AP4809 Datasheet, PDF (5/9 Pages) Anachip Corp – Stereo Headphone Amplifier With Shutdown Mode

Stereo Headphone Amplifier With Shutdown Mode
AP4809
„ Application Information
1. Input Capacitor, C_I
In the typical application, an input capacitor, C_i, is
required to allow the amplifier to bias the input signal
to the proper dc level for optimum operation. In this
case, C_i and R_i form a high-pass filter with the corner
frequency determined in following equation (1).
1
f_co(highpass) =
(1)
2π _RiCi
The value of C_i is important to consider, as it directly
affects the low frequency performance of the circuit.
Consider the example where R_i is 15kΩ and the
specification calls for a flat bass response down to
20Hz. Equation (1) is reconfigured as below:
1
C_i =
(2)
2π _{Ri f co(highpass)}
In this example, C_i is 0.5 µF, so one would likely
choose a value in the range of 1µF to 2.2µF. A
further consideration for this capacitor is the leakage
path from the input source through the input network
(R_i, C_i) and the feedback resistor (R_f) to the load.
This leakage current creates a dc offset voltage at
the input to the amplifier that reduces useful
headroom, especially in high-gain applications (>10).
For this reason a low-leakage tantalum or ceramic
capacitor is the best choice. When polarized
capacitors are used, the positive side of the
capacitor should face the amplifier input in most
applications, as the dc level there is held at V_DD/2,
which is likely higher than the source dc level. It is
important to confirm the capacitor polarity in the
application.
2. Power Supply Decoupling, C_S
The AP4809 is a high-performance CMOS audio
amplifier that requires adequate power supply
decoupling to ensure that the output total harmonic
distortion (THD) is as low as possible. Power supply
decoupling also prevents oscillations for long lead
lengths between the amplifier and the speaker. The
optimum decoupling is achieved by using two
capacitors of different types that target different
types of noise on the power supply leads. For higher
frequency transients, 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_DD lead, works best. For
filtering lower-frequency noise signals, a larger
aluminum electrolytic capacitor of 10µF or greater
placed near the power amplifier is recommended.
3. Mid-rail Bypass Capacitor, C_B
In the consideration of reducing the start-up pop, the
mid-rail voltage should rise at a sub-sonic rate; that
is, less than the rise time of 20Hz waveform and
slower than the charging rate of both C_i & C_C. The
relationship shown in equation (3) should be
maintained to keep the noise as low as possible.
Where C_B is the value of bypass capacitor and
R_SOURCE is the equivalent source impedance of the
voltage divider (the parallel combination of the two
resistors)
1
1
1
<
<<
(3)
C_B ^× R_SOURCE CiRi
R_L R_C
The bypass capacitor, C_B, serves several important
functions. During start-up, C_B determines the rate at
which the amplifier starts up. This helps to push the
start-up pop noise into the sub-audible range (so
slow it can not be heard). The second function is to
reduce noise produced by the power supply caused
by coupling into the output drive signal. This noise is
from the mid-rail generation circuit internal to the
amplifier. The capacitor is fed from the resistor
divider with equivalent resistance of R_SOURCE. On
selection of bypass capacitor, C_B, ceramic or
tantalum low-ESR capacitors are recommended for
the best THD and noise performance.
4. Output Coupling Capacitor, C_C
In the typical single-supply single-ended (SE)
configuration, an output coupling capacitor (C_C) 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 impedance of the load form a
high-pass filter governed by equation (4).
1
f_{(out high)} =
(4)
2π _{RL CC}
The main disadvantage, from a performance
standpoint, is that the typically small load
impedances drive the low-frequency corner higher.
Large values of C_C are required to pass low
frequencies into the load.
The output coupling capacitor required in
single-supply SE mode also places additional
constraints on the selection of other components in
the amplifier circuit. With the rules described earlier
still valid, add the following relationship:
Anachip Corp
www.anachip.com.tw
Rev. 1.0 Sep 29, 2004
5/9