简化的 LDO PSRR 测量

1 Input Output Ripple PSRR = 20 log Ripple (1) 80 70 60 50 40 30 20 10 0 10 100 1�k 10�k 100�k 1�M 10�M f�-�Frequency�-�Hz PS RR �-� dB 150�mA 10�mA 75�mA C =�1 F, C =�10�nF OUT NR � Application Report SLAA414–July 2009 LDO PSRR Measurement Simplified Sanjay Pithadia and Scot Lester .................................................................... PMP - LP Linear Regulators ABSTRACT This application report explains different methods of measuring the Power Supply Rejection Ratio (PSRR) of a Low-Dropout (LDO) regulator and includes the pros and cons of these measuring methods. What is PSRR? Power Supply Rejection Ratio or Power Supply Ripple Rejection (PSRR) is a measure of a circuit’s power supply’s rejection expressed as a log ratio of output noise to input noise. PSRR provides a measure of how well a circuit rejects ripple, of various frequencies, injected at its input. The ripple can be either from the input supply such as a 50Hz/60Hz supply ripple, switching ripple from a DC/DC converter, or ripple due to the sharing of an input supply between different circuit blocks on the board. In the case of LDOs, PSRR is a measure of the regulated output voltage ripple compared to the input voltage ripple over a wide frequency range (10Hz to 1MHz is common) and is expressed in decibels (dB). The PSRR is very critical parameter in many audio and RF applications. The basic equation for PSRR is: Historically LDOs have poor high frequency PSRR performance, but currently TI has LDOs with PSRR > 40dB at 5MHz. One important point regarding the PSRR graphs in TI LDO datasheet is that the PSRR axis is inverted (See Figure 1). The PSRR is calculated as rejection so it should be a negative number; however, the graph shows it as positive number so that a higher number denotes higher noise rejection. Figure 1. PSRR Graph of TPS717xx LDOs SLAA414–July 2009 LDO PSRR Measurement Simplified 1 Submit Documentation Feedback 应用 报告 软件系统测试报告下载sgs报告如何下载关于路面塌陷情况报告535n,sgs报告怎么下载竣工报告下载 ZHCA089–2009年7月 LDO PSRR 测量简化说明 作者：Sanjay Pithadia 和 Scot LesterPMP – 低压线性稳压器 摘要 本应用报告解释了测量低压差 (LDO) 稳压器的电源抑制比 (PSRR) 的不同测量方法以及这些测量方法的优点 及缺点。 什么是 PSRR? 电源抑制比或电源纹波抑制 (PSRR) 是一个电路的电源抑制能力的度量值，表示为输出噪声与输入噪声的对数之比。PSRR 提供了 一个电路对从它的输入处引入的不同频率的纹波抑制能力的度量值。纹波可以来自输入电源，比如一个 50Hz/60Hz 的电源纹波， 也可以是来自 DC/DC 转换器的开关纹波，还可以是由于输入电压被电路板上不同的电路块共用所致。在 LDO的情况下，PSRR 是一个在较大的频率范围内（通常为 10Hz 至 1MHz），稳定的输出电压纹波相比于输入电压纹波的度量值，用分贝 (dB) 来表示 其大小。在许多音频和射频应用中，PSRR是一个非常重要的参数。 计算 PSRR 的基本公式是： 在过去，LDO 稳压器件都具有较差的高频 PSRR 性能，但目前 TI 已经拥有了在 5MHz 下 PSRR > 40dB 的 LDO。在 TI 的 LDO 数据表中，关于 PSRR 曲线图很重要的一点是，PSRR 的坐标轴是反向的（如图 1所示）。PSRR 被计算为抑制能力，因此它本应 是个负数；然而，图中显示它是一个正数，这说明一个更高的数值表示了更高的噪声抑制。 f – 频率 - Hz P S R R – d B 图 1. TPS717xx LDO 的 PSRR 曲线图 ZHCA089 – 2009 年 7 月 提交文档反馈 LDO PSRR 测量简化说明 Input Output Ripple PSRR = 20 log Ripple (1) 80 70 60 50 40 30 20 10 0 10 100 1�k 10�k 100�k 1�M 10�M f�-�Frequency�-�Hz PS RR �-� dB 150�mA 10�mA 75�mA C =�1 F, C =�10�nF OUT NR � Application Report SLAA414–July 2009 LDO PSRR Measurement Simplified Sanjay Pithadia and Scot Lester .................................................................... PMP - LP Linear Regulators ABSTRACT This application report explains different methods of measuring the Power Supply Rejection Ratio (PSRR) of a Low-Dropout (LDO) regulator and includes the pros and cons of these measuring methods. What is PSRR? Power Supply Rejection Ratio or Power Supply Ripple Rejection (PSRR) is a measure of a circuit’s power supply’s rejection expressed as a log ratio of output noise to input noise. PSRR provides a measure of how well a circuit rejects ripple, of various frequencies, injected at its input. The ripple can be either from the input supply such as a 50Hz/60Hz supply ripple, switching ripple from a DC/DC converter, or ripple due to the sharing of an input supply between different circuit blocks on the board. In the case of LDOs, PSRR is a measure of the regulated output voltage ripple compared to the input voltage ripple over a wide frequency range (10Hz to 1MHz is common) and is expressed in decibels (dB). The PSRR is very critical parameter in many audio and RF applications. The basic equation for PSRR is: Historically LDOs have poor high frequency PSRR performance, but currently TI has LDOs with PSRR > 40dB at 5MHz. One important point regarding the PSRR graphs in TI LDO datasheet is that the PSRR axis is inverted (See Figure 1). The PSRR is calculated as rejection so it should be a negative number; however, the graph shows it as positive number so that a higher number denotes higher noise rejection. Figure 1. PSRR Graph of TPS717xx LDOs SLAA414–July 2009 LDO PSRR Measurement Simplified 1 Submit Documentation Feedback Input Output Ripple PSRR = 20 log Ripple (1) 80 70 60 50 40 30 20 10 0 10 100 1�k 10�k 100�k 1�M 10�M f�-�Frequency�-�Hz PS RR �-� dB 150�mA 10�mA 75�mA C =�1 F, C =�10�nF OUT NR � Application Report SLAA414–July 2009 LDO PSRR Measurement Simplified Sanjay Pithadia and Scot Lester .................................................................... PMP - LP Linear Regulators ABSTRACT This application report explains different methods of measuring the Power Supply Rejection Ratio (PSRR) of a Low-Dropout (LDO) regulator and includes the pros and cons of these measuring methods. What is PSRR? Power Supply Rejection Ratio or Power Supply Ripple Rejection (PSRR) is a measure of a circuit’s power supply’s rejection expressed as a log ratio of output noise to input noise. PSRR provides a measure of how well a circuit rejects ripple, of various frequencies, injected at its input. The ripple can be either from the input supply such as a 50Hz/60Hz supply ripple, switching ripple from a DC/DC converter, or ripple due to the sharing of an input supply between different circuit blocks on the board. In the case of LDOs, PSRR is a measure of the regulated output voltage ripple compared to the input voltage ripple over a wide frequency range (10Hz to 1MHz is common) and is expressed in decibels (dB). The PSRR is very critical parameter in many audio and RF applications. The basic equation for PSRR is: Historically LDOs have poor high frequency PSRR performance, but currently TI has LDOs with PSRR > 40dB at 5MHz. One important point regarding the PSRR graphs in TI LDO datasheet is that the PSRR axis is inverted (See Figure 1). The PSRR is calculated as rejection so it should be a negative number; however, the graph shows it as positive number so that a higher number denotes higher noise rejection. Figure 1. PSRR Graph of TPS717xx LDOs SLAA414–July 2009 LDO PSRR Measurement Simplified 1 Submit Documentation Feedback 2 COUTCIN + VAC L C VDC LOAD VIN VOUT GND LDO VIN VOUT + Measuring PSRR of LDO www.ti.com Measuring PSRR of LDO The following sections explain different methods of measuring the PSRR of an LDO. 1. Measuring PSRR using LC summing node method: The basic method of measuring PSRR is shown in Figure 2. In this method, DC voltage and AC voltages are summed together and applied at the input of the LDO. VDC is the operating point bias voltage and VAC is the noise source used in the test. Capacitor C prevents VAC from shoring VDC and inductor L prevents VDC from shorting VAC. So L and C are used for isolating both the sources, VDC and VAC, from each other. The L and C will create a high pass filter for VAC which will limit how low in frequency we can measure the PSRR. The 3dB point of this filter is determined by Equation 2. Frequencies below the 3dB point will start to be attenuated which will make measurements more difficult. The highest frequency that can be measured is determined by the self resonant frequencies of the L and C components. Fmin = 1/ 2Ȇ ¥LC (2) A drawback to this method is that it works well only for mid-range frequencies (approximately 1 kHz to 500 kHz). Figure 2. Basic Method of Measuring PSRR of LDO 2. Measuring PSRR using summing amplifier To improve the measurement of PSRR, a recommended method is described using a high-bandwidth amplifier as summing node to inject the signals and provides the isolation between VAC and VDC. This method is tested and verified using TPS72715 LDO and THS3120 high-speed amplifier from Texas Instruments. The basic set-up is shown in Figure 3. The PSRR is measured with a no-load condition and the resulting measured PSRR graph corresponds with the datasheet graph of PSRR. Keep in mind the following while measuring the PSRR using this method: a. The input capacitor of LDO should be removed before the measurement because this capacitor could cause the high-speed amplifier to go unstable. b. Vin and Vout should be measured with high-impedance probes (either scope or network analyzer) immediately at the Vin or Vout pins to minimize the set-up inductance effects. c. There test set-up should not have any long wires since this will add inductance and impact the results. d. While selecting the values of AC and DC inputs, the following conditions should be considered: VAC (max) + VDC < VABS (max) of LDO VDC – VAC > VUVLO of LDO Also, the best results will be obtained if: VDC–VAC>Vout + Vdo + 0.5 where Vout is the output voltage of the LDO and Vdo is the specified drop out voltage at the operating point. e. At very high frequencies, the response of the amplifier will start to attenuate the VAC signal that is applied to the LDO. At some point, the attenuated VAC will be too small to measure on the output of the LDO. f. As load current increases, the open-loop output impedance of LDO decreases (Since a MOSFET output impedance is inversely proportional to the drain current), thus lowering the gain. Increasing the load current also pushes the output pole to higher frequencies, which increases the feedback 2 LDO PSRR Measurement Simplified SLAA414–July 2009 Submit Documentation Feedback LDO的PSRR测量 www.ti.com LDO的PSRR测量 以下各节解释了测量一个LDO的PSRR的不同方法： 1. 使用LC总和节点法测量PSRR： 测量PSRR的基本方法如图2所示。在这个方法中，直流电压和交流电压合在一起作为LDO的输入。电容C防止VAC对VDC产生 高脉冲影响，电感L防止VDC令VAC发生短路。因此L和C用于隔离两个电源，VDC和VAC。 L和C形成一个针对于VAC的高通滤波器，将限制我们所能测量的PSRR的最低频率。这个滤波器的3dB点由公式2确定。低于 3dB点的频率将被减弱，使得测量变得更加困难。能被测量的最高频率由L和C的自谐振频率所确定。 这种方法的缺点是，它只对中频范围内（大约 1 kHz 至 500 kHz）的测量是有效的。 COUTCIN + VAC L C VDC LOAD VIN VOUT GND LDO VIN VOUT + Measuring PSRR of LDO www.ti.com Measuring PSRR of LDO The following sections explain different methods of measuring the PSRR of an LDO. 1. Measuring PSRR using LC summing node method: The basic method of measuring PSRR is shown in Figure 2. In this method, DC voltage and AC voltages are summed together and applied at the input of the LDO. VDC is the operating point bias voltage and VAC is the noise source used in the test. Capacitor C prevents VAC from shoring VDC and inductor L prevents VDC from shorting VAC. So L and C are used for isolating both the sources, VDC and VAC, from each other. The L and C will create a high pass filter for VAC which will limit how low in frequency we can measure the PSRR. The 3dB point of this filter is determined by Equation 2. Frequencies below the 3dB point will start to be attenuated which will make measurements more difficult. The highest frequency that can be measured is determined by the self resonant frequencies of the L and C components. Fmin = 1/ 2Ȇ ¥LC (2) A drawback to this method is that it works well only for mid-range frequencies (approximately 1 kHz to 500 kHz). Figure 2. Basic Method of Measuring PSRR of LDO 2. Measuring PSRR using summing amplifier To improve the measurement of PSRR, a recommended method is described using a high-bandwidth amplifier as summing node to inject the signals and provides the isolation between VAC and VDC. This method is tested and verified using TPS72715 LDO and THS3120 high-speed amplifier from Texas Instruments. The basic set-up is shown in Figure 3. The PSRR is measured with a no-load condition and the resulting measured PSRR graph corresponds with the datasheet graph of PSRR. Keep in mind the following while measuring the PSRR using this method: a. The input capacitor of LDO should be removed before the measurement because this capacitor could cause the high-speed amplifier to go unstable. b. Vin and Vout should be measured with high-impedance probes (either scope or network analyzer) immediately at the Vin or Vout pins to minimize the set-up inductance effects. c. There test set-up should not have any long wires since this will add inductance and impact the results. d. While selecting the values of AC and DC inputs, the following conditions should be considered: VAC (max) + VDC < VABS (max) of LDO VDC – VAC > VUVLO of LDO Also, the best results will be obtained if: VDC–VAC>Vout + Vdo + 0.5 where Vout is the output voltage of the LDO and Vdo is the specified drop out voltage at the operating point. e. At very high frequencies, the response of the amplifier will start to attenuate the VAC signal that is applied to the LDO. At some point, the attenuated VAC will be too small to measure on the output of the LDO. f. As load current increases, the open-loop output impedance of LDO decreases (Since a MOSFET output impedance is inversely proportional to the drain current), thus lowering the gain. Increasing the load current also pushes the output pole to higher frequencies, which increases the feedback 2 LDO PSRR Measurement Simplified SLAA414–July 2009 Submit Documentation Feedback COUTCIN + VAC L C VDC LOAD VIN VOUT GND LDO VIN VOUT + Measuring PSRR of LDO www.ti.com Measuring PSRR of LDO The following sections explain different methods of measuring the PSRR of an LDO. 1. Measuring PSRR using LC summing node method: The basic method of measuring PSRR is shown in Figure 2. In this method, DC voltage and AC voltages are summed together and applied at the input of the LDO. VDC is the operating point bias voltage and VAC is the noise source used in the test. Capacitor C prevents VAC from shoring VDC and inductor L prevents VDC from shorting VAC. So L and C are used for isolating both the sources, VDC and VAC, from each other. The L and C will create a high pass filter for VAC which will limit how low in frequency we can measure the PSRR. The 3dB point of this filter is determined by Equation 2. Frequencies below the 3dB point will start to be attenuated which will make measurements more difficult. The highest frequency that can be measured is determined by the self resonant frequencies of the L and C components. Fmin = 1/ 2Ȇ ¥LC (2) A drawback to this method is that it works well only for mid-range frequencies (approximately 1 kHz to 500 kHz). Figure 2. Basic Method of Measuring PSRR of LDO 2. Measuring PSRR using summing amplifier To improve the measurement of PSRR, a recommended method is described using a high-bandwidth amplifier as summing node to inject the signals and provides the isolation between VAC and VDC. This method is tested and verified using TPS72715 LDO and THS3120 high-speed amplifier from Texas Instruments. The basic set-up is shown in Figure 3. The PSRR is measured with a no-load condition and the resulting measured PSRR graph corresponds with the datasheet graph of PSRR. Keep in mind the following while measuring the PSRR using this method: a. The input capacitor of LDO should be removed before the measurement because this capacitor could cause the high-speed amplifier to go unstable. b. Vin and Vout should be measured with high-impedance probes (either scope or network analyzer) immediately at the Vin or Vout pins to minimize the set-up inductance effects. c. There test set-up should not have any long wires since this will add inductance and impact the results. d. While selecting the values of AC and DC inputs, the following conditions should be considered: VAC (max) + VDC < VABS (max) of LDO VDC – VAC > VUVLO of LDO Also, the best results will be obtained if: VDC–VAC>Vout + Vdo + 0.5 where Vout is the output voltage of the LDO and Vdo is the specified drop out voltage at the operating point. e. At very high frequencies, the response of the amplifier will start to attenuate the VAC signal that is applied to the LDO. At some point, the attenuated VAC will be too small to measure on the output of the LDO. f. As load current increases, the open-loop output impedance of LDO decreases (Since a MOSFET output impedance is inversely proportional to the drain current), thus lowering the gain. Increasing the load current also pushes the output pole to higher frequencies, which increases the feedback 2 LDO PSRR Measurement Simplified SLAA414–July 2009 Submit Documentation Feedback 图 2. 测量LDO的PSRR的基本方法 2. 使用和放大器法测量PSRR 为了改进 PSRR 的测量方法，推荐的方法是，使用一个高带宽放大器作为总和节点来输入信号并提供 VAC 和 VDC 之间的隔离。 德州仪器使用 TPS72715 LDO 和 THS3120 高速放大器对这一方法进行了测试和验证。基本结构如图 3所示。PSRR 在无负载的 条件下进行测量，而测量结果图与数据表中的 PSRR 结果图相一致。 当使用此方法测量 PSRR 时，请记住以下几点： a. 在测量前，应去掉 LDO 的输入电容，因为它将引起高速放大器的不稳定。 b. 当测量 Vin 和 Vout 时，应使用高阻抗探头（示波器或网络分析仪）迅速接触 Vin 或 Vout 引脚，以便使探头电感的影响降至最 c. 在这里测试探头不应具有过长的导线，否则将增大电感并影响测量结果。 d. 当确定交流及直流输入的数值时，应考虑以下条件： VAC（最大值）+ VDC < LDO 的 VABS（最大值） VDC – VAC > LDO 的 VUVLO 此外，若满足以下条件，将会得到最好结果：A VDC–VAC > Vout + Vdo + 0.5 这里 Vout 是 LDO 的输出电压，Vdo 是工作点的额定压降。 e. 在非常高的频率下，放大器的响应将开始减弱应用于 LDO 的 VAC信号。在某些时候，被减弱的 VAC 将会过小而无法在 LDO 的输出端测到。 f. 随着负载电流的增加，LDO 的开环输出阻抗将会减小（因为 MOSFET 的输出阻抗与漏电流成反比），从而降低增益。增加 负载电流也会将输出极点推向更高的频率，从而增大了反馈回路的带宽。负载的增加带来的实际效果是，减小了更低频率的 PSRR（由于增益的减小），而增大了更高频率的 PSRR。 ZHCA089 – 2009 年 7 月 提交文档反馈 LDO PSRR 测量简化说明 低。 3 低。 100 F� 1�k� 100 � DUT COUTDC�out RF�out THS3120 + - +10�V -10�V Built�on�THS3120�EVM High�Impedance Adapters�(Model�#�41802A from Agilent) CH1 CH2 Network Analyzer (Model�# 4395A from Agilent) FB* FB* � Ferrite�Bead�having�impedance�of 10 � @�100�MHz 1�k� www.ti.com Measuring PSRR Using Oscilloscope loop bandwidth. The net effect of increasing the load is therefore reduced PSRR at lower frequencies (because of reduced gain) along with increased PSRR at higher frequencies. Figure 3. Recommended Method of Measuring PSRR of LDO Figure 4 shows the PSRR graph measured with this method. Figure 4. PSRR Measured With Recommended Method The THS3120 is suitable for measuring PSRR up to VDC = 5V, Frequency = 10MHz and Iload = 400mA. Measuring PSRR Using Oscilloscope If the user does not have a network analyzer then there is a simpler but more cumbersome method which uses a signal generator, DC source and oscilloscope to measure the PSRR. An AC signal from signal generator is applied along with DC signal at the input of the LDO, as shown in either of the afore mentioned methods, and the output of the LDO is measured on an oscilloscope at different VAC frequencies. The PSRR is calculated using the Equation 1 where Ripple(input) is the amplitude of the input AC signal and Ripple(output) is the amplitude of output signal. This is then repeated at different frequencies of VAC to generate a piecemeal graph of PSRR. This method can be used along with the set-ups described in the previous section. But this method is only good for LDOs with lower PSRR values due to resolution and sensitivity of oscilloscopes. Since most oscilloscopes can measure down to the millivolt range, the maximum range of PSRR that could realistically be measured using an oscilloscope is about 40dB–50dB. SLAA414–July 2009 LDO PSRR Measurement Simplified 3 Submit Documentation Feedback 用示波器测量PSRR www.ti.com 图 3. 测量 LDO 的 PSRR 的推荐方法 图 4 显示的是使用这种方法测量 PSRR 的结果图。 图 4. 使用推荐方法测量所得到的 PSRR THS3120 适用于测量最大值为 VDC = 5V, Frequency = 10MHz 并且 Iload = 400mA 的 PSRR。 用示波器测量 PSRR 如果用户没有网络分析仪，那么一个更简单但更麻烦的方法是使用一个信号发生器、直流电源和示波器来测量 PSRR。 用信号发生器产生一个交流信号，与直流信号一起作为 LDO 的输入，这与前面所提到的方法是一样的，然后用一个 示波器以不同频率来测量 LDO 的输出。PSRR 由公式 1 计算得到，其中 Ripple(input) 是输入交流信号的幅度，而 Ripple(output) 是输出信号的幅度。以不同频率的 VAC 重复以上测量过程，以产生 PSRR 曲线图片段。 这一方法可以与上一节所描述的结构一起使用。然而由于示波器的分辨率和灵敏度有限，这一方法仅仅对具有更低 PSRR 值的 LDO 适用。由于大多数示波器可以测量到毫伏的范围，因此使用示波器可实际测量到的 PSRR 的最大范围约 为 40dB–50dB。 ZHCA089 – 2009 年 7 月 提交文档反馈 LDO PSRR 测量简化说明 100 F� 1�k� 100 � DUT COUTDC�out RF�out THS3120 + - +10�V -10�V Buil