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ldo [2016/06/02 19:13] gongyu [历史] |
ldo [2016/06/02 19:51] (当前版本) gongyu [输出噪声] |
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| 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。 | 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。 | ||
| - | ====历史==== | + | ====来历==== |
| 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。 | 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。 | ||
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| {{ :low_drop_voltage_regulator.png |}} | {{ :low_drop_voltage_regulator.png |}} | ||
| - | <WRAP centeralign> Schematic of a low-dropout regulator </WRAP> | + | <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。 | |
| - | 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. | + | |
| ====稳压==== | ====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。 |
| ====效率和散热==== | ====效率和散热==== | ||
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| ====指标==== | ====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 | ||
| ====电源抑制比==== | ====电源抑制比==== | ||
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| ====输出噪声==== | ====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==== | ====负载稳压==== | ||
| 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: | ||