The SSM4321 is a fully integrated, high efficiency Class-D audio amplifier with digitized output of output voltage, output current, and PVDD supply. It is designed to maximize performance for mobile phone applications. The application circuit requires a minimum of external components and operates from a 2.5 V to 5.5 V supply for the amplifier and a 1.42 V to 3.6 V supply for IO. It is capable of delivering 2.2 W of continuous output power with less than 1% THD + N driving an 4 Ω load from a 5.0 V supply with a 0.1Ω V/I sense resistor.
The SSM4321 includes circuitry to sense output current, output voltage, and the PVDD supply voltage. Current sense is performed using an external sense resistor that is connected between an output pin and the load. Output current and voltage are sent to an ADC with 16-bit resolution and PVDD is sent to an ADC with 8-bit resolution. The outputs of these ADCs are available on the TDM or I2S output serial port. The SLOT pin is used to determine which of 4 possible output slots is used on the TDM interface. A stereo I2S interface can be selected by swapping the pin connections for BCLK and FSYNC. Alternatively, a direct PDM bitstream of voltage and current data can be selected via the SLOT pin.
Spread spectrum pulse density modulation (PDM) is used to provide lower EMI-radiated emissions compared with other Class-D architectures. The inherent randomized nature of spread spectrum PDM eliminates clock intermodulation (beating effect) of several amplifiers in close proximity. The SSM4321 includes an optional modulation select pin that enables a low EMI mode, which significantly reduces the radiated emissions at the Class-D outputs, particularly above 100 MHz. With this option selected, the SSM4321 can pass FCC Class B radiated emissions testing with a 50 cm unshielded speaker cable without any external filtering.
The device also includes a flexible gain select pin that only requires one series resistor to select among 0 dB, 3 dB, 6 dB, 9 dB, or 12 dB. The benefit of this is to improve gain matching between multiple SSM4321 devices within a single application as compared to using external resistors to set gain.