S-EX3L3403-498837模块备件

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S-EX3L3403-498837
在整个工作温度范围内的稳定性。有一些陶瓷电容器可以保持
±15%电容公差超过温度。
8.1.1.4.2钽
钽电容器可用于LP2985的输出，但也存在明显的缺点
禁止使用：
•在1-μF至4.7-μF范围内，钽电容器比同等陶瓷更昂贵
电容和电压额定值。
•钽电容器的ESR高于同等尺寸的陶瓷电容器。因此，为了满足
ESR要求时，可能需要更高的电容钽，但代价是尺寸更大和更高
费用
•钽电容器的ESR随着温度的下降而增加，从+25°C到–40°C的ESR增加了一倍。
因此，ESR裕度必须在温度范围内保持，以防止调节器不稳定。
8.1.2反向输入输出电压
如图8-1所示，LP2985的PNP pass元件上存在固有二极管。
VIN VOUT
图8-1：。固有PNP车身二极管
当阳极连接到输出端时，由于输入端
电压高于输出。然而，如果出于任何原因，输出电压高于输入电压，则该二极管
正向偏置，可导致寄生可控硅整流器（SCR）锁存，导致高电流
从输出流向输入。因此，为了防止在任何应用中可能损坏调节器
输出可能拉到输入上方，或输入可能对地短路，连接外部肖特基
输出和输入之间的二极管。当阳极位于输出端时，该肖特基二极管限制反向电压
穿过输出和输入引脚至约0.3 V（如图8-2所示），防止调节器
正向偏置的内部二极管。
VIN VOUT
肖特基
LP2985
图8-2：。外部肖特基二极管，防止反向电流通过设备
此外，该输入电容器必须位于输入引脚1 cm以内，并连接到干净的模拟电路
地该电容器没有等效串联电阻（ESR）要求
可以无限增加。
8.1.1.2输出电容器（COUT）
作为与其他调节器相比的优势，LP2985允许在输出端使用低ESR电容器，
包括ESR低至5 m的陶瓷电容器Ω. 钽和薄膜电容器也可以
如果尺寸和成本不是问题，则使用。输出电容器必须位于输出引脚1 cm以内，并且
返回到干净的模拟接地。
与其他PNP LDO一样，稳定性条件要求输出电容器具有小电容和
在一定范围内的ESR。
•小COUT:2.2μF（可无限增加，以提高瞬态响应稳定裕度）
•ESR范围：见图6-18至图6-20
在整个工作温度下，必须满足小电容和ESR要求
范围根据所用电容器的类型，这两个参数可能会随温度发生显著变化
（参见电容器特性部分）。
8.1.1.3噪声旁路电容器（CBYPASS）
LP2985通过使用连接到内部的旁路电容器实现低噪声性能
通过旁路引脚的带隙基准。这种高阻抗带隙电路的偏置为微安
因此，不能显著加载范围，否则，其输出（以及相应的
调节器）变更。因此，为了获得佳的输出精度，通过CBYPASS的直流泄漏电流必须小化为
尽可能多，且不得超过100 nA。
CBYPASS建议使用10 nF电容器。陶瓷和薄膜电容器非常适合此用途。
8.1.1.4电容器特性
8.1.1.4.1陶瓷
陶瓷电容器是LP2985输出上使用的理想选择，原因有几个。对于电容
在2.2μF至4.7μF的范围内，陶瓷电容器具有低的成本和低的ESR，使其
选择用于过滤高频噪声的候选者。例如，典型的2.2μF陶瓷电容器具有ESR
10米范围内Ω 至20米Ω 因此，满足调节器的低ESR要求。
陶瓷电容器有一个必须考虑的主要缺点——温度低
系数，其中电容可随温度显著变化。例如，大价值陶瓷
电容器(≥ 当温度从25°C上升到85°C时，2.2μF）会失去一半以上的电容。
因此，25°C下的2.2μF电容器远远低于环境温度下稳定性所需的低温度
温度升高。因此，请选择一个输出电容器，以保持所需的小2.2μF
S-EX3L3403-498837

S-EX3L3403-498837
stability over the entire operating temperature range. There are some ceramic capacitors that can maintain a
±15% capacitance tolerance over temperature.
8.1.1.4.2 Tantalum
Tantalum capacitors can be used at the output of the LP2985, but there are significant disadvantages that can
prohibit their use:
• In the 1-μF to 4.7-μF range, tantalum capacitors are more expensive than ceramics of the equivalent
capacitance and voltage ratings.
• Tantalum capacitors have higher ESRs than their equivalent-sized ceramic counterparts. Thus, to meet the
ESR requirements, a higher-capacitance tantalum may be required, at the expense of larger size and higher
cost.
• The ESR of a tantalum capacitor increases as temperature drops, as much as double from +25°C to –40°C.
Thus, ESR margins must be maintained over the temperature range to prevent regulator instability.
8.1.2 Reverse Input-Output Voltage
As shown in Figure 8-1, there is an inherent diode present across the PNP pass element of the LP2985.
VIN VOUT
Figure 8-1. Inherent PNP Body Diode
With the anode connected to the output, this diode is reverse biased during normal operation, since the input
voltage is higher than the output. However, if the output is pulled higher than the input for any reason, this diode
is forward biased and can cause a parasitic silicon-controlled rectifier (SCR) to latch, resulting in high current
flowing from the output to the input. Thus, to prevent possible damage to the regulator in any application where
the output may be pulled above the input, or the input may be shorted to ground, connect an external Schottky
diode between the output and input. With the anode on the output, this Schottky diode limits the reverse voltage
across the output and input pins to approximately 0.3 V (as shown in Figure 8-2), preventing the regulator
internal diode from forward biasing.
VIN VOUT
Schottky
LP2985
Figure 8-2. External Schottky Diode to Prevent Reverse Current Through the Device
In addition, this input capacitor must be located within 1 cm of the input pin and connected to a clean analog
ground. There are no equivalent series resistance (ESR) requirements for this capacitor, and the capacitance
can be increased without limit.
8.1.1.2 Output Capacitor (COUT)
As an advantage over other regulators, the LP2985 permits the use of low-ESR capacitors at the output,
including ceramic capacitors that can have an ESR as low as 5 mΩ. Tantalum and film capacitors also can be
used if size and cost are not issues. The output capacitor must be located within 1 cm of the output pin and be
returned to a clean analog ground.
As with other PNP LDOs, stability conditions require the output capacitor to have a minimum capacitance and an
ESR that falls within a certain range.
• Minimum COUT: 2.2 μF (can be increased without limit to improve transient response stability margin)
• ESR range: see Figure 6-18 through Figure 6-20
Both the minimum capacitance and ESR requirement are critical to be met over the entire operating temperature
range. Depending on the type of capacitors used, both these parameters can vary significantly with temperature
(see the Capacitor Characteristics section).
8.1.1.3 Noise Bypass Capacitor (CBYPASS)
The LP2985 allows for low-noise performance with the use of a bypass capacitor that is connected to the internal
band-gap reference via the BYPASS pin. This high-impedance band-gap circuitry is biased in the microampere
range and, thus, cannot be loaded significantly, otherwise, its output (and, correspondingly, the output of the
regulator) changes. Thus, for best output accuracy, dc leakage current through CBYPASS must be minimized as
much as possible and must never exceed 100 nA.
A 10-nF capacitor is recommended for CBYPASS. Ceramic and film capacitors are well suited for this purpose.
8.1.1.4 Capacitor Characteristics
8.1.1.4.1 Ceramics
Ceramic capacitors are ideal choices for use on the output of the LP2985 for several reasons. For capacitances
in the range of 2.2 μF to 4.7 μF, ceramic capacitors have the lowest cost and the lowest ESR, making them
choice candidates for filtering high-frequency noise. For instance, a typical 2.2-μF ceramic capacitor has an ESR
in the range of 10 mΩ to 20 mΩ and, thus, satisfies minimum ESR requirements of the regulator.
Ceramic capacitors have one major disadvantage that must be taken into account—a poor temperature
coefficient, where the capacitance can vary significantly with temperature. For instance, a large-value ceramic
capacitor (≥ 2.2 μF) can lose more than half of its capacitance as the temperature rises from 25°C to 85°C.
Thus, a 2.2-μF capacitor at 25°C drops well below the minimum COUT required for stability, as ambient
temperature rises. For this reason, select an output capacitor that maintains the minimum 2.2 μF required for