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现实中的电源抑制比(PSRR) - 第四部分

由  Petr Novobilsky 发布  - 07-10-2020 

继续我们有关低压降稳压器(LDO) PSRR的系列文章,请查看我们以前的博客以回顾-什么是PSRR -第三部分和第四部分一样,我们将继续讲解LDO的行为及其有趣的参数。在当前的文章中,我们将从实际的角度关注电源抑制比(PSRR)。它可帮助将数据表编号与示波器测量值连接起来。

首先,必须说的是,在每个电子系统中(即使只有线性稳压器),如果存在多个负载点,则可能会产生输出电压纹波,并可能影响其他部件。因此,让我们在实际测量中讲解PSRR。

数据表上显示的PSRR是测量的输入和输出电压纹波之间的比率。如果牢记一些规则,则测量本身相对简单。低压降(LDO)稳压器应由干净的直流电源供电,带有耦合的正弦波纹波电压。输出负载应严格电阻,以防止电子负载与LDO稳压器之间互相影响。必须谨慎选择纹波电压幅值,以使LDO保持稳定并具有足够的电压裕量。例如,当VIN = 3.6 V和VOUT = 3.3 V时,AC信号的幅值不能为300 mV,因为LDO处于压差状态,只是将输入纹波传递到输出。

例如,在图1中,请参见安森美半导体LDO NCP163NCP161的PSRR图表。相似的图表广泛用于整个半导体行业的许多LDO数据表中。该实际示例显示了达10 MHz频率范围内的PSRR。对于我们的案例研究,我们标记了两个点,如下图的时域中所示-1 kHz和100 kHz。 1 kHz是数据表电气特性表中PSRR规范的最常见频率,而100 kHz是许多DC-DC转换器工作的区域。

 

图1. NCP163和NCP161和PSRR图表

在点1,PSRR约为90 dB,这意味着输入纹波被衰减了约32,000次。下图是针对200 mVpp的输入电压纹波而捕获的,这意味着LDO输出上的纹波仅为6 uV,且不可能在示波器上看到它。如图2所示-一条直线。

图2. 1 kHz的示波器图–两个器件的PSRR均约90 dB

图3中的情况显示了在100 kHz频率下会发生什么。从图1中我们可以看出,两个器件的PSRR都远低于90 dB,并且两者之间存在很大差异。仔细研究时域,例如,应用要求输入电压纹波低于1 mVpp。 NCP161处于临界点,而NCP163满足要求有很大余量,纹波峰峰值低于200 uVpp。

图3. 100 kHz的示波器图– PSRR有很大不同

如图1-3所示,电气表中的PSRR参数是一个单独的数字,不足以决定其在设计应用中的适用性。系统工程师必须考虑输入电压纹波频率,并在所需的位置检查PSRR图表。应该仔细匹配在MHz范围内的现代DC-DC转换器开关和LDO,以提供最佳性能。大多数LDO数据手册都将PSRR指定为仅1 kHz,并会提高高值以获得良好的性能,但由于在较高频率范围内的性能较差,它们可能不足以作为DC-DC后置稳压器。因此,考虑应用条件并选择合适的LDO非常重要。安森美半导体在我们所有的数据表中都提供了类似图1的图表,因此客户可以找到必要的信息来完成他们的设计。

最后,让我们看看DC-DC转换器结合LDO如何在系统方案中一起发挥作用。FAN2356被选作DC-DC转换器。它具有高达96%的能效和高达1.5 MHz的可调开关频率。在我们的示例中,开关频率设置为500 kHz。后稳压器LDO是NCP163,它是具有超低噪声输出的高PSRR LDO。 DC-DC转换器的输出电压为3.6 V,LDO输出为3.3V。基于此,电压裕量为300 mV,LDO负载电流为250 mA。在图4中,我们可看到黄色迹线DC-DC输出电压,蓝色迹线LDO输出电压。两条迹线的垂直刻度均设置为30 mV / div,以易于比较LDO的影响。

图4. DC-DC转换器及LDO后稳压器

我们可清楚地看到DC-DC输出的大尖峰和纹波电压,这可能会引起对电源敏感的器件出现问题。 LDO输出更加清晰和稳定。纹波减小到mV范围,尖峰抑制到〜30-40 mV。凭借这种性能,输出电压可用于为各种电源敏感型应用供电。

请确保订阅我们的博客,以继续获取来自我们专家团队的最新信息,并及时了解我们在各细分市场的最新技术和方案。

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