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MP7731 Datasheet(PDF) 6 Page - Monolithic Power Systems |
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MP7731 Datasheet(HTML) 6 Page - Monolithic Power Systems |
6 / 9 page MP7731 – 5W – 30W CLASS D MONO BRIDGED AUDIO AMPLIFIER MP7731 Rev. 1.8 www.MonolithicPower.com 6 9/25/2006 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2006 MPS. All Rights Reserved. APPLICATION INFORMATION COMPONENT SELECTION The MP7731 uses a minimum number of external components to complete a fully bridged Class D audio amplifier. The circuit in Figure 1 shows a typical application. Use the following sections to customize the amplifier for your particular application. Setting the Voltage Gain The voltage gain sets the output voltage swing for a given input voltage swing and is set by the following equation: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = 4 1 V R R 2 A Where: R1 = R8 R4 = R6 The maximum output voltage swing is limited by the power supply. The MP7731 is a bridged amplifier and the output load is driven differentially. Each side of the load is limited to a maximum peak-to-peak voltage swing of approximately VDD. To achieve the maximum output power of the MP7731 amplifier, set the amplifier gain such that the maximum peak-to-peak input signal results in at least the maximum peak-to- peak output voltage swing. Setting the Switching Frequency The idle switching frequency (the switching frequency with no audio input signal) is a function of the supply voltage, VDD, the capacitors C4, C6 and C10 and resistors R1 and R8. Lower switching frequencies result in more inductor ripple, causing more quiescent output voltage ripple, increasing the output noise.. Higher switching frequencies result in more power loss. The optimum quiescent switching frequency is approximately 400KHz-600KHz. C6 and C14 are typically 1pF to 2.2pF. C10 is used to program the idle switching frequency. Choosing the LC Filter Two identical LC filters are required in the typical application. The inductor-capacitor (LC) filter is a second order filter that converts the pulse train at SW (Pins 14, 19) to the output differential signal that drives the speaker. Typical values for the LC filters are shown in Figure 1. The characteristic frequency of the LC filter needs to be high enough to allow high frequency audio to reach the output, yet needs to be low enough to filter out high frequency contents of the pulses from SW. The characteristic frequency of the LC filter is: LC 2 1 f0 π = Where: L = L1 + L2, L1 = L2 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + × + = 17 C 2 C 17 C 2 C 9 C C , C2 = C17 The quality factor (Q) of the LC filter is important. If this is too low, output noise will increase. If the Q factor is too high, then peaking may occur at high signal frequencies reducing the pass-band flatness. The Q is calculated as: C L R Q = Where R is the load (speaker) resistance. Use an LC filter with a Q between 0.7 and 2.0. The actual output ripple and noise is greatly affected by the type of inductor and capacitor used in the LC filter. Use a film capacitor and an inductor with sufficient power handling capability to supply the output current to the load. The inductor should exhibit soft saturation characteristics. If the inductor exhibits hard saturation, it should operate well below the saturation current. Gapped ferrite, MPP, Powdered Iron, or similar type toroidal cores are recommended. If open or shielded bobbin |
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