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IRF6645 데이터시트(PDF) 5 Page - International Rectifier |
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IRF6645 데이터시트(HTML) 5 Page - International Rectifier |
5 / 44 page www.irf.com 4 RD-0617 Functional Description Class D Operation Referring to CH1 as an example, the op-amp U1 forms a front-end second-order integrator with C11, C13 & R25 + R29P. This integrator receives a rectangular feedback waveform from the Class D switching stage and outputs a quadratic oscillatory waveform as a carrier signal. To create the modulated PWM signal, the input signal shifts the average value of this quadratic waveform (through gain relationship between R13 and R31 + R33) so that the duty varies according to the instantaneous value of the analog input signal. The IRS20955 input comparator processes the signal to create the required PWM signal. This PWM signal is internally level-shifted down to the negative supply rail where this signal is split into two signals, with opposite polarity and added deadtime, for high-side and low-side MOSFET gate signals, respectively. The IRS20955 drives two IRF6645 DirectFET MOSFETs in the power stage to provide the amplified PWM waveform. The amplified analog output is re-created by demodulating the amplified PWM. This is done by means of the LC low-pass filter (LPF) formed by L1 and C23, which filters out the Class D switching carrier signal. Figure 2. Simplified Block Diagram of Class D Amplifier Power Supplies The IRAUDAMP4 has all the necessary housekeeping power supplies onboard and only requires a pair of symmetric power supplies ranging from ±25 V to ±35 V (+B, GND, -B) for operation. The internally-generated housekeeping power supplies include a ±5 V supply for analog signal processing (preamp, etc.), while a +12 V supply (VCC), referenced to –B, is included to supply the Class D gate-driver stage. For the externally-applied power, a regulated power supply is preferable for performance measurements, but not always necessary. The bus capacitors, C31 and C32 on the motherboard, along with high-frequency bypass-caps C15-C18 on daughter board, address the high-frequency ripple current that result from switching action. In designs involving unregulated power supplies, the designer should place a set of bus capacitors, having enough capacitance to handle the audio-ripple current, externally. Overall regulation and output voltage ripple for the power supply design are not critical when using the IRAUDAMP4 Class D amplifier as the power supply rejection ratio (PSRR) of the IRAUDAMP4 is excellent (Figure 3). IRF6645 Direct-FET Feedback GND LPF +B -B Σ IRS20955S Gate Driver U1 U1 Daughter-board Integrator PWM Modulator and Level Shifter |
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