125W Class D Subwoofer Power Amplifier

The 125W Class D subwoofer power amplifier circuit designed with the newly launched LM4651 and LM4652 of National Semiconductor is shown in the figure. The output power of the amplifier is 125W under the total harmonic distortion THD=1%, the load impedance RL=4Ω, the maximum level of the input signal Vin(rms) is 3V, the input signal bandwidth is 10～150Hz, the ambient temperature is 50℃, and the power supply voltage is ±20V.

The LM4651 in a 28-pin DIP package is a PWM control/driver IC with a built-in oscillator, PWM comparator, error amplifier, feedback amplifier, level shifter, high-side driver, low-side driver, and undervoltage, overheating, short circuit, and overmodulation protection circuits. The LM4652 is a half-bridge power MOSFET IC in a 15-pin (6, 8, 9, {11}, {12} pins are not connected) TO-220 package. The breakdown voltage of the four MOSFETs is V(BR)DSS=50V, the drain current ID=10A, the on-state resistance RDS(ON)=200mΩ (typical value), and the turn-on voltage VGS(th)=0.85V (typical value).

The oscillator frequency fosc in LM4651 is 1×109/(4000＋Rosc), where Rosc=R6=3.9kΩ, so fosc=125kHz. The input audio signal is input to the error amplifier with a gain of 75V/V (i.e. 175dB) through C1, R1 and pin 10. The half-bridge switch outputs on pins 27 and {15} of IC are filtered by RC filters and fed back to pins {14} and {19} of IC1. After passing through the internal feedback measurement amplifier, they are output from pins 9 to pin 10, providing a single-ended feedback signal for the error amplifier. The audio input signal is compared with the triangle wave generated by the oscillator, and a square wave pulse with a duty factor proportional to the input audio signal level is generated at the output of the PWM comparator to drive the power MOSFET in IC2. The switch outputs of pins 27 and {15} of IC are low-pass filtered by L1, C16 and L2, C17 to drive the speaker.

The resistor R4 on the {18} pin of IC1 is used for short-circuit current limitation. When R4=100kΩ, the short-circuit current level is not less than 10A; the resistor R5 on the {17} pin is used to set the dead time; the capacitor C9 on the {24} pin is used to set the start delay time (1 second). When the {13} pin (STOBY) is set to logic "1" or 5V, the standby function is performed, the pulse width modulation is stopped, and the power switch in the LM4652 is turned off, consuming only very low current. As long as the junction temperature of IC2 reaches 150℃, the output of its 4th pin is fed back to the {12} pin of IC1, and the thermal shutdown function can be performed.