SA58670 Datasheet, PDF (10/18 Pages) NXP Semiconductors – 2.1 W/channel stereo Class D audio amplifier

NXP Semiconductors
SA58670
2.1 W/channel stereo Class D audio amplifier
11.3 PCB layout considerations
Component location is very important for performance of the SA58670. Place all external
components very close to the device. Placing decoupling capacitors directly at the power
supply pins increases efficiency because the resistance and inductance in the trace
between the device power supply 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 terminals (PVDD, PGND and audio output), the trace widths should be
maximized to ensure proper performance and output power. Use at least 500 µm wide
traces.
For the input pins (INRP/INRN and INLP/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 greater than 1 MHz. A ferrite bead low-pass
filter functions well for amplifiers that must pass FCC unintentional radiation requirements
at 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 frequency
(<1 MHz) and there are long leads from amplifier to speaker, it is necessary to use an LC
output filter.
11.5 Efficiency and thermal considerations
The maximum ambient operating temperature depends on the heat transferring ability of
the heat spreader on the PCB layout. In Table 3 “Limiting values”, power dissipation, the
power derating factor is given as 41.6 mW/°C. The device thermal resistance, R_th(j-a) is the
reciprocal of the power derating factor. Convert the power derating factor to R_th(j-a) by the
following equation:
R_th(j-a)
=
--------------------¹--------------------
derating factor
=
-------¹--------
0.0413
=
24
°C/W
(3)
For a maximum allowable junction temperature, T_j = 150 °C and R_th(j-a) = 24 °C/W and a
maximum device dissipation of 1.5 W (750 mW per channel) and for 2.1 W per channel
output power, 4 Ω load, 5 V supply, the maximum ambient temperature is calculated using
Equation 4:
T_amb(max) = T_j(max) – (R_th(j-a) × P_D(max)) = 150 – (24 × 1.5) = 114 °C
(4)
The maximum ambient temperature is 114 °C at maximum power dissipation for 5 V
supply and 4 Ω load. If the junction temperature of the SA58670 rises above 150 °C, the
thermal protection circuitry turns the device off; this prevents damage to IC. Using
speakers greater than 4 Ω further enhances thermal performance and battery lifetime by
reducing the output load current and increasing amplifier efficiency.
SA58670_1
Objective data sheet
Rev. 01 — 22 June 2007
© NXP B.V. 2007. All rights reserved.
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