APA3541/4
Application Note
pacitors are recommended for the best THD and
noise performance .
Input Capacitor , Ci
In the typical application an input capacitor , Ci , is
required to allow the amplifier to bias the input signal
to the proper DC level for optimum operation . In this
case , the external capacitor Ci and the internal re-
sistance Ri form a high-pass filter with the corner fre-
quency determined in the follow equation:
Output Coupling Capacitor, Cc
In the typical single-supply SE configuration , an out-
put coupling capacitor (Cc) is required to block the
DC bias at the output of the amplifier thus preventing
DC currents in the load . As with the input coupling
capacitor , the output coupling capacitor and imped-
ance of the load form a high-pass filter governed by
equation .
fc (highpass)= 1/ (2πRiCi)
(1)
The value of Ci is important to consider as it directly
affects the low frequency performance of the circuit.
Consider the APA3541 where Ri is 180kΩ and
APA3544 is 90kΩ internal fixed . Equation is
reconfigured as follow:
fc(highpass)= 1/(2πRLCc)
(4)
For example , a 220µF capacitor with an 32Ω speaker
would attenuate low frequencies below 22Hz . The
main disadvantage , from a performance standpoint
, is the load impedance is typically small , which drives
the low-frequency corner higher degrading the bass
response . Large values of Cc are required to pass
low frequencies into the load .
Ci= 1/(2π*180kΩ*fc) for APA3541
Ci= 1/(2π*90kΩ*fc) for APA3544
(2)
And the ceramic capacitor is recommanded.
Bias Capacitor , Cb
Optimizing Depop Circuitry
As with any power amplifier , proper supply bypass-
ing is critical for low noise performance and high
power supply rejection . The capacitor location on
both the bypass and power supply pins should be as
close to the device as possible . The effect of a larger
half supply bias capacitor is improved PSRR due to
increased half-supply stability . Typical applications
employ a 5V regulator with 10µF and a 0 . 1µF bias
capacitors which aid in supply filtering .
When the amplifier is in mute mode , both of the out-
put stage and input bypass continues to be biased .
And no pop noise will be heard during the transition
out of mute mode .
Power Supply Decoupling, Cs
APA3541/4 is a high-performance CMOS audio am-
plifier that requires adequate power supply decoupling
to ensure the output total harmonic distortion (THD)
is as low as possible . Power supply decoupling also
prevents the oscillations causing by long lead length
between the amplifier and the speaker . The optimum
decoupling is achieved by using two different type
capacitors that target on different type of noise on
the power supply leads . For higher frequency tran-
sients , spikes , or digital hash on the line , a good
low equivalent-series-resistance (ESR) ceramic
capacitor, typically 0.1µF placed as close as possible
to the device V lead works best . For filtering lower-
This does not eliminate the need for bypassing the
supply nodes of the APA3541/4 . The selection of
bias capacitors , especially Cb , is thus dependent
upon desired PSRR requirements , click and pop per-
formance . The capacitor is fed from a 95kΩ source
inside the amplifier . To keep the start-up pop as low
as possible , the relationship shown in equation should
be maintained .
1/(Cb*95kΩ)≤ 1/{Ci*Ri}
(3)
As an example , consider a circuit where Cb is
4.7µF, Ci is 1µF and APA3541 Ri is 180kΩ . Inserting
these values into the equation we get 2.24≤ 5.55
which satisfies the rule . Bias capacitor , Cb , values
of 2.2µF to 10µF ceramic or tantalum low-ESR ca-
DD
frequency noise signals , a large aluminum electro-
lytic capacitor of 10µF or greater placed near the audio
power amplifier is recommended .
Copyright ANPEC Electronics Corp.
Rev. B.1 -Apr., 2003
8
www.anpec.com.tw
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
