ZHCSFP7A November 2016 – February 2017 TAS2557
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The TAS2557 is a digital input high efficiency Class-D audio power amplifier with advanced battery current management and an integrated Class-H boost converter. In auto-passthrough mode, the Class-H boost converter generates the Class-D amplifier supply rail. During low Class-D output power, the boost improves efficiency by deactivating and connecting VBAT directly to the Class-D amplifier supply. When high power audio is required, the boost quickly activates to provide louder audio than a stand-alone amplifier connected directly to the battery. To enable load monitoring, the TAS2557 constantly measures the current and voltage across the load and provides a digital stream of this information back to a processor.
|L1||Boost Converter Inductor||Inductance, 20% Tolerance||1||2.2||µH|
|L2, L3||EMI Filter Inductors (optional). These are not recommended as it degrades THD+N performance. The TAS2557 device is a filter-less Class-D and does not require these bead inductors.||Impedance at 100MHz||120||Ω|
|C1||Boost Converter Input Capacitor||Capacitance, 20% Tolerance||10||µF|
|C2||Boost Converter Output Capacitor||Type||X5R|
|Capacitance, 20% Tolerance||22||47||µF|
|Capacitance at 8.5 V derating||3.3||µF|
|C3, C4||EMI Filter Capacitors (optional, must use L2, L3 if C3, C4 used)||Capacitance||1||nF|
For this design example, use the parameters shown in Table 24.
|DESIGN PARAMETER||EXAMPLE VALUE|
|Audio Input||Digital Audio, I2S|
|Current and Voltage Data Stream||Digital Audio, I2S|
|Mono or Stereo Configuration||Mono|
|Max Output Power at 1% THD+N||3.8 W|
In this application, the device is assumed to be operating in mono mode. See General I2C Operation for information on changing the I2C address of the TAS2557 to support stereo operation. Mono or stereo configuration does not impact the device performance.
The boost converter requires three passive devices that are labeled L1, C1 and C2 in Figure 43 and whose specifications are provided in Table 23. These specifications are based on the design of the TAS2557 and are necessary to meet the performance targets of the device. In particular, L1 should not be allowed to enter in the current saturation region. The saturation current for L1 should be > ILIM to deliver Class-D peak power.
Additionally, the ratio of L1/C2 (the derated value of C2 at 8.5 V should be used in this ratio) has to be lesser than 1/3 for boost stability. This 1/3 ratio should be maintained including the worst case variation of L1 and C2. To satisfy sufficient energy transfer, L1 needs to be >= 1µH at the boost switching frequency (~1.7 MHz). Using a 1µH will have more boost ripple than a 2.2µH but the PSRR should minimize the effect from the additional ripple. Finally, the minimum C2 (derated value at 8.5 V) should be > 3.3µF for Class-D power delivery specification.
The TAS2557 supports edge-rate control to minimize EMI, but the system designer may want to include passive devices on the Class-D output devices. These passive devices that are labeled L2, L3, C3 and C4 in Figure 43 and their recommended specifications are provided in Table 23. If C3 and C4 are used, they must be placed after L2 and L3 respectively to maintain the stability of the output stage.
|Freq = 1kHz VBAT = 3.6 V, AVDD = IOVDD = 1.8 V, RESET = IOVDD, RL = 8 Ω + 33 µH, I2S digital input, ROM mode 1|