AN-135 AAT5101 AAT2868 AN-134 AP-SYS2722 SYS2722

AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Introduction This application note explains the

APPLICATION NOTE AN-135 AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Introduction This application note explains the importance of PSRR and shows how to calculate and measure it. Two examples using the AAT5101 AAT5101 Class D audio amplifier and the AAT2868 AAT2868 CABC Compatible 4 Channel Backlight Driver with Dual LDO Regulators are used to explain how to measure the PSRR of a Class D audio amplifier and a low dropout regulator (LDO). Additionally, a non-inverting amplifier adder circuit used in PSRR measurement is introduced at the end of this note. What is PSRR? Power Supply Rejection Ratio (PSRR) is the ratio of the output ripple voltage to the power supply ripple voltage; PSRR indicates how well a circuit rejects ripple coming from the power supply input at various frequencies. PSRR is an important parameter of amplifier and LDO performance in many applications, especially RF and wireless application. The Importance of PSRR In a typical portable RF application, the RF amplifier is turned on/off at a rate of 217Hz. At each of these events, a high current (typically, up to 1.7A) is drawn from the power supply, creating a sudden voltage drop (about 200mV) through the battery's equivalent series resistance (ESR)as shown in Figure 1. The battery also provides power to other devices such as an audio amplifier. The supply ripple caused by the RF amplifier will be injected into the audio amplifier, which then will appear at the same frequency as a voltage ripple in the audio amplifier output. Since 217Hz is within the audio bandwidth (20Hz to 20kHz) the ripple can be heard by the human ear as is observed as a fixed-frequency noise. The level of this noise depends on the PSRR performance of the audio amplifier. With excellent PSRR, the 217Hz noise will be rejected well and not disturb the audio performance. IRF ESR LDO RF Amplifier V BAT Audio In Audio Amplifier Figure 1: RF Subsystem Diagram. AN-134 AN-134.2010.08.1.0 www.analogictech.com 1 APPLICATION NOTE AN-135 AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Calculating PSRR The PSRR of the audio amplifier can be calculated using Equation 1: Eq. 1: PSRR = 20 · log VRIPPLE(OUT) VRIPPLE(IN) For example, if there is 200mV peak-to-peak ripple in the power supply input and the same frequency voltage ripple in the amplifier output at 200V peak-to-peak. The PSRR of the LDO can then be calculated using Equation 2: Eq. 2: PSRR = 20 · log 200 · 10-6 200 · 10-3 = -60dB However, in an LDO, the PSRR is always expressed as a positive value. Measuring PSRR As shown previously, when the amplitude of the input ripple and relative output ripple are known, the device PSRR can be calculated easily. The common way to perform the PSRR measurement is to add a fixed amplitude sinusoid ripple on the power supply input and measure the amplitude of the same frequency ripple on the output. The PSRR can then be calculated using Equation 1 or 2. VRIPPLE(IN) VRIPPLE(IN) VDD Input + Inputs Amplifier Output VRIPPLE(OUT) LDO Output VRIPPLE(OUT) - a: Amplifier PSRR Measurement b: LDO PSRR Measurement Figure 2: PSRR Measurement. An Audio Precision audio analyzer or a network analyzer is needed to perform the PSRR measurement. The PSRR value can be expressed in dB directly by using the crosstalk or A/R function in the measurement instrument instead of calculating it. 2 www.analogictech.com AN-134 AN-134.2010.08.1.0 APPLICATION NOTE AN-135 AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs PSRR Measurement of Class D Audio Amplifier Figure 3 shows the PSRR measurement set-up for the AAT5101 AAT5101 Class D audio amplifier. In this case, the DC power supply, non-inverting amplifier adder, an 8 load, 30kHz low pass filter, and Audio Precision analyzer AP-SYS2722 AP-SYS2722 are needed. DC Supply DC Input Voltage VS + Adder Audio Precision OUT (SYS2722 SYS2722) Sine Ripple + INA VS - LPF VDD + INN Audio Precision (SYS2722 SYS2722) OUTP AAT5101 AAT5101 INP Load INB OUTN GND Figure 3: AAT5101 AAT5101 PSRR Measurement Set-up. The inputs of the AAT5101 AAT5101 should be AC grounded as shown. A 200mV peak to peak sinusoid ripple generated from the Audio Precision analyzer output is added onto the power supply input through the non-inverting amplifier adder. The output ripple can be measured by the Audio Precision (AP) analyzer through a 30kHz low pass filter (LPF). Connect the power supply input (VDD) to the AP's channel A and connect the output of the low pass filter to the AP's channel B. Choose the crosstalk function and scan frequency from 20Hz to 20kHz, then a resulting curve of PSRR over the audio bandwidth will be measured as shown in Figure 4. Adjust the power supply DC value to the intended system operating range. Figure 4 shows the AAT5101 AAT5101 PSRR measurement results for DC input values of VDD at 2.5V, 3.6V and 5V. Figure 4: AAT5101 AAT5101 PSRR Result. AN-134 AN-134.2010.08.1.0 www.analogictech.com 3 APPLICATION NOTE AN-135 AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs PSRR Measurement of LDO Figure 5 shows the PSRR measurement set-up for the AAT2868 AAT2868 which contains two LDO regulators. In this example, the DC power supply, non-inverting amplifier adder, DC electronic load, and a network analyzer such as the Agilent 4395A are needed. DC Supply DC Input Voltage INA VS + Adder Network Analyzer (4395A) RF OUT Sine Ripple VS - Network Analyzer (4395A) AAT2868 AAT2868 INLDO LDOA INR GND eLoad Figure 5: AAT2868 AAT2868 LDO Part PSRR Measurement Set-up. A 200mV peak-to-peak sinusoid ripple waveform generated from network analyzer output is added onto the power supply input (INLDO) through the non-inverting amplifier adder. The output voltage can be measured by the network analyzer. Connect the LDO input to the network port INA and connect the LDO output to the network port INR. Chose the A/R function and a scan frequency from 100Hz to 100kHz. Adjust the power supply DC value to the intended system operating range. Figure 6 shows the PSRR measurement result using the AAT2868 AAT2868 LDO for the condition: VIN = 3.6V, VRIPPLE = 200mVpp, ILOAD =10mA. Figure 6: AAT2868 AAT2868 LDO PSRR Measurement Result. 4 www.analogictech.com AN-134 AN-134.2010.08.1.0 APPLICATION NOTE AN-135 AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Design and Application of the Non-inverting Amplifier Adder To generate a DC power with a certain sinusoid ripple waveform added, a non-inverting amplifier adder is adopted. The output of the adder will be DC+AC as shown in Figure 7. DC Input Voltage DC Input Voltage Adder Ripple Peak Value Sine Ripple Figure 7: Adder for PSRR Measurement. Figures 8 and 9 show the schematic and board picture of the non-inverting amplifier adder used in the PSRR measurement. DC Input Voltage +12V C3 C4 R1 0.1F 10F 20 U1A Sine Ripple V+ OPA C1 R2 DC Input Voltage Ripple Peak Value OUT V- 470F C2 0.1F 0 C5 10F -12V Figure 8: Schematic of the Non-inverting Amplifier Adder in PSRR Measurement. AN-134 AN-134.2010.08.1.0 www.analogictech.com 5 APPLICATION NOTE AN-135 AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Figure 9: Board Picture of the Non-inverting Amplifier Adder in PSRR Measurement. Connect ±12V DC power to +12V and -12V on the board. Connect the power supply input voltage of the test device to VDD. Connect the output of audio precision or network analyzer to In to supply the desire sinusoid ripple. The DC+AC voltage of VDD + In signal will be generated at OUT. Figure 10 shows the system connecting of the set-up described above. Yellow trace is the DC+AC output of the adder. Power supply provides ±12V to the adder board. +5V DC power supply input. Sine ripple fed into adder generated by Audio Precision or Network output. Figure 10: Real Connect of the Non-inverting Amplifier Adder Board. The yellow curve on the scope is the sinusoid ripple waveform before the non-inverting amplifier adder and the blue curve is the DC+AC signal of the Adder output. 6 www.analogictech.com AN-134 AN-134.2010.08.1.0 APPLICATION NOTE AN-135 AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Conclusion The PSRR measurement indicates how well a circuit rejects ripple or noise from the power supply input. When making a PSRR measurement, the biggest challenge is that the output ripple amplitude is very small. This makes it difficult to measure accurately using common equipment such as an oscilloscope or multi-meter. An Audio Precision analyzer (e.g. AP-SYS2722 AP-SYS2722) or a network analyzer (e.g. Agilent 4395A) is necessary to do the measurement correctly. Table 1 shows the required equipment and calculations for the Class D audio amplifier and the LDO PSRR measurement. PSRR Calculation Class D Audio Amplifier LDO PSRR = 20 · log Measurement Equipment VRIPPLE(OUT) VRIPPLE(IN) VRIPPLE(IN) PSRR = 20 · log V RIPPLE(OUT) Audio Precision (e.g. AP-SYS2722 AP-SYS2722), Adder Network Analyzer (e.g. Agilent-4395A), Adder Table 1: PSRR Calculation Equation and Measurement Equipment. Advanced Analogic Technologies, Inc. 3230 Scott Boulevard, Santa Clara, CA 95054 Phone (408) 737-4600 Fax (408) 737-4611 © Advanced Analogic Technologies, Inc. AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice. 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