Datasheet

LM4818
SNAS163B –APRIL 2002–REVISED MAY 2013 www.ti.com
In order to eliminate "clicks and pops", all capacitors must be discharged before turn-on. Rapidly switching V
DD
may not allow the capacitors to fully discharge, which may cause "clicks and pops".
AUDIO POWER AMPLIFIER DESIGN EXAMPLE
The following are the desired operational parameters:
Given:
Power Output 100mW
Load Impedance 16Ω
Input Level 1Vrms (max)
Input Impedance 20kΩ
Bandwidth 100Hz–20kHz ± 0.25dB
The design begins by specifying the minimum supply voltage necessary to obtain the specified output power. To
find this minimum supply voltage, use the Output Power vs. Supply Voltage graph in the TYPICAL
PERFORMANCE CHARACTERISTICS section. From the graph for a 16Ω load, (graphs are for 8Ω, 16Ω, and
32Ω loads) the supply voltage for 100mW of output power with 1% THD+N is approximately 3.15 volts.
Additional supply voltage creates the benefit of increased headroom that allows the LM4818 to reproduce peaks
in excess of 100mW without output signal clipping or audible distortion. The choice of supply voltage must also
not create a situation that violates maximum dissipation as explained above in the Power Dissipation section. For
example, if a 3.3V supply is chosen for extra headroom then according to Equation 3 the maximum power
dissipation point with a 16Ω load is 138mW. Using Equation 4 the maximum ambient temperature is 126°C for
the D package.
After satisfying the LM4818's power dissipation requirements, the minimum differential gain is found using
Equation 6.
(6)
Thus a minimum gain of 1.27 V/V allows the LM4818 to reach full output swing and maintain low noise and
THD+N performance. For this example, let A
VD
= 1.27. The amplifier's overall gain is set using the input (R
i
) and
feedback (R
F
) resistors. With the desired input impedance set to 20kΩ, the feedback resistor is found using
Equation 7.
R
F
/R
i
= A
VD
/2 (V/V) (7)
The value of R
F
is 13kΩ.
The last step in this design example is setting the amplifier's -3dB frequency bandwidth. To achieve the desired
±0.25dB pass band magnitude variation limit, the low frequency response must extend to at least one-fifth the
lower bandwidth limit and the high frequency response must extend to at least five times the upper bandwidth
limit. The gain variation for both response limits is 0.17dB, well with in the ±0.25dB desired limit.
The results are:
f
L
= 100Hz/5 = 20Hz (8)
f
H
= 20 kHz*5 = 100kHz (9)
As mentioned in the PROPER SELECTION OF EXTERNAL COMPONENTS section, R
i
and C
i
create a high
pass filter that sets the amplifier's lower band pass frequency limit. Find the coupling capacitor's value using
Equation 10.
C
i
≥ 1/(2πR
i
f
c
) (F) (10)
C
i
≥ 0.398µF, a standard value of 0.39µF will be used. The product of the desired high frequency cutoff (100kHz
in this example) and the differential gain, A
VD
, determines the upper pass band response limit. With A
VD
= 1.27
and f
H
= 100kHz, the closed-loop gain bandwidth product (GBWP) is 127kHz. This is less than the LM4818's
900kHz GBWP. With this margin the amplifier can be used in designs that require more differential gain while
avoiding performance restricting bandwidth limitations.
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