TDA7297

The TDA7297 from STMicroelectronics is a dual bridge (stereo) Class-AB audio power amplifier designed primarily for TV and portable radio applications. Housed in a 15-pin Multiwatt through-hole package, it delivers up to 15W per channel into 8-ohm loads when powered from a 16.5V supply, or 15W per channel into 4-ohm loads at 14.4V supply.

The device is characterized by its minimal external component requirement. Unlike many audio power amplifiers, the TDA7297 requires no SVR (supply voltage rejection) capacitor, no bootstrap capacitor, and no Boucherot cell (Zobel network) for stability. The gain is internally fixed at 32dB (approximately 40x voltage gain), eliminating external gain-setting resistors. This makes the TDA7297 one of the simplest audio amplifier ICs to implement, ideal for cost-sensitive consumer electronics.

The BTL (Bridge-Tied Load) output configuration doubles the voltage swing across the speaker compared to a single-ended output, enabling higher output power from a single supply voltage without requiring a coupling capacitor or split supply. Each channel uses two output stages in a push-pull H-bridge configuration, with the speaker connected between the two outputs (OUT+ and OUT-).

The mute and standby functions provide microprocessor-compatible control of the amplifier state. The standby function reduces power consumption by shutting down the output stages while keeping the bias circuits alive for fast recovery. The mute function silences the output without entering standby. Both functions can be controlled independently or tied together for simple on/off operation. A recommended RC network on the mute/standby pins provides pop-free turn-on by delaying the release of mute until the internal circuits have stabilized.

The TDA7297 includes comprehensive protection: short-circuit protection (between outputs and to ground) with internally limited peak output current of 2A, and thermal shutdown that activates when the junction temperature exceeds approximately 150C. The thermal resistance from junction to case (Rth j-case) is typically 1.4C/W (maximum 2C/W), requiring an appropriate heatsink for most applications.

For 8-ohm loads, the recommended supply voltage is 12V to 16.5V, with 16.5V providing the maximum output power. For 4-ohm loads, the supply voltage should be limited to 14V maximum to prevent excessive power dissipation and distortion. The quiescent current is 50mA typical, making the device relatively efficient for a Class-AB amplifier.

The device separates power ground (PW-GND, Pin 8) from signal ground (S-GND, Pin 9), allowing proper star-grounding practices to minimize ground-loop noise. The signal inputs (IN1 and IN2) are referenced to S-GND, while the output stages and supply pins are referenced to PW-GND.

The TDA7297 operates as a dual-channel Class-AB bridge-tied load (BTL) audio power amplifier with internally fixed gain.

Class-AB Output Stage: Each channel uses a Class-AB push-pull output stage, where both NPN and PNP (or N-channel and P-channel) output transistors are biased to conduct slightly at idle, eliminating the crossover distortion that characterizes pure Class-B designs. The small quiescent bias current (contributing to the 50mA total quiescent current) ensures smooth transition between the push and pull halves of the output waveform. This provides the low distortion (THD typically 0.1% or less at moderate power levels) expected from Class-AB operation.

Bridge-Tied Load (BTL) Configuration: Each channel uses two output amplifiers in an H-bridge configuration. The input signal is fed to the non-inverting input of one amplifier (driving OUT+) and simultaneously to the inverting input of the second amplifier (driving OUT-). The speaker is connected between OUT+ and OUT-, with no ground reference. This BTL configuration doubles the effective voltage across the speaker compared to a single-ended output: Vspeaker = 2 x Vsingle-ended. Since power is proportional to V-squared, the BTL configuration provides four times the output power of a single-ended amplifier from the same supply voltage (theoretically; in practice, current limiting and supply sag reduce this to approximately 3x).

The BTL configuration also eliminates the need for a large output coupling capacitor, as the DC offset across the speaker is nominally zero (both outputs sit at VCC/2). This is a significant advantage in TV and portable audio applications where size and cost are critical.

Internally Fixed Gain: The voltage gain is set internally at 32dB (approximately 40x) through internal feedback resistors. This simplifies the external circuit but means the gain cannot be adjusted. The 32dB gain implies that a 500mV RMS input signal produces a 20V RMS output, which would exceed the supply voltage. In practice, the maximum output is limited by the supply voltage and load impedance. For a 16.5V supply into 8 ohms, the maximum output power is approximately 15W, corresponding to about 11V RMS across the speaker, which requires approximately 275mV RMS input at 32dB gain.

Mute Function: When the MUTE pin (Pin 6) is pulled LOW (below approximately 2V), the output stages are muted – the output transistors are turned off, and the outputs present a high impedance. When MUTE is HIGH (above approximately 3.5V), the amplifier operates normally. The mute function provides a clean way to silence the output without disconnecting the power supply.

Standby Function: When the STANDBY pin (Pin 7) is pulled LOW, the entire amplifier enters a low-power standby state, reducing quiescent current to a few mA. When STANDBY is HIGH, the amplifier is fully operational. The standby state disables the bias circuits and output stages, but the internal reference and protection circuits remain active for fast recovery.

Pop-Free Turn-On: To achieve pop-free operation, the mute and standby pins are typically connected together and driven through an RC network. During power-up, the capacitor charges slowly, keeping the amplifier in standby+mute until the supply voltage has stabilized. Once the capacitor voltage exceeds the standby threshold, the bias circuits activate. Once it exceeds the mute threshold, the outputs are un-muted. This sequencing (standby releases before mute) ensures the amplifier is fully biased before the output is enabled, preventing turn-on transients (pops) from reaching the speaker.

Protection Circuits: The short-circuit protection limits the peak output current to approximately 2A. If the output current exceeds this limit (e.g., due to a shorted speaker wire), the output stage reduces the drive to maintain the current at or below the limit. The thermal protection monitors the junction temperature and shuts down the output stages if the temperature exceeds approximately 150C, preventing permanent damage. The amplifier automatically recovers when the temperature drops below the threshold.