SA58672_15 Datasheet, PDF (13/27 Pages) NXP Semiconductors – 3.0 W mono class-D audio amplifier

NXP Semiconductors
SA58672
3.0 W mono class-D audio amplifier
11. Application information
11.1 Power supply decoupling considerations
The SA58672 is a mono class-D audio amplifier that requires proper power supply
decoupling to ensure the rated performance for THD+N and power efficiency. To decouple
high frequency transients, power supply spikes and digital noise on the power bus line, a
low Equivalent Series Resistance (ESR) capacitor, of typically 1 µF is placed as close as
possible to the PVDD terminals of the device. It is important to place the decoupling
capacitor at the power pins of the device because any resistance or inductance in the
PCB trace between the device and the capacitor can cause a loss in efficiency. Additional
decoupling using a larger capacitor, 4.7 µF or greater may be done on the power supply
connection on the PCB to filter low frequency signals. Usually this is not required due to
high PSRR of the device.
11.2 Voltage gain
The SA58672 is comprised of an analog amplifier stage and a comparator stage. The
output of the analog amplifier stage is compared with the periodic ramp signal from the
sawtooth ramp generator. The resulting output of the comparator is a Pulse Width
Modulated (PWM) signal. The final stage is a power NMOS and PMOS H-bridge that
converts the PWM into a high power output signal capable of driving low-impedance
loads.
The input resistor, R_i sets the gain of the amplifier according to Equation 1:
Gain = ²----⁽--¹---⁵---⁰-----^k---^Ω-----⁾-
(1)
R_i
11.3 Input capacitor selection
The SA58672 does not require input coupling capacitors when used with a differential
audio source that is biased from 0.5 V to V_DD − 0.8 V. In other words, the input signal must
be biased within the common-mode input voltage range. If high-pass filtering is required
or if it is driven using a single-ended source, input coupling capacitors are required.
The 3 dB cut-off frequency created by the input coupling capacitor and the input resistors
is calculated by Equation 2:
f _–3dB = -₂---_π-----_×-----_R1---_i---_×-----_C----_i
(2)
Using an input resistor of 150 kΩ, the gain is set to 2 V/V. At this gain setting, for input
capacitor values from 220 nF to 2.2 µF, the 3 dB cut-off frequency may be set between
22 Hz and 220 Hz. Since the values of the input coupling capacitor and the input resistor
affects the low frequency performance of the audio amplifier, it is important to consider in
the system design. Small speakers in wireless and cellular phones usually do not respond
well to low frequency signals. Their low frequency response may be only 600 Hz; typically
1 kHz. Thus, the 3 dB cut-off frequency should be increased to block the low frequency
signals to the speakers.
SA58672_4
Product data sheet
Rev. 04 — 8 June 2009
© NXP B.V. 2009. All rights reserved.
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