Power Supply Chip Selection Guide

In modern electronic devices, power supply chips play a crucial role. They provide a stable power supply for a variety of devices to ensure the normal operation of the equipment. However, a variety of different types and specifications of power supply chips exist in the market, what are the areas that should be considered when choosing a power supply chip? Input voltage linear adjustment rate, the relative impact on the output voltage when the input voltage changes linearly? Here are a few conceptual issues to understand first:

What is a power chip? What is its role?

• Output Voltage Load Adjustment Ratio: the relative change of output voltage when the load current changes.
• Output voltage accuracy: the device output voltage error range
• Load transient response: load current from a small value to the maximum flow of rapid changes in output voltage fluctuations.

Should I choose DC/DC or LDO for my power supply chip?

This depends on your application. For example, used in boost occasions, of course, can only use DC/DC, because the LDO is a voltage drop type, can not boost.

Also look at the main features of each:

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

So if it is used in the case of large voltage drop, choose DC/DC, because of its high efficiency, and LDO will be because of the large voltage drop and its own loss of a large part of the efficiency;

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

LDO is lowdropoutregulator, meaning low dropout linear regulator, is relative to the traditional linear regulator. Traditional linear regulators, such as the 78xx series of chips are required to input voltage than the output voltage is higher than the output voltage of more than 2V ~ 3V, otherwise it can not work properly. But in some cases, such a condition is obviously too harsh, such as 5v to 3.3v, the input and output voltage difference is only 1.7v, obviously does not meet the conditions. For this situation, there is a LDO class of power conversion chips.

LDO linear buck chip: the principle is equivalent to a resistor voltage divider to realize the voltage reduction, energy loss, the lowering of the voltage is converted into heat, the greater the voltage difference between the buck and the load current, the more pronounced the chip heat. The package of this type of chip is relatively large, which is convenient for heat dissipation.

LDO linear buck chips such as: 2596, L78 series and so on.

DC/DC buck chips: energy loss in the buck process is relatively small, 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, meaning low dropout linear regulator, is relative to the traditional linear regulator. Traditional linear regulators, such as the 78xx series of chips are required to input voltage than the output voltage is higher than the output voltage of more than 2V ~ 3V, otherwise it will not work properly.

But in some cases, such a condition is obviously too harsh, such as 5v to 3.3v, the input and output voltage difference is only 1.7v, obviously does not meet the conditions. For this situation, there is a LDO class of power conversion chips. The production of LDO chip companies, common ALPHA, Linear (LT), Micrel, Nationalsemiconductor, TI and so on.

What is an LDO (Low Dropout) Regulator?

An LDO is a linear voltage regulator. Linear regulators use transistors or FETs operating in 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.

LDO (Low Dropout) regulators for positive output voltages 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 a traditional linear regulator using an NPN composite power transistor. 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 device load current. If the load is small, the voltage drop produced in this way is only tens of millivolts.

DCDC means DC to (to) DC (conversion of different DC power supply values), as long as it meets this definition, it can be called a DCDC converter, including LDO. However, it is generally said that the device that converts DC (to) DC by switching is called DCDC.

LDO means low dropout, here is a description: Low dropout (LDO) linear regulator has low cost, low noise, and low quiescent current, these are its outstanding advantages. It also requires very few external components, usually just one or two bypass capacitors. The new LDO linear regulator can achieve the following indicators: the output noise is 30μV, the PSRR is 60dB, the quiescent current is 6μA, and the voltage drop is only 100mV.

The main reason why the performance of the LDO linear regulator can reach this level is that the adjustment tube uses a P-channel MOSFET, while the ordinary linear regulator uses a PNP transistor. 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 a circuit using a PNP transistor, in order to prevent the PNP transistor from entering a saturated state and reduce the output capability, between input and output The voltage drop between them cannot be too low; and the voltage drop on the P-channel MOSFET is roughly equal to the product of the output current and the on-resistance. Due to the small on-resistance of the MOSFET, the voltage drop across it is very low.

If the input voltage is very close to the output voltage, it is best to use an LDO regulator, which can achieve very high efficiency. Therefore, LDO regulators are mostly used in applications that convert the lithium-ion battery voltage to a 3V output voltage. Although the last 10% of the battery's energy is not used, the LDO regulator can still ensure the battery's working time is longer and the noise is lower.

If the input voltage and the output voltage are not very close, you should consider using a switching DCDC, because from the above principle, you can know 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 not very efficient.

DC-DC converters include step-up, step-down, step-up/down and inverting 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 pulsation and switching noise of this type 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 a MOSFET with a small on-resistance can output a large amount of power, no external high-power FET is 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 low-to-medium power applications, small, low-cost packages can be used. In addition, if the switching frequency is increased to 1MHz, the cost can be reduced and smaller inductors and capacitors can be used. Some new devices also add many new functions, such as soft start, current limiting, PFM or PWM mode selection, etc.

In general, DCDC must be selected for step-up, 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 less in interference. When the input and output voltage difference 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 voltage regulator--LowDropOut, while DC/DC is a general term for linear and switching voltage 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 an LDO regulator (the advantage is that the ripple of the output voltage 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 is not well controlled, it will easily affect the system work).

Selection of LDOs

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

1. High noise and ripple suppression;

2. Occupies a small PCB area, 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 the function of instantaneous calibration and output state self-test;

5. The voltage regulator is required to have low dropout and low power consumption;

6. Low line cost and simple scheme are required;

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

When choosing a power chip, we need to consider multiple factors, such as power requirements, efficiency, stability, and cost, in order to choose the most suitable power chip for our equipment. Hope the guidance provided in this article can help you to be more confident and wise when choosing power chips.

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