AT73C240 - Audio Power Amplifier Filtering and Gain 1. Scope This document decribes how to determine the capacitance of the Input Capacitors of the Audio Power Amplifier for the product AT73C240. AT73C240 has an Audio Power Amplifier, bridge-tied loaded, designed in CMOS technology, capable of driving an 8 Ohm Loudspeaker, making it suitable as a handsfree speaker driver in Wireless Handset Application. High quality mono output is provided. The DAC output can be connected trough a buffer stage to the input of the Audio Power Amplifier, using two coupling capacitors. This document defines how to calculate the required capacitance of these two coupling capacitors. Caution: the Audio power amplifier is not internally protected against short-circuit. The user should avoid any short-circuit on the load. Power Management and Analog Companions (PMAAC) Application Note 2. Operating Conditions Table 2-1. Operating Condition Table For AT73C240 Product Parameter/Function Pads Min Max Unit Storage Temperature -- -55 150 °C Operating Temperature -- -40 85 °C Audio Power Input Voltage VBAT 3.0 5.5 V Digital Input Voltage VDIG 2.4 3.3 V Analog Input Voltage AVDD, AVDDHS 2.7 3.3 V 3. Glossary BW -- Bandwith CMRR -- Common Mode Rejection Ratio DAC -- Digital to Analog Converter PA -- Audio Power Amplifier 6465A–PMAAC–04-Jun-09 4. AT73C240 Block Diagram -36 to +12dB / 3dB step AT73C240 GNDB VREF PAINN VBAT CBP HPP Audio PA HPN AT73C240 Block Diagram PAINP Figure 4-1. VDIG AVDD Voltage Reference AVDDHS SPI_DOUT LINER PGA SPI_DIN / TWD Status Registers INGND -36 to +12dB / 3dB step SPI SPI_CLK / TWCK LINEL SPI_CSB / TW_ADD PGA -46.5dB to 0dB / 1.5dB step -34.5dB to +12dB / 1.5dB step MCLK -6 to +6dB / 3dB step 32 driver + DAC Volume Control + Volume Control RSTB Digital Filter VCM -6 to +6dB / 3dB step HSR 32 driver + DAC Volume Control + Volume Control Digital Filter Serial Audio I/F HSL SMODE SDIN LRFS BCLK GNDA -46.5dB to 0dB / 1.5dB step -34.5dB to +12dB / 1.5dB step MONOP MONO + GNDD MONON 2 Application Note 6465A–PMAAC–04-Jun-09 Application Note 5. Power Amplifier Inupt Capacitor Selection 5.1 Use Needs The two input capacitors, C8 and C15 in Figure 5-1, connected between MONON-PAINN and MONOP-PAINP, affect two parameters: • The low frequency cut-off • The common mode rejection at low frequencies. The low frequency cut-off depends on those capacitor values and the PA input impedance. This impedance also depends on the gain selected (the higher the gain, the lower the input impedance). The low frequency cut-off is generally chosen to allow full bandwidth reproduction in all cases. The worst case is the highest low frequency cut-off available. Power Amplifier Apllication Diagram Figure 5-1. AT73C240 Power Amplifier Application Diagram PAINN AT73C240 C16 3.6 V VBAT CBP C7 HPP Battery (Li-Ion or 3 x NiMh or NiCd) 5.2 Audio PA Loudspeaker HPN C15 PAINP MONOP C8 MONON 5.3 Calculation Since minimum input impedance value is 10kOhms at maximum gain, the input capacitance regarding the lower frequency to reproduce (Fcl) is calculated at follows: 5 Cin = 10 ⁄ ( 6283 × Fcl ) • Fcl is expressed in Hz • Cin is expressed in µF For a 50Hz low frequency cut-off, the calculated value is 0.318µF, or a 330nF as a standard capacitor value. Another parameter is the CMRR: to ensure a 40dB minimum rejection at low frequency, the matching between the two input capacitors must be better than 1%. 3 6465A–PMAAC–04-Jun-09 6. Power Amplifier Input Impedance Versus Gain The input impedance depends on the power amplifier gain. The minimum input impedance (15kOhms typ, 10kOhms min) occurs with maximum gain value selected (+20dB), and at the opposite the maximum input impedance (150kOhms typ, 100kOhms min) is obtained with the minimum gain value (-22dB). The following table gives all typical process input impedance values versus the gain selected. Table 6-1. 4 Input Impedance vs Power Amplifier Gain Input Impedance (Ohms) Gain (dB) Input Impedance (Ohms) Gain (dB) 15k +20 100k -4 20k +17 113k -7 27k +14 124k -10 36k +11 133k -13 46k +8 140k -16 59k +5 145k -19 71k +2 150k -22 86k -1 Application Note 6465A–PMAAC–04-Jun-09 Application Note Revision History Doc. Rev Date Comments 6465A 04-Jun-09 First issue Change Request Ref. 5 6465A–PMAAC–04-Jun-09 Headquarters International Atmel Corporation 2325 Orchard Parkway San Jose, CA 95131 USA Tel: 1(408) 441-0311 Fax: 1(408) 487-2600 Atmel Asia Unit 1-5 & 16, 19/F BEA Tower, Millennium City 5 418 Kwun Tong Road Kwun Tong, Kowloon Hong Kong Tel: (852) 2245-6100 Fax: (852) 2722-1369 Atmel Europe Le Krebs 8, Rue Jean-Pierre Timbaud BP 309 78054 Saint-Quentin-enYvelines Cedex France Tel: (33) 1-30-60-70-00 Fax: (33) 1-30-60-71-11 Atmel Japan 9F, Tonetsu Shinkawa Bldg. 1-24-8 Shinkawa Chuo-ku, Tokyo 104-0033 Japan Tel: (81) 3-3523-3551 Fax: (81) 3-3523-7581 Technical Support AT91SAM Support Atmel techincal support Sales Contacts www.atmel.com/contacts/ Product Contact Web Site www.atmel.com www.atmel.com/AT91SAM www.atmel.com/Iproducts/ASIC Literature Requests www.atmel.com/literature Disclaimer: The information in this document is provided in connection with Atmel products. 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