Audio Power Amplifier Detailed Design By: Brian Felsmann

Audio Power Amplifier Detailed Design Design Issues: Thermal Protection Circuits Maximum Power Dissipation Determining the Correct Heat Sink Voltage Gain Frequency Response Total Harmonic Distortion + Noise (THD + N) Common Mode Rejection Ratio (CMRR) Signal-to-Noise Ratio (SNR) Over Voltage / Under Voltage Protection

Audio Power Amplifier Detailed Design Thermal Protection Circuits (on LM4780) Protection to prevent long-term thermal stress When die temperature exceeds 150°C, the LM4780 shuts down until temperature falls below 145°C, then amp restarts Helps prevent thermal cycling Improves reliability Still need an adequate heat sink to prevent IC from approaching 150°C

Audio Power Amplifier Detailed Design Maximum Power Dissipation Calculation Power dissipation is the power that is converted to heat within the amplifier Important parameter used to determine heat sinking requirements and output power Pi + Ps = Po + Pd Where: Pi = input signal power, Ps = DC supply power, Po = output signal power, and Pd = dissipated power Pd should be minimized so Po is maximized

Audio Power Amplifier Detailed Design Maximum Power Dissipation Calculation Determines size of heat sink For Parallel Amplifier Configuration the equivalent impedance at the load is: RL(parallel) = RL(total) * Number of Amps on IC RL(parallel) = 8 ohms * 2 amps on IC = 16 ohms Pdmax = (Vcc^2)/(2π^2*RL(parallel)) Pdmax = (70V^2)/(2π^2*16 ohms) = 15.51 W Pdmax = 2 * 15.51W = 31.02 W for both amps on IC

Audio Power Amplifier Detailed Design Determining the Correct Heat Sink Chosen to keep the die temperature of the amplifier IC below 150°C to prevent the thermal protection circuits to be activated under normal circumstances Need to choose the heat sink with the lowest cost and smallest size for its thermal resistance

Audio Power Amplifier Detailed Design Determining the Correct Heat Sink (con’t) Convection heat flow or power dissipation is analogous to current flow Thermal resistance is analogous to resistance Temperature drops are analogous to voltage drops

Audio Power Amplifier Detailed Design Determining the Correct Heat Sink (con’t) Thermal resistance from die to outside air θJA = θJC + θCS + θSA, where: θJC = thermal resistance (junction to case) θCS = thermal resistance (case to sink) θSA = thermal resistance (sink to air)

Audio Power Amplifier Detailed Design Determining the Correct Heat Sink (con’t) For the audio amplifier IC chosen, LM4780: θJC = 0.8°C/W θCS = 0.2°C/W and θJA = 30°C/W (max)

Audio Power Amplifier Detailed Design Determining the Correct Heat Sink (con’t) Using these analogies, power dissipation can be calculated, Pdmax = (TJmax – TAmb)/θJA TJmax = max junction (die) temp allowed for the LM4780 this is 150ºC TAmb = ambient temperature

Audio Power Amplifier Detailed Design Determining the Correct Heat Sink (con’t) Finally, to calculate the sink to ambient thermal resistance, θSA, the equation for power dissipation can be used, θSA = [(TJmax - TAmb)-PDmax(θJC+θCS)]/Pdmax θSA = [(150°C - 50°C)-31.03(0.8+0.2)]/31.03 = 2.22 θSA = 2.22°C/W for worst case ambient temp of 50ºC

Audio Power Amplifier Detailed Design Voltage Gain Calculation Non-inverting configuration: Av = Rf/Rin + 1 = 20k/1k + 1 = 21 Av (dB) = 20log(21) = 26.4 dB This is the voltage gain for one amplifier on IC

Audio Power Amplifier Detailed Design Under-Voltage Protection of LM4780 allows power supplies and voltage across capacitors to reach full values before amp turned on to prevent DC output spikes Over-Voltage Protection of LM4780 limits the output current while providing voltage clamping

Audio Power Amplifier Detailed Design Power Supply Bypassing LM4780 has excellent power supply rejection To improve performance, bypass capacitors are needed on power supply Eliminates possible oscillations

Audio Power Amplifier Detailed Design Parallel Amplifier Configuration LM4780 has two op-amps internally, so load can be driven with both op-amps on IC for higher output power Design both amplifiers to have close to identical gain Connect inputs in parallel Connect outputs in parallel through a small R Ideally each amplifier shares output current equally

Audio Power Amplifier Detailed Design Electrical Characteristics for the LM4780 Audio Power Amplifier Total Harmonic Distortion + Noise (THD+N) = 0.03% (typical) Conditions: Pout = 30 W, f = 20Hz – 20kHz, Av = 26 dB, Po = 30 W and RL = 8Ω

Audio Power Amplifier Detailed Design Electrical Characteristics for the LM4780 Audio Power Amplifier Output Power (Po) = 60 W (typical) Conditions: f = 1kHz, f = 20 kHz, THD + N = 0.5%, Vcc = 35V and RL = 8Ω

Audio Power Amplifier Detailed Design Electrical Characteristics for the LM4780 Audio Power Amplifier Common Mode Rejection Ratio (CMRR) = 110 dB (typical) and 85 dB (min) Conditions: Vcc = 35V and Vcm = 20V to –20V

Audio Power Amplifier Detailed Design Electrical Characteristics for the LM4780 Audio Power Amplifier Signal-to-Noise Ratio (SNR) = 114 dB (typical) Conditions: Po = 50W rms

Audio Power Amplifier Detailed Design Functions of Components: R1: sets input terminals DC bias & HPF with C1 R2 (R3): limits current into non-inverting terminal R4 (R5): sets gain along with R6 (R7) R8 (R9): sets high frequency pole to prevent oscillations R10 (R11): limits current to output load C1: input coupling capacitor C2 (C3): HPF with R4 (R5) C4, C5, C6 & C9, C10, C11: power supply filtering capacitors C7 (C8): sets high frequency pole to prevent oscillations

Audio Power Amplifier Detailed Design Offset Error Contribution: Error Voltage due to Vio: Verr = Vio(1+Rf/Rp) Verr = 10mV(1+20k/1k) = 210 mV

Audio Power Amplifier Detailed Design Gain Error: (5% tolerance resistors) Av(nom) = 1 + Rf/Rp = 1 + 20k/1k = 21 Resistor Tolerances: Assume Rf = Rf + 5% and Rp = Rp – 5%, then If Rf = 21k and Rp = 0.95k, then Av = 1 + 21k/0.95k = 23.1 Gain Error = Av(nom) – Av = 21 – 23.1 = 2.1

Audio Power Amplifier Detailed Design For the parallel amplifier configuration, the gain of each amplifier must be matched, so only 1% tolerance resistors can be used Gain Error (1% tolerance resistors) Resistor Tolerances: Assume Rf = Rf + 1% and Rp = Rp – 1% then Rf = 20.2k and Rp = 0.99k The Av = 1 + Rf/Rp = 1 + 20.2k/0.99k = 21.4 Gain Error = Av(nom) – Av = 21 – 21.4 = 0.4

Audio Power Amplifier Detailed Design