TAS5760LD General-Purpose I2S Input Class-D Amplifier With DirectPath™ Headphone and Line Driver

8.3.2.1 Serial Audio Port (SAP)

The serial audio port (SAP) receives audio in either I²S, Left Justified, or Right Justified formats. In Hardware Control mode, the device operates only in 32, 48 or 64 x f_S I²S mode. In Software Control mode, additional options for left-justified and right justified audio formats are available. The supported clock rates and ratios for Hardware Control Mode and Software Control Mode are detailed in their respective sections below.

8.3.2.2 DC Blocking Filter

Excessive DC content in the audio signal can damage loudspeakers and even small amounts of DC offset in the signal path cause cause audible artifacts when muting and unmuting the speaker amplifier. For these reasons, the amplifier employs two separate DC blocking methods for the speaker amplifier. The first is a high-pass filter provided at the front of the data path to remove any DC from incoming audio data before it is presented to the audio path. The –3 dB corner frequencies for the filter are specified in the speaker amplifier electrical characteristics table. In Hardware Control mode, the DC blocking filter is active and cannot be disabled. In Software Control mode, the filter can be bypassed by writing a 1 to bit 7 of register 0x02. The second method is a DC detection circuit that will shutdown the power stage and issue a latching fault if DC is found to be present on the output due to some internal error of the device. This DC Error (DCE) protection is discussed in the Protection Circuitry section below.

8.3.2.3 Digital Boost and Volume Control

Following the high-pass filter, a digital boost block is included to provide additional digital gain if required for a given application as well as to set an appropriate clipping point for a given GAIN[1:0] pin configuration when in Hardware Control mode. The digital boost block defaults to +6dB when the device is in Hardware Mode. In most use cases, the digital boost block will remain unchanged when operating the device in Software Control mode, as the volume control offers sufficient digital gain for most applications. The TAS5760LD's digital volume control operates from Mute to 24 dB, in steps of 0.5 dB. The equation below illustrates how to set the 8-bit volume control register at address 0x04:

Transitions between volume settings will occur at a rate of 0.5 dB every 8 LRCK cycles to ensure no audible artifacts occur during volume changes. This volume fade feature can be disabled via Bit 7 of the Volume Control Configuration Register.

8.3.2.4 Digital Clipper

A digital clipper is integrated in the oversampled domain to provide a component-free method to set the clip point of the speaker amplifier. Through the "Digital Clipper Level x" controls in the I²C control port, the point at which the oversampled digital path clips can be set directly, which in turns sets the 10% THD+N operating point of the amplifier. This is useful for applications in which a single system is designed for use in several end applications that have different power rating specifications. Its place in the oversampled domain ensures that the digital clipper is acoustically appealing and reduces or eliminates tones which would otherwise foldback into the audio band during clipping events. Figure 39 shows a block diagram of the digital clipper.

Figure 39. Digital Clipper Simplified Block Diagram

As mentioned previously, the audio signature of the amplifier when the digital clipper is active is very smooth, owing to its place in the signal chain. Figure 40 shows the typical behavior of the clipping events.

Figure 40. Digital Clipper Example Waveform for Various Settings of Digital Clip Level [19:0]

It is important to note that the actual signal developed across the speaker will be determined not only by the digital clipper, but also the analog gain of the amplifier. Depending on the analog gain settings and the PVDD level applied, clipping could occur as a result of the voltage swing that is determined by the gain being larger than the available PVDD supply rail. The gain structures are discussed in detail below for both Hardware Control Mode and Software Control Mode.

8.3.2.5 Closed-Loop Class-D Amplifier

Following the digital clipper, the interpolated audio data is next sent to the Closed-Loop Class-D amplifier, whose first stage is Digital to PWM Conversion (DPC) block. In this block, the stereo audio data is translated into two pairs of complimentary pulse width modulated (PWM) signals which are used to drive the outputs of the speaker amplifer. Feedback loops around the DPC ensure constant gain across supply voltages, reduce distortion, and increase immunity to power supply injected noise and distortion. The analog gain is also applied in the Class-D amplifier section of the device. The gain structures are discussed in detail below for both Hardware Control Mode and Software Control Mode.

The switching rate of the amplifier is configurable in both Hardware Control Mode and Software Control Mode. In both cases, the PWM switching frequency is a multiple of the sample rate. This behavior is described in the respective Hardware Control Mode and Software Control Mode sections below.

8.3.3.1 Speaker Amplifier Fault Notification (SPK_FAULT Pin)

In both hardware and Software Control mode, the SPK_FAULT pin of the TAS5760LD serves as a fault indicator to notify the system that a fault has occurred with the speaker amplifier by being actively pulled LOW. This pin is an open-drain output pin and, unless one is provided internal to the receiver, requires an external pullup to set the net to a known value. The behavior of this pin varies based upon the type of error which has occurred.

In the case of a latching error, the fault line will remain LOW until such time that the TAS5760LD has resumed normal operation (that is the SPK_SD pin has been toggled and T_{SPK_FAULT} has passed).

With the exception of clock errors, non-latching errors will not cause the SPK_FAULT pin to be pulled LOW. Once a non-latching error has been cleared, normal operation will resume. For clocking errors, the SPK_FAULT line will be pulled LOW, but upon clearing of the clock error normal operation will resume automatically, that is, with no T_{SPK_FAULT} delay.

One method which can be used to convert a latching error into an auto-recovered, non-latching error is to connect the SPK_FAULT pin to the SPK_SD pin. In this way, a fault condition will automatically toggle the SPK_SD pin when the SPK_FAULT pin goes LOW and returns HIGH after the T_{SPK_FAULT} period has passed.

Table 1. Protection Suite Error Handling Summary

8.3.3.2 DC Detect Protection

The TAS5760LD has circuitry which will protect the speakers from DC current which might occur due to an internal amplifier error. The device behavior in response to a DCE event is detailed in the table in the previous section.

A DCE event occurs when the output differential duty-cycle of either channel exceeds 60% for more than 420 msec at the same polarity. The table below shows some examples of the typical DCE Protection threshold for several values of the supply voltage. This feature protects the speaker from large DC currents or AC currents less than 2 Hz.

The minimum output offset voltages required to trigger the DC detect are listed in Table 2. The outputs must remain at or above the voltage listed in the table for more than 420 msec to trigger the DC detect.

8.4.1.1 Speaker Amplifier Shut Down (SPK_SD Pin)

In both Hardware and Software Control mode, the SPK_SD pin is provided to place the speaker amplifier into shutdown. Driving this pin LOW will place the device into shutdown, while pulling it HIGH (to DVDD) will bring the device out of shutdown. This is the lowest power consumption mode that the device can be placed in while the power supplies are up. If the device is placed into shutdown while in normal operation, an audible artifact may occur on the output. To avoid this, the device should first be placed into sleep mode, by pulling the SPK_SLEEP/ADR pin HIGH before pulling the SPK_SD low.

8.4.1.2 Serial Audio Port in Hardware Control Mode

When used in Hardware Control Mode, the Serial Audio Port (SAP) accepts only I²S formatted data. Additionally, the device operates in Single-Speed Mode (SSM), which means that supported sample rates, MCLK rates, and SCLK rates are limited to those shown in the table below. Additional clocking options, including higher sample rates, are available when operating the device in Software Control Mode.

Table 3 details the supported SCLK rates for each of the available sample rate and MCLK rate configurations. For each f_S and MCLK rate, the supported SCLK rates are shown and are represented in multiples of the sample rate, which is written as "x f_S".

8.4.1.3 Soft Clipper Control (SFT_CLIP Pin)

The TAS5760LD has a soft clipper that can be used to clip the output voltage level below the supply rail. When this circuit is active, the amplifier operates as if it was powered by a lower supply voltage, and thereby enters into clipping sooner than if the circuit was not active. The result is clipping behavior very similar to that of clipping at the PVDD rail, in contrast to the digital clipper behavior which occurs in the oversampled domain of the digital path. The point at which clipping begins is controlled by a resistor divider from GVDD_REG to ground, which sets the voltage at the SFT_CLIP pin. The precision of the threshold at which clipping occurs is dependent upon the voltage level at the SFT_CLIP pin. Because of this, increasing the precision of the resistors used to create the voltage divider, or using an external reference will increase the precision of the point at which the device enters into clipping. To ensure stability, and soften the edges of the clipping event, a capacitor should be connected from pin SFT_CLIP to ground.

Figure 41. Soft Clipper Example Wave Form

To move the output stage into clipping, the soft clipper circuit limits the duty cycle of the output PWM pulses to a fixed maximum value. After filtering this limit applied to the duty cycle resembles a clipping event at a voltage below that of the PVDD level. The peak voltage level attainable when the soft clipper circuit is active, called V_P in the example below, is approximately 4 times the voltage at the SFT_CLIP pin, noted as V_{SFT_CLIP}. This voltage can be used to calculate the maximum output power for a given maximum input voltage and speaker impedance, as shown in the equation below.

Equation 2.

Where:

R_S is the total series resistance including R_DS(on), and output filter resistance.
R_L is the load resistance.
V_P is the peak amplitude achievable when the soft clipper circuit is active (As mentioned previously, V_P = [4 x V_{SFT_CLIP}], provided that [4 x V_{SFT_CLIP}] < PVDD.)
P_OUT (10%THD) ≈ 1.25 × P_OUT (unclipped)

If the PVDD level is below (4 x V_{SFT_CLIP}) clipping will occur due to clipping at PVDD before the clipping due to the soft clipper circuit becomes active.

8.4.1.4 Speaker Amplifier Switching Frequency Select (FREQ/SDA Pin)

In Hardware Control mode, the PWM switching frequency of the TAS5760LD is configurable via the FREQ/SDA pin. When connected to the system ground, the pin sets the output switching frequency to 16 × f_S. When connected to DVDD through a pull-up resistor, as shown in the Typical Application Circuits, the pin sets the output switching frequency to 8 × f_S. More switching frequencies are available when the TAS5760LD is used in Software Control Mode.

8.4.1.5 Parallel Bridge Tied Load Mode Select (PBTL/SCL Pin)

The TAS5760LD can be configured to drive a single speaker with the two output channels connected in parallel. This mode of operation is called Parallel Bridge Tied Load (PBTL) mode. This mode of operation effectively reduces the output impedance of the amplifier in half, which in turn reduces the power dissipated in the device due to conduction losses through the output FETs. Additionally, since the output channels are working in parallel, it also doubles the amount of current the speaker amplifier can source before hitting the over-current error threshold.

The device can be placed operated in PBTL mode in either Hardware Control Mode or in Software Control Mode, via the I²C Control Port. For instructions on placing the device in PBTL via the I²C Control Port, see Software Control Mode.

To place the TAS5760LD into PBTL Mode when operating in Hardware Control Mode, the PBTL/SCL pin should be pulled HIGH (that is, connected to the DVDD supply through a pull-up resistor). If the device is to operate in BTL mode instead, the PBTL/SCL pin should be pulled LOW, that is connected to the system supply ground. When operated in PBTL mode, the output pins should be connected as shown in the Typical Application Circuit Diagrams.

In PBTL mode, the amplifier selects its source signal from the right channel of the stereo signal presented on the SDIN line of the Serial Audio Port. To select the right channel of the stereo signal, the LRCK can be inverted in the processor that is sending the serial audio data to the TAS5760LD.

8.4.1.6 Speaker Amplifier Sleep Enable (SPK_SLEEP/ADR Pin)

In Hardware Control mode, pulling the SPK_SLEEP/ADR pin HIGH gracefully transitions the switching of the output devices to a non-switching state or "High-Z" state. This mode of operation is similar to mute in that no audio is present on the outputs of the device. However, unlike the 50/50 mute available in the I²C Control Port, sleep mode saves quiescent power dissipation by stopping the speaker amplifier output transitors from switching. This mode of operation saves quiescent current operation but keeps signal path blocks active so that normal operation can resume more quickly than if the device were placed into shutdown. It is recommended to place the device into sleep mode before stopping the audio signal coming in on the SDIN line or before bringing down the power supplies connected to the TAS5760LD in order to avoid audible artifacts.

8.4.1.7 Speaker Amplifier Gain Select (SPK_GAIN [1:0] Pins)

In Hardware Control Mode, a combination of digital gain and analog gain is used to provide the overall gain of the speaker amplifier. The decode of the two pins "SPK_GAIN1" and "SPK_GAIN0" sets the gain of the speaker amplifier. Additionally, pulling both of the SPK_SPK_GAIN[1:0] pins HIGH places the device into software control mode.

As seen in Figure 42, the audio path of the TAS5760LD consists of a digital audio input port, a digital audio path, a digital to PWM converter (DPC), a gate driver stage, a Class D power stage, and a feedback loop which feeds the output information back into the DPC block to correct for distortion sensed on the output pins. The total amplifier gain is comprised of digital gain, shown as G_DIG in the digital audio path and the analog gain from the input of the analog modulator G_ANA to the output of the speaker amplifier power stage.

Figure 42. Speaker Amplifier Gain Select (SPK_GAIN [1:0] Pins)

As shown in Figure 42, the first gain stage for the speaker amplifier is present in the digital audio path. It consists of the volume control and the digital boost block. The volume control is set to 0dB by default and, in Hardware Control mode, it does not change. For all settings of the SPK_GAIN[1:0] pins, the digital boost block remains at +6 dB as analog gain block is transitioned through 19.2, 22.6, and 25 dBV.

The gain configurations provided in Hardware Control mode were chosen to align with popular power supply levels found in many consumer electronics and to balance the trade-off between maximum power output before clipping and noise performance. These gain settings ensure that the output signal can be driven into clipping at those popular PVDD levels. If the power level required is lower than that which is possible with the PVDD level, a lower gain setting can be used. Additionally, if clipping at a level lower than the PVDD supply is desired, the digital clipper or soft clipper can be used.

The values of G_DIG and G_ANA for each of the SPK_GAIN[1:0] settings are shown in the table below. Additionally, the recommended PVDD level for each gain setting, along with the typical unclipped peak to peak output voltage swing for a 0dBFS input signal is provided. The peak voltage levels in the table below should only be used to understand the peak target output voltage swing of the amplifier if it had not been limited by clipping at the PVDD rail.

8.4.1.8 Considerations for Setting the Speaker Amplifier Gain Structure

Configuration of the gain of the amplifier is important to the overall noise and output power performance of the TAS5760LD. Higher gain settings mean that more power can be driven from an amplifier before it becomes voltage limited. Moreover, when output clipping "at the rail" is desired, it becomes important that there be enough voltage gain in the signal path to drive the output signal above the PVDD level in order to "clip" the output signal at the PVDD level in the output stage. Another desirable aspect of higher gain settings is that the dynamic headroom of an amplifier is increased with higher gain settings, which increases the overall dynamic audio quality of the signal being amplified.

With these advantages in mind, it may seem that setting the gain at the highest setting available would be appropriate. However, there are some drawbacks to having a gain that is set arbitrarily high. The first drawback is that a higher gain setting results in increased amplification of any noise that is present in the signal path. If the gain is set too high, and the speaker is sensitive enough, this may result in an audible "hiss" at the speakers when no audio is playing. Another consideration is that the speakers used in the system may not be rated for operation at the power levels which would be possible for the given PVDD supply that is present in the system. For this reason, it may be necessary to limit the voltage swing of the amplifier via a lower gain setting to reduce the voltage presented, and therefore, the power delivered, to the speaker.

8.4.2.1 Speaker Amplifier Shut Down (SPK_SD Pin)

In both hardware and Software Control mode, the SPK_SD pin is provided to place the speaker amplifier into shutdown. Driving this pin LOW will place the device into shutdown, while driving it HIGH (DVDD) will bring the device out of shutdown. This is the lowest power consumption mode that the device can be placed in while the power supplies are up. If the device is placed into shutdown while in normal operation, an audible artifact may occur on the output. To avoid this, the device should first be placed into sleep mode, by pulling the SPK_SLEEP/ADR pin HIGH before pulling the SPK_SD low.

8.4.2.2 Serial Audio Port Controls

In Software Control mode, additional digital audio data formats and clock rates are made available via the I²C control port. With these controls, the audio format can be set to left justified, right justified, or I²S formatted data.

8.4.2.3 Parallel Bridge Tied Load Mode via Software Control

The TAS5760LD can be configured to drive a single speaker with the two output channels connected in parallel. This mode of operation is called Parallel Bridge Tied Load (PBTL) mode. This mode of operation effectively reduces the on resistance of the amplifier in half, which in turn reduces the power dissipated in the device due to conduction losses through the output FETs. Additionally, since the output channels are working in parallel, it also doubles the amount of current the speaker amplifier can source before hitting the over-current error threshold.

It should be noted that the device can be placed operated in PBTL mode in either Hardware Control Mode or in Software Control Mode, via the I²C Control Port. For instructions on placing the device in PBTL via the PBTL/SCL Pin, see Hardware Control Mode.

To place the TAS5760LD into PBTL Mode when operating in Software Control Mode, the Bit 7 of the Analog Control Register (0x06) should be set in the control port. This bit is cleared by default to configure the device for BTL mode operation. An additional control available in software mode control is PBTL Channel Select, which selects which of the two channels presented on the SDIN line will be used for the input signal for the amplifier. This is found at Bit 1 of the Analog Control Register (0x06). When operated in PBTL mode, the output pins should be connected as shown in the Typical Application Circuit Diagrams.

8.4.2.4 Speaker Amplifier Gain Structure

As shown in Figure 43, the audio path of the TAS5760LD consists of a digital audio input port, a digital audio path, a digital to analog converter, an analog modulator, a gate driver stage, a Class D power stage, and a feedback loop which feeds the output information back into the analog modulator to correct for distortion sensed on the output pins. The total amplifier gain is comprised of digital gain, shown as G_DIG in the digital audio path and the analog gain from the input of the analog modulator G_ANA to the output of the speaker amplifier power stage.

Figure 43. Speaker Amplifier Gain Structure

8.4.2.5 I²C Software Control Port

The TAS5760LD includes an I²C control port for increased flexibility and extended feature set.

8.4.2.5.2 General Operation of the I²C Control Port

The TAS5760LD device has a bidirectional I²C interface that is compatible with the Inter IC (I²C) bus protocol and supports both 100-kHz and 400-kHz data transfer rates. This is a slave-only device that does not support a multimaster bus environment or wait-state insertion. The control interface is used 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 is transferred on the bus serially, one bit at a time. The address and data can be transferred in byte (8-bit) format, with the most significant bit (MSB) transferred first. In addition, each byte transferred on the bus is acknowledged by the receiving device 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. These conditions are shown in Figure 44. 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 TAS5760LD holds SDA LOW during the acknowledge clock period to indicate an acknowledgment. When this occurs, the master transmits the next byte of the sequence. All compatible devices share the same signals via a bidirectional bus using a wired-AND connection. An external pullup resistor must be used for the SDA and SCL signals to set the HIGH level for the bus.

Figure 44. Typical I²C Sequence

There is no limit on the number of bytes that can be transmitted between START and STOP conditions. When the last word transfers, the master generates a STOP condition to release the bus. A generic data transfer sequence is shown in Figure 44.

8.4.2.5.3 Writing to the I²C Control Port

As shown in Figure 45, a single-byte data-write transfer begins with the master device transmitting a START condition followed by the I²C and the read/write bit. The read/write bit determines the direction of the data transfer. For a data-write transfer, the read/write bit is a 0. After receiving the correct I²C and the read/write bit, the TAS5760LD responds with an acknowledge bit. Next, the master transmits the address byte corresponding to the TAS5760LD register being accessed. After receiving the address byte, the TAS5760LD again responds with an acknowledge bit. Next, the master device transmits the data byte to be written to the memory address being accessed. After receiving the data byte, the TAS5760LD again responds with an acknowledge bit. Finally, the master device transmits a STOP condition to complete the single-byte data-write transfer.

Figure 45. Write Transfer

8.4.2.5.4 Reading from the I²C Control Port

As shown in Figure 46, a 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, both a write followed by a read are actually done. Initially, a write is done to transfer the address byte of the internal register to be read. As a result, the read/write bit becomes a 0. After receiving the TAS5760LD address and the read/write bit, TAS5760LD responds with an acknowledge bit. In addition, after sending the internal memory address byte or bytes, the master device transmits another START condition followed by the TAS5760LD address and the read/write bit again. This time, the read/write bit becomes a 1, indicating a read transfer. After receiving the address and the read/write bit, the TAS5760LD again responds with an acknowledge bit. Next, the TAS5760LD transmits the data byte from the register being read. After receiving the data byte, the master device transmits a not-acknowledge followed by a STOP condition to complete the data-read transfer.

Figure 46. Read Transfer

Table 8. Control Port Quick Reference Table

8.5.2.1 Device Identification Register (0x00)

8.5.2.2 Power Control Register (0x01)

8.5.2.3 Digital Control Register (0x02)

8.5.2.4 Volume Control Configuration Register (0x03)

8.5.2.5 Left Channel Volume Control Register (0x04)

8.5.2.6 Right Channel Volume Control Register (0x05)

8.5.2.7 Analog Control Register (0x06)

8.5.2.8 Reserved Register (0x07)

The controls in this section of the control port are reserved and must not be used.

8.5.2.9 Fault Configuration and Error Status Register (0x08)

8.5.2.10 Reserved Controls (9 / 0x09) - (15 / 0x0F)

The controls in this section of the control port are reserved and must not be used.

8.5.2.11 Digital Clipper Control 2 Register (0x10)

8.5.2.12 Digital Clipper Control 1 Register (0x11)

9.2.1.1 Design Requirements

For this design example, use the parameters listed in Table 19 as the input parameters.

9.2.1.2 Detailed Design Procedure

9.2.1.2.5 Headphone and Line Driver Amplifier

Single-supply line-driver amplifiers typically require dc-blocking capacitors. The top drawing in Figure 58 illustrates the conventional line-driver amplifier connection to the load and output signal. DC blocking capacitors are often large in value. The line load (typical resistive values of 600 Ω to 10 kΩ) combines with the dc blocking capacitors to form a high-pass filter. Equation 3 shows the relationship between the load impedance (R_L), the capacitor (C_O), and the cutoff frequency (f_C).

Equation 3.

C_O can be determined using Equation 4, where the load impedance and the cutoff frequency are known.

Equation 4.

If f_C is low, the capacitor must then have a large value because the load resistance is small. Large capacitance values require large package sizes. Large package sizes consume PCB area, stand high above the PCB, increase cost of assembly, and can reduce the fidelity of the audio output signal.

Figure 58. Conventional and DirectPath Line Drivers

The DirectPath amplifier architecture operates from a single supply but makes use of an internal charge pump to provide a negative voltage rail. Combining the user-provided positive rail and the negative rail generated by the IC, the device operates in what is effectively a split-supply mode. The output voltages are now centered at zero volts with the capability to swing to the positive rail or negative rail. Combining this with the built-in click and pop reduction circuit, the DirectPath amplifier requires no output dc blocking capacitors. The bottom block diagram and waveform of Figure 58 illustrate the ground-referenced line-driver architecture. This is the architecture of the headphone / line driver inside of the TAS5760LD.

9.2.1.2.5.1 Charge-Pump Flying Capacitor and DR_VSS Capacitor

The charge-pump flying capacitor serves to transfer charge during the generation of the negative supply voltage. The PVSS capacitor must be at least equal to the charge-pump capacitor in order to allow maximum charge transfer. Low-ESR capacitors are an ideal selection, and a value of 1 µF is typical. Capacitor values that are smaller than 1 µF can be used, but the maximum output voltage may be reduced and the device may not operate to specifications. If the TAS5760LD is used in highly noise-sensitive circuits, it is recommended to add a small LC filter on the DRVDD connection.

9.2.1.2.5.2 Decoupling Capacitors

The TAS5760LD contains a DirectPath line-driver amplifier that requires adequate power supply decoupling to ensure that the noise and total harmonic distortion (THD) are low. A good, low equivalent-series-resistance (ESR) ceramic capacitor, typically 1 µF, placed as close as possible to the device DRVDD lead works best. Placing this decoupling capacitor close to the TAS5760LD is important for the performance of the amplifier. For filtering lower-frequency noise signals, a 10-µF or greater capacitor placed near the audio power amplifier would also help, but it is not required in most applications because of the high PSRR of this device.

9.2.1.2.5.3 Gain-Setting Resistor Ranges

The gain-setting resistors, R_IN and R_fb, must be chosen so that noise, stability, and input capacitor size of the headphone amplifier / line driver inside the TAS5760LD are kept within acceptable limits. Voltage gain is defined as R_fb divided by R_IN.

Selecting values that are too low demands a large input ac-coupling capacitor, C_IN. Selecting values that are too high increases the noise of the amplifier. Table 20 lists the recommended resistor values for different inverting-input gain settings.

Table 20. Recommended Resistor Values

GAIN	INPUT RESISTOR VALUE, RIN	FEEDBACK RESISTOR VALUE, Rfb
–1 V/V	10 kΩ	10 kΩ
–1.5 V/V	8.2 kΩ	12 kΩ
–2 V/V	15 kΩ	30 kΩ
–10 V/V	4.7 kΩ	47 kΩ

9.2.1.2.5.4 Using the Line Driver Amplifier in the TAS5760LD as a Second-Order Filter

Several audio DACs used today require an external low-pass filter to remove out-of-band noise. This is possible with the headphone amplifier / line driver inside the TAS5760LD, as it can be used like a standard operational amplifier. Several filter topologies can be implemented, both single-ended and differential. In Figure 59, multi-feedback (MFB) with differential input and single-ended input are shown.

An ac-coupling capacitor to remove dc content from the source is shown; it serves to block any dc content from the source and lowers the dc gain to 1, helping to reduce the output dc offset to a minimum.

The component values can be calculated with the help of the TI FilterPro™ program available on the TI Web site at: http://focus.ti.com/docs/toolsw/folders/print/filterpro.html.

Figure 59. Second-Order Active Low-Pass Filter

The resistor values should have a low value for obtaining low noise, but should also have a high enough value to get a small-size ac-coupling capacitor. With the proposed values of R1 = 15 kΩ, R2 = 30 kΩ, and R3 = 43 kΩ, a dynamic range (DYR) of 106 dB can be achieved with a 1-mF input ac-coupling capacitor.

9.2.1.2.5.5 External Undervoltage Detection

External undervoltage detection can be used to mute/shut down the heaphone / line driver amplifier in the TAS5760LD before an input device can generate a pop. The shutdown threshold at the UVP pin is 1.25 V. The user selects a resistor divider to obtain the shutdown threshold and hysteresis for the specific application. The thresholds can be determined as follows:

Equation 5. V_UVP = (1.25 – 6 µA × R3) × (R1 + R2) / R2

Equation 6. Hysteresis = 5 µA × R3 × (R1 + R2) / R2

For example, to obtain VUVP = 3.8 V and 1-V hysteresis, we can use R1 = 3 kΩ, R2 = 1 kΩ, and R3 = 50 kΩ.

Figure 60. External Undervoltage Detection

9.2.1.2.5.6 Input-Blocking Capacitors

DC input-blocking capacitors are required to be added in series with the audio signal into the input pins of the headphone amplifier / line driver inside the TAS5760LD. These capacitors block the dc portion of the audio source and allow the headphone / line driver amplifier inside the TAS5760LD.

These capacitors form a high-pass filter with the input resistor, R_IN. The cutoff frequency is calculated using Equation 7. For this calculation, the capacitance used is the input-blocking capacitor, and the resistance is the input resistor chosen from Table 20; then the frequency and/or capacitance can be determined when one of the two values is given.

It is recommended to use electrolytic capacitors or high-voltage-rated capacitors as input blocking capacitors to ensure minimal variation in capacitance with input voltages. Such variation in capacitance with input voltages is commonly seen in ceramic capacitors and can increase low-frequency audio distortion.

Equation 7.

9.2.1.3 Application Curves

9.2.2.1 Design Requirements

For this design example, use the parameters listed in Table 22 as the input parameters.

9.2.2.2 Detailed Design Procedure

9.2.2.3 Application Curves

9.2.3.1 Design Requirements

For this design example, use the parameters listed in Table 24 as the input parameters.

9.2.3.2 Detailed Design Procedure

9.2.3.3 Application Curves

9.2.4.1 Design Requirements

For this design example, use the parameters listed in Table 26 as the input parameters.

9.2.4.2 Detailed Design Procedure

9.2.4.3 Application Curve