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MP7722DF Datasheet(PDF) 8 Page - Monolithic Power Systems |
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MP7722DF Datasheet(HTML) 8 Page - Monolithic Power Systems |
8 / 11 page MP7722 – 2 x 20W CLASS D STEREO SINGLE ENDED AUDIO AMPLIFIER MP7722 Rev. 1.3 www.MonolithicPower.com 8 9/25/2006 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2006 MPS. All Rights Reserved. The quality factor (Q) of the LC filter is important. If this is too low output noise will increase, and if this is too high then peaking may occur at high signal frequencies reducing the passband flatness. The circuit Q is set by the load resistance (speaker resistance, typically 4Ω or 8Ω) and is calculated as: R L f 2 R L Q 0 0 × × π = × ω = Where ω0 is the characteristic frequency in radians per second and f0 is in Hz. Use a LC filter with Q between 0.7 and 2. 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 ferrite cores are used for multi-channel designs, make sure that the start windings of each inductor line up (all starts going toward SW pin or all starts going toward the output) to prevent crosstalk or other channel-to-channel interference. Output Coupling Capacitors The output AC coupling capacitors COUT1 and COUT2 serve to block DC voltages and thus pass only the amplified AC signal from the LC filter to the load. The combination of the coupling capacitor and the load resistance results in a first-order high-pass filter. The values of COUT1 and COUT2 should be selected such that the required minimum frequency is still allowed to pass. The output corner frequency (-3dB point), fOUT, can be calculated as: OUT LOAD OUT C R 2 1 f × × π × = Set the output corner frequency at or below the minimum required frequency. The output coupling capacitor carries the full load current, so the capacitor should be chosen such that its ripple current rating is greater than the maximum load current. Low ESR aluminum electrolytic capacitors are recommended. Input Coupling Capacitors The input coupling capacitors CIN1 and CIN2 are used to pass only the AC signal at the input. In a typical system application, the source input signal is typically centered around the circuit ground, while the MP7722 input is at half the power supply voltage (VDD/2). The input coupling capacitors transmit the AC signal from the source to the MP7722 while blocking the DC voltage. Choose the input coupling capacitors such that the corner frequency (fIN) is less than the passband frequency. The corner frequency is calculated as: IN IN IN C R 2 1 f × × π × = Power Source For maximum output power, the amplifier circuit requires a regulated external power source to supply the power to the amplifier. A higher power supply voltage allows more power to be delivered to a given load resistance. However if the power source voltage exceeds the maximum operating voltage of 24V, the MP7722 may sustain permanent damage. It is very important to bypass the power supply pins with 1µF X7R ceramic capacitors. Power Supply Pumping It is also very important to bypass the power supply with a large aluminum electrolytic capacitor. It is recommended to use a value of at least 2200µF. This is necessary to prevent the supply voltage from getting pumped up to a level that exceeds the absolute maximum rating. Supply pumping occurs in single-ended Class D amplifiers, and is caused by rapid switch transitions where current is pumped back up into the supply line. The large capacitor is necessary to absorb this current and prevent VDD from rising too high. |
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