What is the LDO low pressure difference linear regulator?
LDO is Low Dropout Regulator, is a low pressure difference linear regulator. This is relative to the traditional linear regulator. Traditional linear regulators, such as the 78xx series, require the input voltage to be at least 2V ~ 3V than the output voltage, otherwise it will not work. However, in some cases, such conditions are obviously too harsh, such as 5V to 3.3V, the differential pressure between the input and output is only 1.7V, apparent that this is not satisfied with the working conditions of the conventional linear regulator. In this case, chip manufacturers have developed a voltage conversion chip of the LDO class.
LDO is a linear regulator that uses a transistor or field effect tube (FET) operating in its saturated region, subtracting the excess voltage from the application input voltage, producing an adjusted output voltage. The so-called voltage drop voltage refers to the minimum value of the input voltage and output voltage difference required to maintain the output voltage within 100 mV of its rated value. LDO (low pressure drop) regulator of the positive output voltage typically uses a power transistor (also referred to as a transmission device) as a PNP. This transistor allows saturation, so the regulator can have a very low pressure drop voltage, usually from about 200 mV; compared with the conventional linear regulator of the NPN composite power source transistor is about 2V. Negative output LDO uses NPN as its transfer device, and its operating mode is similar to the PNP device of the positive output LDO.
Updated development uses a MOS power transistor that provides the lowest pressure drop voltage. Using power MOS, the unique voltage pressure drop through the regulator is caused by the ON resistance of the power supply load current. If the load is small, the pressure drop produced in this way is only tens of millivolts.
The voltage difference and the ground current value are mainly determined by the pass element of the linear regulator, the voltage difference and the ground current value are set, and the device type applicable to the regulator can be determined. The five major mainstream linear regulators each have different bypass element and unique properties, respectively, suitable for different equipment.
Principle analysis
As shown in the figure below, the circuit is compared by the series adjustment tube Vt, sampling resistors R1, and R2, and compare the amplifier A.
The sampling voltage is added to the inverted input of the amplifier A, compared to the reference voltage Uref applied to the same phase input, after amplifying the amplifier A, control the voltage drop of the series adjusting tube, thereby stabilizing the output voltage. When the output voltage UOOUT is lowered, the difference between the reference voltage and the sampling voltage increases, the drive current of the comparative amplifier is increased, and the tandem adjustment tube is reduced, thereby enhancing the output voltage. Conversely, if the output voltage UOUT exceeds the set value required, the front drive current of the comparative amplifier output is reduced, thereby lowering the output voltage. During the power supply, the output voltage correction is continuously carried out, and the adjustment time is only limited by the comparative amplifier and the output transistor loop response speed.
It should be noted that the actual linear regulator should also have many other functions, such as load short circuit protection, overpressure shutdown, overheating shutdown, reverse protection, etc., and the series adjustment tube can also use MOSFET.
Technology Analysis of Common Linear Regulators
The advantage of the standard NPN regulator is that the stable ground current of the PNP transistor base current is quite stable even without the output capacitance. Such regulators are more suitable for equipment with high voltage difference, but higher pressure differences make this regulator not suitable for many embedded devices.
For embedded applications, NPN bypass transistor regulators are a nice choice because it is small and very easy to use. However, this regulator is still not suitable for battery-powered equipment with very low pressure differences, as it is not low enough. Its high gain NPN bypass tube can stabilize the ground current in several milliamps, and its common emitter structure has a low output impedance.
The PNP bypass transistor is a low pressure difference regulator, wherein the bypass element is a PNP transistor. Its input and output pressure difference is typically between 0.3 and 0.7V. Because of the low differential pressure, this PNP bypass transistor regulator is very suitable for use in battery-powered embedded devices. However, its large ground current will shorten the life of the battery. In addition, the PNP transistor gain is low, and the unstable ground current of several milliamps is formed. Due to the use of a common emitter structure, its output impedance is relatively high, which means that the capacitance of an external specific range capacity and equivalent series resistance (ESR) can be stabilized.
Since the P-channel FET regulator has a lower differential pressure and ground current, it is widely used in many batteries. This type of regulator uses the P-channel FET as its bypass element. The voltage difference of this regulator can be very low because it is easy to adjust the drain-source impedance to a lower value by adjusting the FET size. Another useful feature is low ground current because the "gate current" of the P-channel FET is very low. However, since the P-channel FET has a relatively large gate capacitance, it needs to be external to capacitance of a specific range capacity and ESR to stabilize.
The N-channel FET regulator is ideal for devices that require low pressure difference, low-ground current, and high load current. The N-channel FET used for the bypass tube is used, so the pressure difference and ground current of this regulator is very low. Although it also requires an external capacitor to stabilize, the capacitance value is not very large, and ESR is not important. The N-channel FET regulator requires a charging pump to establish a gate bias voltage, so the circuit is relatively complex. Fortunately, the N-channel FET size at the same load current can be 50% smaller than the P-channel FET.
What is the difference with the DC-DC converter?
The DC-DC means direct current (to) DC (conversion of different DC power values), as long as this definition can be called DC-DC converter, including LDO. However, the general statement is to call DC-DC to DC Change (to) DC by switching mode.
LDO is the meaning of low voltage drop. This is a paragraph: low pressure drop (LDO) linear regulator is low, low noise, low static current, which is its prominent advantage. The external components it need are very small, usually only one or two bypass capacitors. The new LDO linear regulator can reach the following indicators: output noise 30μV, PSRR is 60dB, and the static current is 6 μA (TPS78001 of Ti reaches IQ = 0.5ua), and the voltage drop is only 100mV (Ti quantity is Name 0.1mV LDO). The performance of the LDO linear regulator can achieve this level. The main reason is that the adjustment tube is used with a P-channel MOSFET, while the normal linear regulator is using a PNP transistor. The P-channel MOSFET is a voltage driven. It does not require a current, which greatly reduces the current consumption of the device itself; on the other hand, in the circuit of the PNP transistor, in order to prevent the PNP transistor from entering the saturation state, the output capacity, input, and output The voltage drop can not be too low; and the voltage drop on the P-channel MOSFET is generally equal to the product of the output current and the on-resistance. Since the on-resistance of the MOSFET is small, it is very low above.
If the input voltage and the output voltage are close, it is best to use the LDO regulator to achieve high efficiency. Therefore, most of the LDO regulator is selected in applications that convert the lithium ion battery voltage to 3V output voltage. Although the energy of the battery is not used by 10%, the LDO regulator can still ensure that the battery has a long working time and the noise is low.
If the input voltage and output voltage are not very close, consider using the switch type DCDC, because from the above principle, the input current of the LDO is substantially equal to the output current, if the pressure drop is too large, consumes on the LDO The energy is too large, and the efficiency is not high.
The DC-DC converters include boost, buck, liter / buck, and reverse phase. The advantage of the DC-DC converter is that the efficiency is high, and the large current can be output, the static current is small. As integration improves, many new DC-DC converters require only several external sensors and filter capacitors. However, the output pulsation and switching noise of such power controllers is relatively high.
In recent years, with the development of semiconductor technology, surface-mounted inductors, capacitors, and high-intensive power control chips have been reduced, and the volume is getting smaller. Since the MOSFET with a small on-resistance can output a lot of power, there is no need to externally high power FET. For example, for 3V input voltages, the output of 5V / 2A can be obtained by using the NFET on the chip. Second, for medium and small power applications, cost low-small packaging can be used. In addition, if the switching frequency is increased to 1 MHz, it is also possible to reduce costs, which can be used in size inductors and capacitors. Some new devices also add a number of new features such as soft start, current limiting, PFM, or PWM mode.
In general, boosting is a must choose DCDC, buck, is to choose DCDC or LDO, to compare at cost, efficiency, noise, and performance.
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