APA2068
Application Descriptions (Cont.)
long leadlengthbetweentheamplifier andthespeaker.
Effective Bypass Capacitor, Cbypass (Cont.)
The optimum decoupling is achieved by using two
different typecapacitors that target on different type of
noise on the power supply leads.
maintained.
1
1
<<
(6)
Cbypass x 125kW
100kW x Ci
The bypass capacitor isfed thru from a 125kW resistor
For higher frequency transients, spikes, or digital hash
inside the amplifier and the 100kW is maximum input on the line, a good low equivalent-series-resistance
resistance of (Ri+ Rf). Bypass capacitor, Cb, values of
3.3mF to 10mF ceramic or tantalum low-ESR capacitors close as possible to the device VDD lead works best.
(ESR) ceramic capacitor, typically 0.1mF placed as
are recommended for the best THD and noise
performance.
For filtering lower-frequency noise signals, a large
aluminum electrolytic capacitor of 10mF or greater
placednear theaudiopoweramplifier isrecommended.
The bypass capacitance also effects to the start up
time. It is determined in the following equation :
Optimizing Depop Circuitry
Tstart up = 5 x (Cbypass x 125KW)
(7)
Circuitry hasbeen includedin theAPA2068to 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 clicksand 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.
Output Coupling Capacitor, Cc
In the typical single-supply SE configuration, an
output 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 andimpedance
of theloadform ahigh-passfiltergovernedbyequation.
The value of Ci will also affect turn-on pops (Refer to
Effective Bypass Capacitance). The bypass voltage
1
(8)
FC(highpass)=
2pRLCC
For example, a 330mF capacitor with an 8W speaker ramp up should be slower than input bias voltage.
would attenuate low frequencies below 60.6Hz. The
main disadvantage, from aperformance standpoint, is modified, the size of Cbypass can be changed to alter
the load impedance is typically small, which drives
the low-frequency corner higher degrading the bass pops. By increasing the value of Cbypass, turn-on pop
Although the bypass pin current source cannot be
the device turn-on time and the amount of clicks and
response. Large values of CC are required to passlow
frequencies into the load.
canbereduced. However, thetradeoff for usingalarger
bypass capacitor is to increase the turn-on time for
this device. There is a linear relationship between the
size of Cbypass and the turn-on time. In a SE
configuration, the output coupling capacitor, CC, is of
particular concern.
Power Supply Decoupling, Cs
The APA2068 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
decoupling also prevents the oscillations causing by
This capacitor discharges through the internal 10kW
resistors. Depending on the size of CC, the time
Copyright ã ANPEC Electronics Corp.
20
www.anpec.com.tw
Rev. A.2 - Jul., 2005
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
