低压差(LDO)监管机构简单,inexpensive way to provide a regulated output voltage powered from a higher voltage input. An LDO regulator can regulate the output voltage even when the supply voltage is very close to the output voltage. For instance, it’s able to generate 3.3 V from a 3.6-V Li-on battery. Such regulators are easier to implement than switching converters, and unlike switching regulators, LDOs don’t create high levels of switching noise.
Choosing the right LDO can be critical to maximizing an application’s performance. Low-noise LDOs help ensure a clean dc power supply. It is, as a result, important to both select an LDO with low-noise properties as well as implementing techniques to ensure the cleanest output possible.
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Noise and PSSR
Although an LDO can be simple to implement, its self-generated noise is often confused with its power-supply rejection ratio (PSRR), which measures how much noise from the input couples into the output. Put another way, PSRR is a measure of how well a circuit suppresses extraneous signals (noise and ripple) that appear on the power-supply input to keep them from affecting the circuit output（图。1）。
噪声和PSRR彼此不同。噪声可能是LDO内部和外部的东西，而PSRR是LDO的内部参数。
PSRR表示LDO抑制输入电压波纹的能力。它以分贝（DB）为单位的输出噪声与噪声的比率表示。输出电容器以及输入电压和输出电压之间的差异是可能影响PSRR值的参数。
High PSRR over a wide bandwidth enables the LDO to filter switching noise emanating from upstream dc-dc converters. Low-noise (<5 µVRMS）输出最小化低频噪声，以维持敏感模拟电路（例如ADC或DAC）的信号完整性，从而帮助为精确应用产生清洁功率。
确定PSRR的一种好方法是通过PSRR与频率图，可以在数据表上找到
什么条件会影响噪声性能？
让我们从输出电压开始。总LDO输出电压噪声由输入噪声组成，该输入噪声与输出耦合以及LDO产生的固有噪声。
由于LDO是电子设备，因此它们会自行产生一定数量的噪音。LDO中的主要噪声来源是内部参考电压和误差放大器。参考电压设置了误差放大器的工作点。误差放大器调整了通行元件的电阻，该通行元件负责将电流从输入转移到负载。
系统将看到的噪音是LDO内部产生的噪声的组合，以及LDO电源的噪音。噪声是由LDO内部电路和外部组件中的晶体管和电阻产生的。
这type of noise may include thermal, flicker, and shot noise. Transistors generate shot noise and flicker noise. Flicker noise is caused by trapped charges at the gate of the MOSFET. The resistive element of MOSFETs also generates thermal noise. Thermal and shot noise are random in nature and flat over frequency. Unlike thermal noise, shot noise depends on the current flowing and has no relationship to the temperature at which the system is operating.
A common way for noise to sneak into your system is through the power supply. This includes both the power supply for the LDO and the power supply for any amplifiers used to gain up the LDO’s output. To measure the noise that a system will see due to its power supply, the LDO should be powered by the circuitry used in the actual application. ATI申请注释深入了解电源噪声的细微差别，并提供了了解LDO数据表中可能显示的规格的实用技巧。
数据表通常提供单个集成的噪声值。在选择频率范围内集成可以帮助掩盖某些噪声属性，因此除了集成值外，要检查噪声曲线很重要。
Another good strategy is to employ techniques to ensure that your LDO has the lowest noise characteristics.
例如noise-reduction pincan improve system performance by filtering reference noise and controlling the slew rate. Many low-noise LDOs in the TI portfolio have a special pin designated as “NR/SS.” This noise-reduction pin filters the bandgap noise using an external capacitor, referred to as the NR cap.
在该引脚处添加电容器形成电阻器电容器（RC）滤波器，该滤波器有助于分流电压参考产生的不良噪声。该引脚的功能是双重的。它用于过滤从内部电压参考中发出的噪声，并在启动过程中降低振荡速率。
这most dominant source of noise in an LDO is the internal bandgap. A low-pass filter (LPF) can be inserted between the bandgap output and the input of the error amplifier. This reduces the bandgap noise before it’s gained up by the error amplifier.
通常，该LPF由内部电阻和外部电容器形成。该过滤器的截止频率设置为尽可能低的频率，以滤除几乎所有来自带隙的噪声。
用低噪声LDO清理供应栏
Designers of ultra-sensitive applications such as medical, test and measurement, or telecommunications can leverage an ultra-low-noise (0.46 μVRMS），高PSRR LDO电压调节器，能够用仅150 mV的辍学来采购1 A。这TPS7A94（图2）再加上宽带误差放大器，在低操作室（500 mV）和高输出电流（750 mA）下，在低操作室（500 mV）下，PSRR（1 kHz为110 dB，1 MHz为110 dB）。
设备输出可通过外部电阻从0调节到5.5 V。由于其宽的输入电压范围，该设备支持低至1.7 V且最高5.7 V的操作。它很容易平行以实现较低的噪声和较高的电流。总体而言，它非常适合为敏感的模拟低压设备供电。
As we’ve seen, selecting a low-noise LDO and taking steps to reduce internal noise are key to generating clean supply rails. Engineers can find the industry's lowest-noise LDOs in the TI portfolio, including parts whose integrated noise values measure as low as 0.47 μVRMS。
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