APA2057A
Application Information (Cont.)
Please note that it is important to confirm the capaci-
tor polarity in the application.
Gain Setting (Cont.)
For headphone driver, the internal feedback resistor is
40kW (Rf(HP) external, 10% variation by process), so the
headphone driver’s gain is set by the input resistor (Ri(HP)
external), the Table 1 lists the reference gain settings
with external resistor for headphone driver (HP Mode).
Note: The headphone dirver’s input is ground reference,
so please check the C ’s polarized at design.
i(HP)
Effective Bias Capacitor, CB
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection.
HP Mode Gain Setting Table for Reference
Ri(HP),external *Rf(HP),internal
HP OUT (V/V) HP Gain(dB)
(kW)
62
(kW)
40
0.65
0.80
1.03
1.33
1.67
2.00
-3.8
-1.9
0.2
2.5
4.4
6.0
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 bypass capacitor is improved PSRR
due to increased 1.8V bias voltage stability. Typical
applications employ a 5V regulator with 2.2mF and a
0.1mF bypass capacitor, which aids in supply filtering.
This does not eliminate the need for bypassing the
supply nodes of the APA2057A. The selection of by-
pass capacitors, especially CB, is thus dependent upon
desired PSRR requirements and click-and-pop
performance.
50
40
39
40
30
40
24
40
20
40
*The internal Rf's value has 10% variation by process.
Table 1: Gain Setting Table for Reference
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, Ci and the
minimum input impedance Ri from a high-pass filter with
the corner frequency are determined by the following
equation:
Power Supply Decoupling, Cs
The APA2057A is a high-performance CMOS audio amplifier
that requires adequate power supply decoupling to
ensure the output total harmonic distortion (THD+N)
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 types
of capacitor that target on 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
0.1mF, is placed as close as possible to the device VDD
1
(1)
FC (highpass) =
(2pRi(MIN) ´ Ci)
The value of Ci is important to consider as it directly
affects the low frequency performance of the circuit.
Consider the example where Ri is 10kW and the
specification calls for a flat bass response down to
10Hz. Equation is reconfigured as below:
1
Ci =
(2)
(2pRiFc)
Consider to input resistance variation, the Ci is 1.6mF, so
one would likely choose a value in the range of 2.2mF
to 3.3mF. A further consideration for this capacitor is
the leakage path from the input source through the input
network (Ri+Rf, Ci) 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. 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. As the DC level is held
at VDD/2, which is likely higher than the source DC level.
lead works best (the pin1 (V ) and pin2 (GND)’s capaci-
DD
tor must short less than 1cm). For filtering lower-frequency
noise signals, a large aluminum electrolytic capacitor of
10mF or greater is placed near the audio power amplifier
is recommended.
Shutdown Function
In order to reduce power consumption while not in use,
the APA2057A contains a shutdown pin to externally turn
off the amplifier bias circuitry. This shutdown feature
turns the amplifier off when a logic low is placed on the
Copyright ã ANPEC Electronics Corp.
23
www.anpec.com.tw
Rev. A.1 - Aug., 2007
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
