Table 5. Design Requirements, BTL Application

10.2.1.2.1 Decoupling Capacitor Recommendations

In order to design an amplifier that has robust performance, passes regulatory requirements, and exhibits good audio performance, good quality decoupling capacitors should be used. In practice, X7R should be used in this application.

10.2.1.2.2 PVDD Capacitor Recommendation

The PVDD decoupling capacitors must be placed as close to the device pins a possible to insure short trace length and low a low inductance path. Likewise the ground path for these capacitors must provide a good reference and should be substantial. This will keep voltage ringing on PVDD to a minimum.

The voltage of the decoupling capacitors should be selected in accordance with good design practices. Temperature, ripple current, and voltage overshoot must be considered. This fact is particularly true in the selection of the 1μF that is placed on the power supply to each full-bridge. It must withstand the voltage overshoot of the PWM switching, the heat generated by the amplifier during high power output, and the ripple current created by high power output. A minimum voltage rating of 100 V is required for use with a 51-V power supply.

The large capacitors used in conjunction with each full-bridge, are referred to as the PVDD Capacitors. These capacitors should be selected for proper voltage margin and adequate capacitance to support the power requirements. In practice, with a well designed system power supply, 1000 μF, 80 V supports most applications. The PVDD capacitors should be low ESR type because they are used in a circuit associated with high-speed switching.

10.2.1.2.3 PCB Material Recommendation

FR-4 Glass Epoxy material with 2 oz. (70 μm) copper is recommended for use with the TPA3255. The use of this material can provide for higher power output, improved thermal performance, and better EMI margin (due to lower PCB trace inductance.

10.2.1.2.4 Oscillator

The built in oscillator frequency can be trimmed by an external resistor from the FREQ_ADJ pin to GND. Changes in the oscillator frequency should be made with resistor values specified in Recommended Operating Conditions while RESET is low.

To reduce interference problems while using a radio receiver tuned within the AM band, the switching frequency can be changed from nominal to lower or higher values. These values should be chosen such that the nominal and the alternate switching frequencies together result in the fewest cases of interference throughout the AM band. The oscillator frequency can be selected by the value of the FREQ_ADJ resistor connected to GND in master mode.

For slave mode operation, turn off the oscillator by pulling the FREQ_ADJ pin to DVDD. This configures the OSC_I/O pins as inputs to be slaved from an external differential clock. In a master/slave system inter-channel delay is automatically set up between the switching of the audio channels, which can be illustrated by no idle channels switching at the same time. This will not influence the audio output, but only the switch timing to minimize noise coupling between audio channels through the power supply. Inter-channel delay is needed to optimize audio performance and to get better operating conditions for the power supply. The inter-channel delay will be set up for a slave device depending on the polarity of the OSC_I/O connection as follows:

• Slave 1 mode has normal polarity (master + to slave + and master - to slave -)
• Slave 2 mode has reverse polarity (master + to slave - and master - to slave +)

The interchannel delay for interleaved channel idle switching is given in the table below for the master/slave and output configuration modes in degrees relative to the PWM frame.

Table 8. Design Requirements, SE Application

Table 9. Relevant Performance Plots, SE Configuration

Table 10. Design Requirements, PBTL Application

Table 11. Relevant Performance Plots, PBTL Configuration