Transcript Audio Power Amplifier

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