases, such conditions are obviously too harsh, such as 5v to 3.3v, the voltage difference between input and output is only 1.7v, which obviously does not meet the conditions. In view of this situation, there are LDO-type power conversion chips.

LDO linear step-down chip: The principle is equivalent to a resistor divider to achieve step-down, the energy loss is large, and the dropped voltage is converted into heat. The package of this type of chip is relatively large, which is easy to dissipate heat.

LDO linear step-down chips such as: 2596, L78 series, etc.

DC/DC step-down chip: During the step-down process, the energy loss is relatively small, and the chip heat is not obvious. The chip package is relatively small and can realize PWM digital control.

DC/DC step-down chips such as: TPS5430/31, TPS75003, MAX1599/61, TPS61040/41

LDO is lowdropoutregulator, which means low dropout linear regulator, which is relative to traditional linear regulators. Traditional linear regulators, such as the 78xx series of chips, require the input voltage to be higher than the output voltage by more than 2v~3V, otherwise it will not work properly.

But in some cases, such conditions are obviously too harsh, such as 5v to 3.3v, the voltage difference between input and output is only 1.7v, which obviously does not meet the conditions. In view of this situation, there are LDO-type power conversion chips. There are many companies producing LDO chips, such as ALPHA, Linear(LT), Micrel, National semiconductor, TI and so on.

2. What is an LDO (low dropout) regulator?

LDO is a linear regulator. Linear regulators use transistors or FETs operating within their 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 100mV of its rated value.

Positive output voltage LDO (low dropout) regulators typically use power transistors (also known as pass devices) as PNPs. This transistor is allowed to saturate, so the regulator can have a very low dropout voltage, typically around 200mV; compared to around 2V for traditional linear regulators using NPN compound power transistors. The negative output LDO uses NPN as its pass device, and its operation mode is similar to the PNP device of the positive output LDO.

Newer developments use CMOS power transistors, which provide the lowest dropout voltage. With CMOS, the only voltage drop across the regulator is due to the ON resistance of the power supply’s load current. If the load is small, the voltage drop generated in this way is only tens of millivolts.

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

LDO is the meaning of low dropout, which has a description: Low dropout (LDO) linear regulators have low cost, low noise, and low quiescent current. These 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 specifications: output noise 30μV, PSRR is 60dB, quiescent current 6μA, the voltage drop is only 100mV.

The main reason why the performance of LDO linear regulators can reach this level is that the pass transistors use P-channel MOSFETs, while ordinary linear regulators use PNP transistors. The P-channel MOSFET is voltage-driven and does not require current, so the current consumed by the device itself is greatly reduced; on the other hand, in circuits using PNP transistors, in order to prevent the PNP transistor from entering a saturated state and reducing the output capability, the difference between input and output The voltage drop between MOSFETs should not be too low; while the voltage drop across a P-channel MOSFET is roughly equal to the product of the output current and the on-resistance. Since the on-resistance of the MOSFET is small, the voltage drop across it is very low.

If the input voltage and output voltage are very close, it is best to choose an LDO regulator, which can achieve high efficiency. Therefore, LDO regulators are mostly used in applications that convert lithium-ion battery voltage to 3V output voltage. Although 10% of the battery’s energy is ultimately unused, the LDO regulator can still ensure a longer battery life and lower noise.

If the input voltage and output voltage are not very close, it is necessary to consider using a switch-type DCDC, because from the above principle, it can be known that the input current of the LDO is basically equal to the output current. If the voltage drop is too large, it will be consumed on the LDO. Too much energy and inefficient.

DC-DC converters include boost, buck, boost/buck, and inverse equalization circuits. The advantages of DC-DC converters are high efficiency, high output current, and low quiescent current. With increased integration, many new DC-DC converters require only a few external inductors and filter capacitors. However, the output ripple and switching noise of this type of power controller are relatively high, 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 a MOSFET with a small on-resistance can output a large power, an external high-power FET is not required. 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, low-cost small packages can be used. In addition, if the switching frequency is increased to 1MHz, the cost can also be reduced, and smaller inductors and capacitors can be used. Some new devices also add many new functions, such as soft start, current limit, PFM or PWM mode selection.

In general, DCDC must be selected for boost, and DC-DC or LDO for step-down should be compared in terms of cost, efficiency, noise and performance.

LDO is small in size and small in interference. When the difference between the input and output voltages is large, the conversion efficiency is low.

The advantage of DC-DC is that the conversion efficiency is high and the current can be large, but the output interference is large and the volume is relatively large.

LDO generally refers to a linear regulator – LowDropOut, and DC/DC is a general term for linear and switching regulators.

If your output current is not very large (such as within 3A), and the input and output voltage difference is not large (such as 3.3V to 2.5V, etc.), you can use the LDO regulator (the advantage is that the output voltage ripple is very small). Otherwise, it is best to use a switching regulator. If it is a boost, you can only use a switching regulator (if the ripple control is not good, it will easily affect the system work).

Third, the choice of LDO

When the designed circuit has the following requirements for the shunt power supply:

1. High noise and ripple rejection;
2. It occupies a small area of ​​the PCB board, such as mobile phones and other handheld Electronic products;
3. The circuit power supply does not allow the use of inductors, such as mobile phones;
4. The power supply needs to have instantaneous calibration and output status self-checking functions;
5. The voltage regulator is required to have a low voltage drop, and its own power consumption is low;
6. The line cost is required to be low and the scheme is simple;

At this time, the choice of LDO is the most appropriate choice, while meeting various requirements of product design. The above is the selection method of the power chip. I hope it can help you. You need to choose according to different projects when designing.