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Tpa324x and Tpa325x Post-Filter Feedback (Rev. A)

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Tpa324x and Tpa325x Post-Filter Feedback (Rev. A) Application Report SLAA788A–September 2017–Revised March 2018 TPA324x and TPA325x Post-Filter Feedback Dan Kisling, Matthew Beardsworth ABSTRACT The TPA324x and TPA325x (TPA3244, TPA3245, TPA3250, TPA3251, TPA3255) Class-D audio amplifier families deliver high audio performance with less than 0.01% total harmonic distortion and noise (THD+N) to clipping. The high level of audio performance makes this device an ideal candidate for high resolution and high fidelity audio applications, which previously could only be achieved by Class-AB amplifiers. The TPA324x and TPA325x devices are analog input, closed loop (internal feedback network) Class-D audio amplifiers that can be further enhanced by adding an additional post-filter feedback (PFFB) or PFFB loop. This application report shows one optional implementation of PFFB for the TPA3244, TPA3245, TPA3250, TPA3251, and TPA3255 amplifiers. PFFB offers many benefits including lower output noise, improved THD+N performance, improved IMD performance, lower output impedance, frequency response less affected by load impedance, and suppression of nonlinearities of the LC filter. Contents 1 PFFB Implementation ....................................................................................................... 2 2 Closed Loop Gain............................................................................................................ 4 3 LC Filter Distortion........................................................................................................... 5 4 Performance Results ........................................................................................................ 6 5 Output Noise ................................................................................................................. 6 6 SNR and DNR................................................................................................................ 6 7 THD+N vs Power ............................................................................................................ 7 8 THD+N vs Frequency ....................................................................................................... 9 9 SMPTE IMD................................................................................................................. 11 10 CCIF IMD.................................................................................................................... 12 11 Stability Analysis ........................................................................................................... 13 12 Stability Testing............................................................................................................. 14 13 Overshoot for Square Wave Input ....................................................................................... 14 14 Calculating Phase Margin ................................................................................................. 15 15 TPA3245 PFFB Stability................................................................................................... 16 16 Frequency Reponse at Full Scale........................................................................................ 18 Appendix A TPA3244............................................................................................................ 19 Appendix B TPA3245............................................................................................................ 25 Appendix C TPA3250............................................................................................................ 31 Appendix D TPA3251............................................................................................................ 37 Appendix E TPA3255............................................................................................................ 43 Trademarks All trademarks are the property of their respective owners. SLAA788A–September 2017–Revised March 2018 TPA324x and TPA325x Post-Filter Feedback 1 Submit Documentation Feedback Copyright © 2017–2018, Texas Instruments Incorporated PFFB Implementation www.ti.com 1 PFFB Implementation Post filter feedback is implemented by adding a secondary feedback loop external to the amplifier. This feedback loop takes a fraction of the output voltage signal of the amplifier after the external LC filter and sends an error signal back to the input of the amplifier. See Figure 1. GND R_fb_gnd C_fb_in C_fb_out R_fb R_in L_out In A Out A TPA324x/5x C_out Class-D Amplifier GND Copyright © 2017, Texas Instruments Incorporated Figure 1. Post-Filter Feedback Loop The following components are required for PFFB implementation: • R_fb – Feedback resistor • C_fb_in – Capacitor on input side of the feedback network • C_fb_out – Capacitor on output side of the feedback network • R_fb_gnd – Resistor between C_fb_in and C_fb_out to GND in the feedback network • R_in – Input summing junction resistor • C_z – Zobel network capacitor • R_z – Zobel network resistor • C_op – Op-Amp feedback capacitor • R_op-fb – Op-Amp feedback resistor 2 TPA324x and TPA325x Post-Filter Feedback SLAA788A–September 2017–Revised March 2018 Submit Documentation Feedback Copyright © 2017–2018, Texas Instruments Incorporated www.ti.com PFFB Implementation Figure 2 shows the passive PFFB implementation used throughout the document for TPA3244, TPA3245, TPA3250, TPA3251, and TPA3255 amplifiers. GND R_fb_gnd C_fb_in C_fb_out R_fb GND R_fb_gnd C_fb_in C_fb_out R_fb C_op R_op-fb C_z R_z 10 k R_in ± L_out GND In A Out A + C_out GND C_op R_op-fb C_out GND 10 k ± R_in L_out In B Out B + C_z R_z GND C_op TPA324x/5x R_op-fb Class-D Amplifier C_z R_z GND 10 k R_in ± L_out GND In C Out C + C_out GND C_op R_op-fb C_out 10 k GND ± R_in L_out In D Out D + GND C_z R_z R_fb GND C_fb_in C_fb_out R_fb_gnd GND R_fb C_fb_in C_fb_out R_fb_gnd GND Copyright © 2017, Texas Instruments Incorporated Figure 2. Passive PFFB Implementation SLAA788A–September 2017–Revised March 2018 TPA324x and TPA325x Post-Filter Feedback 3 Submit Documentation Feedback Copyright © 2017–2018, Texas Instruments Incorporated Closed Loop Gain www.ti.com Table 1 lists the component values that have improved audio performance and sufficient stability. Table 1. Recommended PFFB Component Values TPA3244 TPA3245 TPA3250 TPA3251 TPA3255 EVM Schematic PFFB (PVDD = 30 V, (PVDD = 30 V, (PVDD = 36 V, (PVDD = 36 V, (PVDD = 51 V, Location and Designator Fpwm = 450 Fpwm = 600 Fpwm = 450 Fpwm = 600 Fpwm = 450 Designator kHz) kHz) kHz) kHz) kHz) L_out L2, L3, L4, L5 10 µH 10 µH 10 µH 7 µH 10 µH C24, C35, C43, C_out 1 µF 1 µF 1 µF 680 nF 1 µF C59 R47, R49, R50, R_fb 18 kΩ 18 kΩ 18 kΩ 18 kΩ 33 kΩ R51 C_fb_in N/A 220 pF 220 pF 220 pF 220 pF 220 pF C_fb_out N/A 220 pF 1 nF 220 pF 1 nF 220 pF R_fb_gnd N/A 2 kΩ 2 kΩ 2 kΩ 2 kΩ 10 kΩ R4, R12, R44, R_in 2.7 kΩ 2.7 kΩ 2.7 kΩ 2.7 kΩ 2.7 kΩ R46 C77, C78, C79, C_z 220 nF 220 nF 220 nF 220 nF 220 nF C80 R54, R55, R56, R_z 1 Ω 1 Ω 1 Ω 1 Ω 1 Ω R57 C18, C23, C57, C_op 330 pF 330 pF 330 pF 330 pF 330 pF C65 R8, R41, R21, R_op_fb 10 kΩ 10 kΩ 10 kΩ 10 kΩ 10 kΩ R25 • R_fb and R_in controls the amount of negative feedback used in this system. • C_fb_in, C_fb_out, and R_fb_gnd make a feedback network that helps with open load response and overall network stability. • C_z and R_z create the Zobel Network and are required for network stability. The Zobel network helps attenuate the high-frequency ringing by damping the amplitude response of the output of the LC filter and lowering the quality factor (Q). This is especially required for open-load stability where the Q of the LC filter is extremely large due to the lack of dampening from the load impedance. Reducing the Q of the output filter also increases stability, by relaxing the phase shift associated with a high Q system. It is best to keep the feedback components close to the device so the feedback signals are not subject to significant distortion or noise. 2 Closed Loop Gain With the components for PFFB selected and the gain on the amplifier known, the new closed loop gain can be estimated. Equation 1 shows the closed-loop gain. A A 0 f 1A+E ()0 (1) Use Equation 2 to calculate the feedback factor. R _ in E R _ in+ R _ fb (2) 4 TPA324x and TPA325x Post-Filter Feedback SLAA788A–September 2017–Revised March 2018 Submit Documentation Feedback Copyright © 2017–2018, Texas Instruments Incorporated www.ti.com LC Filter Distortion Using Equation 1 and Equation 2, the PFFB gain and the negative feedback gain can be calculated. Equation 3 shows an example for the TPA3251. 2.7 k A 20 dB 10E 0.13 0 ()2.7 k+ 18 k 10 A 4.35 12.8 dB f ()1+u() 10 0.13 where • The closed loop gain has been reduced to 12.8 dB due to PFFB • 7.2 dB of PFFB has been applied to the amplifier (3) Approximately 6 to 7 dB of PFFB has been applied to each amplifier. Table 2 lists the results. Table 2. PFFB Parameters TPA3244 TPA3245 TPA3250 TPA3251 TPA3255 Feedback (PVDD = 30 V, (PVDD = 30 V, (PVDD = 36 V, (PVDD = 36 V, (PVDD = 51 V, Parameters Fpwm = 450 kHz) Fpwm = 600 kHz) Fpwm = 450 kHz) Fpwm = 600 kHz) Fpwm = 450 kHz) Gain (dB) 18 18 20 20 21.5 Feedback factor 0.130 0.130 0.130 0.130 0.076 PFFB gain Af (dB) 11.82 11.82 12.75 12.75 15.93 Negative feedback 6.18 6.18 7.25 7.25 5.57 3 LC Filter Distortion The LC filter extracts a continuous analog audio signal from the PWM output for the Class-D to suppress radiating EMI and ripple current in the load connected. However, since the TPA324x and TPA325x families offer such a high level of performance, the inductor used in the LC filter is the primary contributor
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