1. Audio Power Amplifier Detailed Design
By: Brian Felsmann

2. 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

3. 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

4. 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

5. 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) = W
Pdmax = 2 * 15.51W = W for both amps on IC

6. 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

7. 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

8. 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)

9. 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)

10. 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

11. 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( )]/31.03 = 2.22
θSA = 2.22°C/W for worst case ambient temp of 50ºC

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

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

23. 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 = k/0.99k = 21.4
Gain Error = Av(nom) – Av = 21 – 21.4 = 0.4

24. Audio Power Amplifier Detailed Design