NXP Semiconductors SA58670A 11.3 PCB layout considerations, 11.4 Filter-free operation and ferrite bead lters

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 maybe 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 Ci:

(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 500µm 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 1MHz. A ferrite bead

low-pass ﬁlter functions well for ampliﬁers that must pass FCC unintentional radiation

requirements for frequencies greater than 30MHz. 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 necessar y to use an LC

output ﬁlter.

Table 6. Gain selection

G1 G0 Gain (V/V) Gain (dB) Input impedance (kΩ)

LOW LOW 2 6 28.1

LOW HIGH 4 12 17.3

HIGH LOW 8 18 9.8

HIGH HIGH 16 24 5.2

Ci1

2πRi

×f3dB–

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