APA0710/0711
Application Descriptions (Cont.)
Effective Bypass Capacitor, Cbypass(Cont.)
bypass capacitors, especially Cbypass, is thus
dependent upon desired PSRR requirements, click
andpopperformance.
Power Supply Decoupling, Cs
The APA0710/1 is a high-performance CMOS audio
amplifier that requires adequate power supply
decoupling to ensure the output total harmonic
distortion (THD) is as low as possible. Power supply
To avoid start-up pop noise occurred, the bypass
voltage should rise slower than the input bias voltage decoupling also prevents the oscillations causing by
long leadlengthbetweentheamplifier andthespeaker.
The optimum decoupling is achieved by using two
different type capacitors that target on different type
of noiseonthepowersupplyleads. Forhigherfrequency
transients, spikes, or digital hash on the line, a good
low equivalent-series-resistance (ESR) ceramic
capacitor, typically 0.1mF placed as close as possible
to the device VDD lead works best. For filtering lower-
frequencynoise signals, a large aluminum electrolytic
capacitor of 10mF or greater placed near the audio
power amplifier is recommended.
and the relationship shown in equation (4) should be
1
1
maintained.
Cbypass ´ 80kO
<<
(4)
(RI + RF)´ CI
The bypass capacitor is fed from a 80kW resistor
inside the amplifier. Bypass capacitor, Cbypass, values
of 0.1mF to 2.2mF ceramic or tantalum low-ESR
capacitors are recommended for the best THD and
noiseperformance.
The bypass capacitance also effects to the start up
time. It is determined in the following equation :
(5)
Tstart up = 5 x (Cbypass x 80kW)
Optimizing Depop Circuitry
Output Coupling Capacitor, Cc (for APA0710 only)
Circuitry has been included in the APA0710/1 to
minimize the amount of popping noise at power-up
and when coming out of shutdown mode. Popping
occurs whenever a voltage step is applied to the
speaker. In order to eliminate clicks and pops, all
capacitors must be fully discharged before turn-on.
Rapid on/off switching of the device or the shutdown
function will cause the click and pop circuitry. The
valueof Ciwillalsoaffect turn-onpops(refer toEffective
Bypass Capacitance). The bypass voltage rise up
should be slower than input bias voltage.
In the typical single-supply (SE) configuration on a
APA0710, an output coupling capacitor (Cc) isrequired
to block theDC 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
impedanceof the loadform ahigh-passfilter governed
by equation.
1
(6)
FC(highpass)=
2pRLCC
For example, a 330mF capacitor with an 8W speaker
would attenuate low frequencies below 60.6Hz. The
main disadvantage, from aperformance 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.
Although the bypass pin current source cannot be
modified, the size of Cbypass can be changed to alter
the device turn-on time and the amount of clicks and
pops. By increasing the value of Cbypass, turn-on pop
canbereduced. However,thetradeoff forusingalarger
bypass capacitor is to increase the turn-on time for
this device. There isa linear relationship between the
Copyright ã ANPEC Electronics Corp.
Rev. A.5 - Oct., 2005
22
www.anpec.com.tw
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
