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| - | A low-dropout or LDO regulator is a DC linear voltage regulator that can regulate the output voltage even when the supply voltag | + | 低压差或LDO稳压器是一种能够在输入电压很接近输出要求电压的情况下也能够保持稳压的直流线性稳压器。相较于其他的直流－直流稳压器其优点是没有开关噪声（没有开关过程）、器件体积较小（不需要大电感或变压器），设计极其简单；最大的缺点是线性直流稳压器必须在器件上以发热的方式消耗能量以保证输出电压的稳定。 |
| - | e is very close to the output voltage. The advantages of a low dropout voltage regulator over other DC to DC regulators include the absence of switching noise (as no switching takes place), smaller device size (as neither large inductors nor transformers are needed), and greater design simplicity (usually consists of a reference, an amplifier, and a pass element). A significant disadvantage is that, unlike switching regulators, linear DC regulators must dissipate power across the regulation device in order to regulate the output voltage. | + | |
| - | History[edit] | + | ====来历==== |
| - | The adjustable low-dropout regulator debuted on April 12, 1977 in an Electronic Design article entitled "Break Loose from Fixed IC Regulators". The article was written by Robert Dobkin, an IC designer then working for National Semiconductor. Because of this, National Semiconductor claims the title of "LDO inventor".[3] Dobkin later left National Semiconductor in 1981 and founded Linear Technology where he is currently chief technology officer.[4] | + | 可调节输出的低压差线性稳压器最早于1977年4月12日发布，标题为"Break Loose from Fixed IC Regulators“文章，作者是大名鼎鼎的Robert Dobkin,当时他在美国国家半导体公司（[[http://www.national.com/|National Semiconductor]]）做IC设计师，由此美国国家半导体公司宣称自己是“LDO的发明者”。 Dobkin于1981年离开美国国家半导体公司创办了[[http://www.linear.com/|Linear Technology]]公司，目前他还任Linear Technology的CTO。 |
| - | Components[edit] | + | ====器件==== |
| - | Schematic of a low-dropout regulator | + | {{ :low_drop_voltage_regulator.png |}} |
| - | The main components are a power FET and a differential amplifier (error amplifier). One input of the differential amplifier monitors the fraction of the output determined by the resistor ratio of R1 and R2. The second input to the differential amplifier is from a stable voltage reference (bandgap reference). If the output voltage rises too high relative to the reference voltage, the drive to the power FET changes to maintain a constant output voltage. | + | <WRAP centeralign> **LDO的原理示意图** </WRAP> |
| + | 主要的器件是一个功率FET和一个差分放大器（误差放大）。差分放大器的一个输入端监测由输出电压经取样电阻R1/R2的比率决定的输出，另一个输入端连接一个稳定的电压基准源。如果输出电压相对于参考电压升的太高，则驱动功率FET以保持一个稳定的输出电压。 | ||
| - | Regulation[edit] | + | ====稳压==== |
| - | Low-dropout (LDO) regulators work in the same way as all linear voltage regulators. The main difference between LDO and non-LDO regulators is their schematic topology. Instead of an emitter follower topology, low-dropout regulators use open collector or open drain topology. In this topology, the transistor may be easily driven into saturation with the voltages available to the regulator. This allows the voltage drop from the unregulated voltage to the regulated voltage to be as low as the saturation voltage across the transistor.[2]:Appendix A | + | LDO和其它的线性稳压器工作模式是一样的，LDO和非LDO的稳压器的主要区别在于它们的原理拓扑结构，LDO采用了集电极开路或漏极开路的方式，而不是一个发射极跟随的方式。采用这种结构稳压器上的电压很容易驱动晶体管进入饱和状态，这就使得从待稳定的电压到稳定的输出电压之间的压差可以低到晶体管上的饱和电压。 |
| - | For the circuit given in the figure to the right, the output voltage is given as: | + | 如上图，输出电压为: |
| V_{OUT}= \left( 1 + \frac{R_1}{R_2} \right) V_{REF} | V_{OUT}= \left( 1 + \frac{R_1}{R_2} \right) V_{REF} | ||
| - | If a bipolar transistor is used, as opposed to a field-effect transistor or JFET, significant additional power may be lost to control it, whereas non-LDO regulators take that power from voltage drop itself. For high voltages under very low In-Out difference there will be significant power loss in the control circuit.[5] | + | 如果采用的是三极管，而不是场效应管或JFET，为控制它需要比较大的附加能量，非LDO会自身消耗掉压降导致的能量，For high voltages under very low In-Out difference there will be significant power loss in the control circuit. |
| - | Because the power control element functions as an inverter, another inverting amplifier is required to control it, which increases schematic complexity compared to simple linear regulator.[citation needed] | + | 因为功率控制的部分起到逆变器的作用，需要另外一个逆变的放大器来控制它，这样相对于普通的线性稳压器就增加了电路的复杂度。 |
| - | Power FETs may be preferable to reduce power consumption, but this poses problems when the regulator is used for low input voltage, as FETs usually require 5 to 10 V to close completely. Power FETs may also increase the cost. | + | 一般选用功率FET来降低功耗，但这在输入电压比较低的时候也会带来问题，因为FET一般需要5－10V才能彻底关断，功率FET也会导致成本增加。 |
| - | Efficiency and heat dissipation[edit] | + | ====效率和散热==== |
| The power dissipated in the pass element and internal circuitry (P_{LOSS}) of a typical LDO is calculated as follows: | The power dissipated in the pass element and internal circuitry (P_{LOSS}) of a typical LDO is calculated as follows: | ||
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| It is important to keep thermal considerations in mind when using a low drop-out linear regulator. Having high current and/or a wide differential between input and output voltage could lead to large power dissipation. Additionally, efficiency will suffer as the differential widens. Depending on the package, excessive power dissipation could damage the LDO or cause it to go into thermal shutdown. | It is important to keep thermal considerations in mind when using a low drop-out linear regulator. Having high current and/or a wide differential between input and output voltage could lead to large power dissipation. Additionally, efficiency will suffer as the differential widens. Depending on the package, excessive power dissipation could damage the LDO or cause it to go into thermal shutdown. | ||
| - | Quiescent current[edit] | + | ====静态电流==== |
| Among other important characteristics of a linear regulator is the quiescent current, also known as ground current or supply current, which accounts for the difference, although small, between the input and output currents of the LDO, that is: | Among other important characteristics of a linear regulator is the quiescent current, also known as ground current or supply current, which accounts for the difference, although small, between the input and output currents of the LDO, that is: | ||
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| P_{LOSS} = V_{IN} \times I_{Q} | P_{LOSS} = V_{IN} \times I_{Q} | ||
| - | Filtering[edit] | + | ====滤波==== |
| Torex XC6206 3.3V LDO voltage regulator in SOT23-3 package | Torex XC6206 3.3V LDO voltage regulator in SOT23-3 package | ||
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| Two specifications that should be considered when using an LDO as a filter are power supply rejection ratio (PSRR) and output noise. | Two specifications that should be considered when using an LDO as a filter are power supply rejection ratio (PSRR) and output noise. | ||
| - | Specifications[edit] | + | ====指标==== |
| - | An LDO is characterized by its drop-out voltage, quiescent current, load regulation, line regulation, maximum current (which is decided by the size of the pass transistor), speed (how fast it can respond as the load varies), voltage variations in the output because of sudden transients in the load current, output capacitor and its equivalent series resistance.[10] Speed is indicated by the rise time of the current at the output as it varies from 0 mA load current (no load) to the maximum load current. This is basically decided by the bandwidth of the error amplifier. It is also expected from an LDO to provide a quiet and stable output in all circumstances (example of possible perturbation could be: sudden change of the input voltage or output current). Stability analysis put in place some performance metrics to get such a behaviour and involve placing poles and zeros appropriately. Most of the time, there is a dominant pole that arise at low frequencies while other poles and zeros are pushed at high frequencies. | + | 评价一个LDO主要的指标有： |
| + | * 电压差 | ||
| + | * 静态电流 | ||
| + | * 负载调整率 | ||
| + | * 线调整率 | ||
| + | * 最大电流（取决于导通晶体管的大小） | ||
| + | * 对负载变化的响应速度 | ||
| + | * 由于负载电流的突变、输出电容以及等效的串行阻抗对输出电压的影响 | ||
| - | Power supply rejection ratio[edit] | + | ====电源抑制比==== |
| PSRR refers to the LDO's ability to reject ripple it sees at its input.[11] As part of its regulation, the error amplifier and bandgap attenuate any spikes in the input voltage that deviate from the internal reference to which it is compared.[12] In an ideal LDO, the output voltage would be solely composed of the DC frequency. However, the error amplifier is limited in its ability to gain small spikes at high frequencies. PSRR is expressed as follows:[11] | PSRR refers to the LDO's ability to reject ripple it sees at its input.[11] As part of its regulation, the error amplifier and bandgap attenuate any spikes in the input voltage that deviate from the internal reference to which it is compared.[12] In an ideal LDO, the output voltage would be solely composed of the DC frequency. However, the error amplifier is limited in its ability to gain small spikes at high frequencies. PSRR is expressed as follows:[11] | ||
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| Most LDOs have relatively high PSRR at lower frequencies (10 Hz - 1 kHz). However, a Performance LDO is distinguished in having high PSRR over a broad frequency spectrum (10 Hz - 5 MHz). Having high PSRR over a wide band allows the LDO to reject high-frequency noise like that arising from a switcher. Similar to other specifications, PSRR fluctuates over frequency, temperature, current, output voltage, and the voltage differential. | Most LDOs have relatively high PSRR at lower frequencies (10 Hz - 1 kHz). However, a Performance LDO is distinguished in having high PSRR over a broad frequency spectrum (10 Hz - 5 MHz). Having high PSRR over a wide band allows the LDO to reject high-frequency noise like that arising from a switcher. Similar to other specifications, PSRR fluctuates over frequency, temperature, current, output voltage, and the voltage differential. | ||
| - | Output noise[edit] | + | ====输出噪声==== |
| - | The noise from the LDO itself must also be considered in filter design. Like other electronic devices, LDOs are affected by thermal noise, bipolar shot noise, and flicker noise.[9] Each of these phenomena contribute noise to the output voltage, mostly concentrated over the lower end of the frequency spectrum. In order to properly filter AC frequencies, an LDO must both reject ripple at the input while introducing minimal noise at the output. Efforts to attenuate ripple from the input voltage could be in vain if a noisy LDO just adds that noise back again at the output. Texas Instruments' TPS7A47 is an example of an LDO with both very low noise and high PSRR over a broad frequency band.[13] | + | LDO自身产生的噪声也应该被考虑在滤波器的设计中，与其它电子器件一样，LDO也受到热噪声、双极散射噪声、以及闪变噪声的影响。所有这些噪声叠加在一起影响了输出的电压，这些噪声主要分布在低频段。为了滤除交流频率，在输入端和输出断都要进行纹波抑制。 |
| - | + | ====负载稳压==== | |
| - | Load regulation[edit] | + | |
| Load regulation is a measure of the circuit’s ability to maintain the specified output voltage under varying load conditions. Load regulation is defined as: | Load regulation is a measure of the circuit’s ability to maintain the specified output voltage under varying load conditions. Load regulation is defined as: | ||
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| The worst case of the output voltage variations occurs as the load current transitions from zero to its maximum rated value or vice versa.[6] | The worst case of the output voltage variations occurs as the load current transitions from zero to its maximum rated value or vice versa.[6] | ||
| - | Line regulation[edit] | + | ====Line regulation==== |
| Line regulation is a measure of the circuit’s ability to maintain the specified output voltage with varying input voltage. Line regulation is defined as: | Line regulation is a measure of the circuit’s ability to maintain the specified output voltage with varying input voltage. Line regulation is defined as: | ||
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| Like load regulation, line regulation is a steady state parameter—all frequency components are neglected. Increasing DC open-loop current gain improves the line regulation.[6] | Like load regulation, line regulation is a steady state parameter—all frequency components are neglected. Increasing DC open-loop current gain improves the line regulation.[6] | ||
| - | Transient response[edit] | + | ====Transient response==== |
| The transient response is the maximum allowable output voltage variation for a load current step change. The transient response is a function of the output capacitor value ({\textstyle C_{OUT} }), the equivalent series resistance (ESR) of the output capacitor, the bypass capacitor ({\textstyle C_{BYP} }) that is usually added to the output capacitor to improve the load transient response, and the maximum load-current ({\textstyle I_{OUT,MAX} }). The maximum transient voltage variation is defined as follows: | The transient response is the maximum allowable output voltage variation for a load current step change. The transient response is a function of the output capacitor value ({\textstyle C_{OUT} }), the equivalent series resistance (ESR) of the output capacitor, the bypass capacitor ({\textstyle C_{BYP} }) that is usually added to the output capacitor to improve the load transient response, and the maximum load-current ({\textstyle I_{OUT,MAX} }). The maximum transient voltage variation is defined as follows: | ||
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| Where {\textstyle \Delta t_{1} } corresponds to the closed-loop bandwidth of an LDO regulator. {\textstyle \Delta V_{ESR} } is the voltage variation resulting from the presence of the ESR ({\textstyle R_{ESR} }) of the output capacitor. The application determines how low this value should be. | Where {\textstyle \Delta t_{1} } corresponds to the closed-loop bandwidth of an LDO regulator. {\textstyle \Delta V_{ESR} } is the voltage variation resulting from the presence of the ESR ({\textstyle R_{ESR} }) of the output capacitor. The application determines how low this value should be. | ||
| - | See also[edit] | ||
| - | Portal icon Electronics portal | ||
| - | Linear regulator | ||
| - | Voltage regulator | ||
| - | Switched-mode power supply | ||
| - | List of linear integrated circuits | ||
| - | References[edit] | ||
| - | Jump up ^ Paul Horowitz and Winfield Hill (1989). The Art of Electronics. Cambridge University Press. pp. 343–349. ISBN 978-0-521-37095-0. | ||
| - | ^ Jump up to: a b Jim Williams (March 1, 1989). "High Efficiency Linear Regulators". Linear Technology. Retrieved 2014-03-29. | ||
| - | Jump up ^ LDOs, Low Dropout Regulators, Linear Regulators, CMOS Linear Regulator | ||
| - | Jump up ^ Don Tuite (September 1, 2007). "Inventor Updates A Classic 30 Years Later". | ||
| - | Jump up ^ Simpson, Chester. "Linear and Switching Voltage Regulator Fundamentals" (PDF). ti.com. Texas Instruments. Retrieved 18 June 2015. | ||
| - | ^ Jump up to: a b c d Lee, Bang S. "Understanding the Terms and Definitions of LDO Voltage Regulators" (PDF). Texas Instruments. Retrieved 30 August 2013. | ||
| - | Jump up ^ Mohammed,, Habeeb. "Supply Noise Effect on Oscillator Phase Noise" (PDF). | ||
| - | Jump up ^ Ramus,, Xavier. "Measuring PSR in an ADC". | ||
| - | ^ Jump up to: a b Pithadia, Sanjay. "LDO Noise Demystified" (PDF). Texas Instruments. | ||
| - | Jump up ^ Current Efficient, Low Voltage LDO A Thesis by Rincon-Mora | ||
| - | ^ Jump up to: a b Pithadia, Sanjay. "LDO PSRR Measurement Simplified" (PDF). Texas Instruments. | ||
| - | Jump up ^ Day, Michael. "Understanding Low Drop Out (LDO) Regulators" (PDF). Retrieved 16 September 2013. | ||
| - | Jump up ^ "36-V, 1-A, 4.17-µVRMS, RF LDO Voltage Regulator" (PDF). Texas Instruments. | ||