NXP Semiconductors SA58670A - Manual (Page 17)

SA58670A_2 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 02 — 23 October 2008 17 of 24
NXP Semiconductors
SA58670A
2.1 W/channel stereo class-D audio amplifier
Since the value of the input decoupling capacitor and the input resistance determined by
the gain setting affects the low frequency performance of the audio amplifier, it is
important to consider this during the system design. Small speakers in wireless and
cellular phones usually do not respond well to low frequency signals, so the 3 dB cut-off
frequency may be increased to block the low frequency signals to the speakers. Not using
input coupling capacitors may increase the output offset voltage.
Equation 2 is solved for C
i
:
(2)
11.3 PCB layout considerations
Component location is very important for performance of the SA58670A. Place all
external components very close to the SA58670A. Placing decoupling capacitors directly
at the power supply voltage pins increases efficiency because the resistance and
inductance in the trace between the SA58670A power supply voltage pins and the
decoupling capacitor causes a loss in power efficiency.
The trace width and routing are also very important for power output and noise
considerations.
For high current pins (PVDD, PGND and audio output), the trace widths should be
maximized to ensure proper performance and output power. Use at least 500
µ
m wide
traces.
For the input pins (INRP, INRN, INLP and INLN), the traces must be symmetrical and run
side-by-side to maximize common-mode cancellation.
11.4 Filter-free operation and ferrite bead filters
A ferrite bead low-pass filter can be used to reduce radio frequency emissions in
applications that have circuits sensitive to frequencies greater than 1 MHz. A ferrite bead
low-pass filter functions well for amplifiers that must pass FCC unintentional radiation
requirements for frequencies greater than 30 MHz. Choose a bead with high-impedance
at high frequencies and very low-impedance at low frequencies. In order to prevent
distortion of the output signal, select a ferrite bead with adequate current rating.
For applications in which there are circuits that are EMI sensitive to low frequencies
(< 1 MHz) and there are long leads from amplifier to speaker, it is necessary to use an LC
output filter.
Table 6. Gain selection
G1 G0 Gain (V/V) Gain (dB) Input impedance (k
Ω
)
LOW LOW 2 6 28.1
LOW HIGH 4 12 17.3
HIGH LOW 8 18 9.8
HIGH HIGH 16 24 5.2
C
i
1
2
π
R
i
×
f
3dB –
Ч
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