How to choose a suitable power chip, do you know this?

1. Foreword

What is a power chip? What is its function? What should be considered when choosing a power supply chip? Input voltage ramp rate, relative effect of input voltage ramp on output voltage? Let's deal with a few conceptual issues first:

1.Output voltage load adjustment: relative change in output voltage when load current changes

2.Output Voltage Accuracy: Device output voltage error range

3.Load transient response: When load current rapidly changes from a small value to a maximum value, output voltage fluctuates.

4.Does the power supply chip select DC/DC or LDO?

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

Also check out main features of each:

DC/DC: High Efficiency, High Noise

LDO: low noise, low quiescent current

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

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

LDO is a low dropout regulator, i.e. a low dropout linear regulator which is different from traditional linear regulator. Traditional linear regulators, such as 78xx series, require input voltage to be 2-3V higher than output voltage or they will not work properly. But in some cases, such conditions are clearly too harsh, for example, 5v to 3.3v, voltage difference between input and output is only 1.7v, which clearly does not meet conditions. In response to this situation, there are LDO type power conversion chips.

Linear step-down LDO chip: The principle is equivalent to a resistive divider to realize step-down. The power loss is large, and reduced voltage is converted into heat. The greater difference between step-down voltage and current load, more obvious that chip heats up. The packaging of this type of chips is relatively large, which is convenient for heat dissipation.

Linear buck LDO ICs such as: 2596 series, L78, etc.

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

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

LDO is a low dropout regulator, i.e. a low dropout linear regulator which is different from traditional linear regulator. Traditional linear regulators, such as 78xx series, require input voltage to be 2-3V higher than output voltage or they will not work properly.

But in some cases, such conditions are clearly too harsh, for example, from 5 V to 3.3 V, voltage difference between input and output is only 1.7 V, which is clearly not consistent with conditions. In response to this situation, there are LDO type power conversion chips. There are many companies that make LDO chips, most common ones are ALPHA, Linear (LT), Micrel, Nationalsemiconductor, TI, etc.

Second, what is an LDO (Low Dropout) Voltage Regulator?

LDO is a linear voltage regulator. Linear regulators use transistors or FETs operating in their linear region to subtract excess voltage from an applied input voltage to produce a regulated output voltage. The so-called voltage drop refers to minimum difference between input voltage and output voltage required by regulator to keep output voltage within 100 mV of its nominal value.

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

New designs use CMOS power transistors to provide lowest voltage drop. In CMOS, only voltage drop across regulator is due to on-resistance of power device. If load is small, then voltage drop created in this way is only a few tens of millivolts.

DCDC means direct current to direct current (conversion of different values ​​of DC power), if it meets this definition, it can be called a DC/DC converter including LDO. However, it is commonly said that a device that converts DC(to)DC by switching is called a DCDC.

LDO means low voltage drop. Here's explanation: A linear low dropout (LDO) regulator is low cost, low noise, and low quiescent current are its outstanding advantages. It also requires very few external components, typically just one or two shunt capacitors. The new LDO linear regulator achieves following performance: 30uV output noise, 60dB PSRR, 6uA quiescent current, and only 100mV voltage drop.

The main reason why performance of LDO linear regulator can reach this level is because control tube uses a P-channel MOSFET, while conventional linear regulator uses a PNP transistor. The P-channel MOSFET is voltage controlled and does not require current, so current drawn by device itself is greatly reduced; on other hand, in a circuit using a PNP transistor, to prevent PNP transistor from going into saturation and reducing output power between input and output. The voltage drop between them cannot be too low, and voltage drop across a P-channel MOSFET is approximately equal to product of output current and on-state resistance. Due to low resistance of MOSFET in on state, voltage drop across it is very small.

If input voltage and output voltage are very close, it is better to use an LDO regulator, which can provide high efficiency. Therefore, LDO voltage regulators are mainly used to convert lithium-ion battery voltage to 3V output voltage. Even though last 10% of battery power is not used, LDO regulator can still provide longer battery life and lower noise.

If input voltage and output voltage are not very close, you should consider using a switching DCDC, because from above principle, we can know that LDO input current is basically equal to output current, if voltage drop is too large, power consumption is too a lot of power on LDO to be efficient.

DC converters include boost, buck, boost/buck, and invert circuits. The advantage of DC-DC converters is high efficiency, high output current and low quiescent current. Thanks to advanced integration, many new DC/DC converters require only a few external inductors and filter capacitors. However, output ripple and switching noise of this type of power controller are large, and cost is relatively high.

In recent years, with development of semiconductor technology, cost of inductors, capacitors and highly integrated power management circuits has steadily decreased, and their volume has become smaller and smaller. Because low on-resistance MOSFET can deliver high power, an external high power FET is not required. For example, with an input voltage of 3 V, an output voltage of 5 V / 2 A can be obtained using an NFET on chip. Second, for low to medium power applications, small, inexpensive kits can be used. In addition, by increasing switching frequency to 1 MHz, cost can be reduced and smaller inductors and capacitors can be used. Some new devices alsoadd many new features such as soft start, current limit, PFM or PWM mode selection, etc.

In general, DCDC should be chosen for upscaling and DC-DC or LDO for downsampling, which should be compared in terms of cost, efficiency, noise, and performance.

LDO is small in size and low in noise. When difference between input and output voltage is large, conversion efficiency is low.

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

LDO usually 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 (for example, within 3A) and difference between input and output voltage is small (for example, from 3.3V to 2.5V, etc.), you can use an LDO regulator (the advantage is that output voltage ripple is very small. Otherwise, it is better to use a switching regulator, if it is a boost, then only a switching regulator can be used (if ripple is poorly controlled, it will easily affect operation of system).

Third, LDO selection

When circuit being designed has following shunt power supply requirements:

1. Suppression of high noise and ripple;

2. Occupies a small area of ​​circuit board, such as mobile phones and other portable electronic devices;

3. The power circuit does not allow use of inductors such as mobile phones;

4. The power supply must have function of instant calibration and self-checking of output status;

5. The regulator requires low voltage drop and low power consumption;

6. Requires low line cost and simple circuitry.

Currently, LDO is most suitable choice because it can meet different product design requirements at same time. The above power chip selection method, I hope it can help you, you need to choose according to different projects when designing.