7.3.1 Analog Audio Input and Preamplifier

The differential input stage of the amplifier cancels common-mode noise that appears on the inputs. For a differential audio source, connect the positive lead to IN_P and the negative lead to IN_N. The inputs must be ac-coupled to minimize the output dc-offset and ensure correct ramping of the output voltages. For good transient performance, the impedance seen at each of the two differential inputs should be the same.

The gain setting impacts the analog input impedance of the amplifier. See Table 1 for typical values.

Table 1. Input Impedance and Gain

7.3.2 Pulse-Width Modulator (PWM)

The PWM converts the analog signal from the preamplifier into a switched signal of varying duty cycle. This is the critical stage that defines the class-D architecture. In the TAS5421-Q1, the modulator is an advanced design with high bandwidth, low noise, low distortion, and excellent stability.

The pulse-width modulation scheme allows increased efficiency at low power. Each output is switching from 0 V to PVDD. The OUTP and OUTN pins are in phase with each other with no input, so that there is little or no current in the speaker. The duty cycle of OUTP is greater than 50% and OUTN is less than 50% for positive output voltages. The duty cycle of OUTN is greater than 50% and that of OUTP is less than 50% for negative output voltages. The voltage across the load is at 0 V through most of the switching period, reducing power loss.

Figure 10. BD Mode Modulation

7.3.3 Gate Drive

The gate driver accepts the low-voltage PWM signal and level shifts it to drive a high-current, full-bridge, power FET stage. The device uses proprietary techniques to optimize EMI and audio performance.

7.3.4 Power FETs

The BTL output comprises four matched N-channel FETs for high efficiency and maximum power transfer to the load. By design, the FETs withstand large voltage transients during a load-dump event.

7.3.5 Load Diagnostics

The device incorporates load diagnostic circuitry designed for detecting and determining the status of output connections. The device supports the following diagnostics:

• Short to GND
• Short to PVDD
• Short across load
• Open load

The device reports the presence of any of the short or open conditions to the system via I²C register read.

1. Load Diagnostics—The load diagnostic function runs on de-assertion of STANDBY or when the device is in a fault state (dc detect, overcurrent, overvoltage, undervoltage, and overtemperature). During this test, the outputs are in a Hi-Z state. The device determines whether the output is a short to GND, short to PVDD, open load, or shorted load. The load diagnostic biases the output, which therefore requires limiting the capacitance value for proper functioning; see the Recommended Operating Conditions. The load diagnostic test takes approximately 229 ms to run. Note that the check phase repeats up to 5 times if a fault is present or a large capacitor to GND is present on the output. On detection of an open load, the output still operates. On detection of any other fault condition, the output goes into a Hi-Z state, and the device checks the load continuously until removal of the fault condition. After detection of a normal output condition, the audio output starts. The load diagnostics run after every other overvoltage (OV) event. The load diagnostic for open load only has I²C reporting. All other faults have I²C and FAULT pin assertion.
   
   The device performs load diagnostic tests as shown in Figure 11.
   Figure 12 illustrates how the diagnostics determine the load based on output conditions.
Figure 11. Load Diagnostics Sequence of Events
Figure 12. Load Diagnostic Reporting Thresholds
2. Faults During Load Diagnostics—If the device detects a fault (overtemperature, overvoltage, undervoltage) during the load diagnostics test, the device exits the load diagnostics, which may result in a pop or click on the output.

7.3.6 Protection and Monitoring

• Overcurrent Shutdown (OCSD)—The overcurrent shutdown forces the output into Hi-Z. The device asserts the FAULT pin and updates the I²C register.
• DC Detect—This circuit checks for a dc offset continuously during normal operation at the output of the amplifier. If a dc offset occurs, the device asserts the FAULT pin and updates the I²C register. Note that the dc detection threshold follows PVDD changes.
• Overtemperature Shutdown (OTSD)—The device shuts down when the die junction temperature reaches the overtemperature threshold. The device asserts the FAULT pin asserts and updates I²C register. Recovery is automatic when the temperature returns to a safe level.
• Undervoltage (UV)—The undervoltage (UV) protection detects low voltages on PVDD. In the event of an undervoltage condition, the device asserts the FAULT pin and resets the I²C register.
• Power-On Reset (POR)—Power-on reset (POR) occurs when PVDD drops below the POR threshold. A POR event causes the I²C bus to go into a high-impedance state. After recovery from the POR event, the device restarts automatically with default I²C register settings. The I²C is active as long as the device is not in POR.
• Overvoltage (OV) and Load Dump—OV protection detects high voltages on PVDD. If PVDD reaches the overvoltage threshold, the device asserts the FAULT pin and updates the I²C register. The device can withstand 40-V load-dump voltage spikes.
• SpeakerGuard protection circuitry limits the output voltage to the value selected in I²C register 0x03. This value determines both the positive and negative limits. One can use this feature to improve battery life or protect the speaker from exceeding its excursion limits.
• Adjacent-Pin Shorts—The device design is such that shorts between adjacent pins do not cause damage.

7.3.7 I²C Serial Communication Bus

The device communicates with the system processor via the I²C serial communication bus as an I²C slave-only device. The processor can poll the device via I²C to determine the operating status. All reports of fault conditions and detections are via I²C. The system can also set numerous features and operating conditions via I²C. The I²C interface is active approximately 1 ms after the STANDBY pin is high.

The I²C interface controls the following device features:

• Changing gain setting to 20 dB, 26 dB, 32 dB, or 36 dB.
• Controlling peak voltage value of SpeakerGuard protection circuitry
• Reporting load diagnostic results
• Changing of switching frequency for AM radio avoidance

7.3.7.1 I²C Bus Protocol

The device has a bidirectional serial control interface that is compatible with the Inter IC (I²C) bus protocol and supports 400-kbps data transfer rates for random and sequential write and read operations. This is a slave-only device that does not support a multimaster bus environment or wait-state insertion. The master device uses the I²C control interface to program the registers of the device and to read device status.

The I²C bus employs two signals, SDA (data) and SCL (clock), to communicate between integrated circuits in a system. Data transfer on the bus is serial, one bit at a time. The transfer of address and data is in byte (8-bit) format with the most-significant bit (MSB) transferred first. In addition, the receiving device acknowledges each byte transferred on the bus with an acknowledge bit. Each transfer operation begins with the master device driving a start condition on the bus and ends with the master device driving a stop condition on the bus. The bus uses transitions on the data pin (SDA) while the clock is HIGH to indicate start and stop conditions. A HIGH-to-LOW transition on SDA indicates a start, and a LOW-to-HIGH transition indicates a stop. Normal data bit transitions must occur within the low time of the clock period. Figure 13 shows these conditions. The master generates the 7-bit slave address and the read/write (R/W) bit to open communication with another device and then waits for an acknowledge condition. The device holds SDA LOW during the acknowledge clock period to indicate an acknowledgment. When this occurs, the master transmits the next byte of the sequence. The address for each device is a unique 7-bit slave address plus R/W bit (1 byte). All compatible devices share the same signals via a bidirectional bus using a wired-AND connection. The SDA and SCL signals require the use of an external pullup resistor to set the HIGH level for the bus. There is no limit on the number of bytes that the communicating devices can transmit between start and stop conditions. After transfer of the last word, the master generates a stop condition to release the bus.

Figure 13. Typical I²C Sequence

To communicate with the device, the I²C master uses addresses shown in Figure 13. Transmission of read and write data can be by single-byte or multiple-byte data transfers.

7.3.7.2 Random Write

As shown in Figure 14, a single-byte data-write transfer begins with the master device transmitting a start condition followed by the I²C device address and the read/write bit. The read/write bit determines the direction of the data transfer. For a write data transfer, the read/write bit is a 0. After receiving the correct I²C device address and the read/write bit, the device responds with an acknowledge bit. Next, the master transmits the address byte corresponding to the internal memory address being accessed. After receiving the address byte, the device again responds with an acknowledge bit. Next, the master device transmits the data byte for writing to the memory address being accessed. After receiving the data byte, the device again responds with an acknowledge bit. Finally, the master device transmits a stop condition to complete the single-byte data-write transfer.

Figure 14. Random Write Transfer

7.3.7.3 Random Read

As shown in Figure 15, a single-byte data-read transfer begins with the master device transmitting a start condition followed by the I²C device address and the read/write bit. For the data-read transfer, the master device performs both a write and a following read. Initially, the master device performs a write to transfer the address byte of the internal memory address to be read. As a result, the read/write bit is a 0. After receiving the address and the read/write bit, the device responds with an acknowledge bit. In addition, after sending the internal memory address byte, the master device transmits another start condition followed by the device address and the read/write bit again. This time, the read/write bit is a 1, indicating a read transfer. After receiving the address and the read/write bit, the device again responds with an acknowledge bit. Next, the device transmits the data byte from the memory address being read. After receiving the data byte, the master device transmits a not-acknowledge followed by a stop condition to complete the single-byte data-read transfer.

Figure 15. Random Read Transfer

7.3.7.4 Sequential Read

A sequential data-read transfer is identical to a single-byte data-read transfer except that the TAS5421-Q1 transmits multiple data bytes to the master device as shown in Figure 16. Except for the last data byte, the master device responds with an acknowledge bit after receiving each data byte and automatically increments the I²C subaddress by one. After receiving the last data byte, the master device transmits a not-acknowledge followed by a stop condition to complete the transfer.

Figure 16. Sequential Read Transfer

7.4.1 Hardware Control Pins

There are three discrete hardware pins for real-time control and indication of device status.

FAULT pin: This active-low open-drain output pin indicates the presence of a fault condition which requires the device to go into the Hi-Z mode. On assertion of this pin, the device has protected itself and the system from potential damage. The system can read the exact nature of the fault via I²C with the exception of PVDD undervoltage faults below POR, in which case the I²C bus is no longer operational.

STANDBY pin: Assertion of this active-low pin sends the device goes into a complete shutdown, limiting the current draw.

MUTE pin: On assertion of this active-high pin, the device is in mute mode. The output pins stop switching and audio does not pass from the input to the output. To place the device back into play mode, it is necessary to deassert this pin.

7.4.2 EMI Considerations

Automotive-level EMI performance depends on both careful integrated-circuit design and good system-level design. Controlling sources of electromagnetic interference (EMI) was a major consideration in all aspects of the design.

The design has minimal parasitic inductances due to the short leads on the package. This dramatically reduces the EMI that results from current passing from the die to the system PCB. The design incorporates circuitry that optimizes output transitions that cause EMI.

7.4.3 Operating Modes and Faults

The following tables list operating modes and faults.

Table 4. I²C Address

Table 5. I²C Address Register Definitions

Table 6. Fault Register (0x01)

Table 7. Status and Load Diagnostic Register (0x02)

Table 8. Control Register (0x03)