What is a power chip? What is its function? When choosing a power chip, what should be considered? Input voltage linear adjustment rate, the relative influence of the input voltage linear change on the output voltage? Let’s first understand a few conceptual issues:
- Output voltage load adjustment rate: the relative change of output voltage when the load current changes
- Output voltage accuracy: error range of device output voltage
- Load transient response: when the load current changes rapidly from a small value to the maximum current, the output voltage fluctuates.
- Choose DC/DC or LDO for the power chip?
This depends on your application. For example, when used in boost applications, of course, only DC/DC can be used, because LDOs are of a voltage drop type and cannot be boosted.
Main features of DC/DC and LDO
Also take a look at their main features:
DC/DC: high efficiency and high noise;
LDO: low noise, low 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 the 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 its noise is low, the power supply is clean, and the peripheral circuit is simple and the cost is low.
LDO is low-dropout regulator, which means a low dropout linear regulator, which is relative to a traditional linear regulator. Traditional linear regulators, such as 78xx series 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 pressure difference between input and output is only 1.7v, which obviously does not meet the conditions. In response to this situation, there is an LDO-type power conversion chip.
LDO linear step-down chip: The principle is equivalent to a resistor divider to achieve a step-down. The energy loss is large, and the dropped voltage is converted into heat. The greater the voltage drop and the load current of the step-down, the more obvious the chip heats up. The package of this type of chip is relatively large, which is convenient for heat dissipation.
LDO linear step-down chips such as 2596, L78 series, etc.
DC/DC step-down chip: In the step-down process, the energy loss is relatively small, and the chip heating 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 low-dropout regulator, which means a low dropout linear regulator, which is relative to a traditional linear regulator. Traditional linear regulators, such as 78xx series 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 pressure difference between input and output is only 1.7v, which obviously does not meet the conditions. In response to this situation, there is an LDO-type power conversion chip. There are many companies that produce LDO chips, such as ALPHA, Linear (LT), Micrel, Nationalsemiconductor, TI, and so on.
What is an LDO (low dropout) regulator?
LDO is a linear regulator. A linear regulator uses a transistor or FET operating in its linear region to subtract the 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 with positive output voltage usually use power transistors (also called transfer devices) as PNPs. This kind of transistor allows saturation, so the regulator can have a very low drop voltage, usually around 200mV; in comparison, the voltage drop of a traditional linear regulator using NPN composite power transistors is around 2V. The negative output LDO uses NPN as its transfer device, and its operating mode is similar to the PNP device of the positive output LDO.
Newer developments use CMOS power transistors, which can provide the lowest drop voltage. With CMOS, the only voltage drop across 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 produced by this method is only tens of millivolts.
What’s the mean of DCDC?
DCDC means DC to (to) DC (conversion of different DC power values). we can call it DC/DC converter, as long as it meets this definition, including LDO. But the general saying is that DC/DC is the device that converts (to) direct current by switching.
LDO means low dropout. This is 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 indicators: output noise is 30μV, PSRR is 60dB, the quiescent current is 6μA, and the voltage drop is only 100mV.
Why LDO can reach this level?
The main reason why the performance of the LDO linear regulator can reach this level is that the pass tube is 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 it greatly reduces the current consumed by the device itself; on the other hand, in a circuit that uses PNP transistors, in order to prevent the PNP transistor from entering a saturated state and reduce the output capacity, the input and output are The voltage drop 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. Since the on-resistance of the MOSFET is very small, 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 to achieve high efficiency. Therefore, we mostly used LDO regulators in applications that convert the lithium-ion battery voltage to a 3V output voltage. Although we don’t use 10% of the battery’s energy in the end, the LDO regulator can still ensure the battery’s working time is longer and the noise is low.
If the input voltage and the output voltage are not very close, you should consider using a switching DCDC. Because 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. The energy is too large and the efficiency is not high.
What does DC-DC converters include?
DC-DC converters include boost, buck, boost/buck, and reverse circuits. The advantages of DC-DC converters are high efficiency, high output current, and low quiescent current. With the improvement of integration, many new DC-DC converters only need a few external inductors and filter capacitors. However, the output pulsation 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 capacitors is low, and the size is smaller. Due to the emergence of a MOSFET, we don’t require an external high-power FET. For example, for an input voltage of 3V, we can use an on-chip NFET to obtain an output of 5V/2A. Secondly, for small and medium power applications, we can use low-cost small packages. In addition, if we increase the switching frequency to 1MHz, we can reduce costs and use smaller inductors and capacitors. Some new devices also add many new functions, such as soft start, current limit, PFM or PWM mode selection.
How to select converter?
In general, we must select DCDC for step-up, and DC-DC or LDO for step-down. And compared in terms of cost, efficiency, noise, and performance.
The LDO is small in size and has less interference. When the voltage difference between the input and the output becomes larger, the conversion efficiency is low.
The advantage of DC-DC is that the conversion efficiency is high, and it can be large current, but the output interference is large, and the volume is relatively large.
LDO generally refers to a linear regulator-LowDropOut, while DC/DC is the 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 an LDO regulator (the adv antage is that the ripple of the output voltage is small). Otherwise, it is best to use a switching regulator. If it is a boost, you can only use a switching regulator.
How to choose LDO?
When the designed circuit has the following requirements for the shunt power supply:
- High noise and ripple suppression;
- The small footprint of PCB board, such as handheld electronic products such as mobile phones;
- The circuit power supply does not allow the use of inductors, such as mobile phones;
- The power supply needs to have instantaneous calibration and output state self-checking functions;
- The low voltage drop of voltage stabilizer is required, and its own power consumption is low;
- Low line cost and simple scheme are required;
At this time, choosing 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 to help you, you need to choose according to the project when designing.
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