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SSM2335
Rev. 0 | Page 13 of 1
6
LAYOUT
As output power continues to increase, care must be taken to
lay out PCB traces and wires properly among the amplifier,
load, and power supply. A good practice is to use short, wide
PCB tracks to decrease voltage drops and minimize inductance.
Ensure that track widths are at least 200 mil for every inch of
track length for lowest DCR, and use 1 oz or 2 oz of copper PCB
traces to further reduce IR drops and inductance. A poor layout
increases voltage drops, consequently affecting efficiency. Use
large traces for the power supply inputs and amplifier outputs to
minimize losses due to parasitic trace resistance.
Proper grounding guidelines help to improve audio performance,
minimize crosstalk between channels, and prevent switching
noise from coupling into the audio signal. To maintain high
output swing and high peak output power, the PCB traces that
connect the output pins to the load, as well as the PCB traces to
the supply pins, should be as wide as possible to maintain the
minimum trace resistances. It is also recommended that a large
ground plane be used for minimum impedances.
In addition, good PCB layout isolates critical analog paths from
sources of high interference. High frequency circuits (analog
and digital) should be separated from low frequency circuits.
Properly designed multilayer PCBs can reduce EMI emission
and increase immunity to the RF field by a factor of 10 or more,
compared with double-sided boards. A multilayer board allows
a complete layer to be used for the ground plane, whereas the
ground plane side of a double-sided board is often disrupted by
signal crossover.
If the system has separate analog and digital ground and power
planes, the analog ground plane should be directly beneath the
analog power plane, and, similarly, the digital ground plane should
be directly beneath the digital power plane. There should be no
overlap between analog and digital ground planes or between
analog and digital power planes.
INPUT CAPACITOR SELECTION
The SSM2335 does not require input coupling capacitors if the
input signal is biased from 1.0 V to VDD 1.0 V. Input capacitors
are required if the input signal is not biased within this recom-
mended input dc common-mode voltage range, if high-pass
filtering is needed, or if a single-ended source is used. If high-
pass filtering is needed at the input, the input capacitor and the
input resistor of the SSM2335 form a high-pass filter whose
corner frequency is determined by the following equation:
fC = 1/(2π × RIN × CIN)
The input capacitor can significantly affect the performance of
the circuit. Not using input capacitors degrades both the output
offset of the amplifier and the dc PSRR performance.
POWER SUPPLY DECOUPLING
To ensure high efficiency, low total harmonic distortion (THD),
and high PSRR, proper power supply decoupling is necessary.
Noise transients on the power supply lines are short-duration
voltage spikes. Although the actual switching frequency can range
from 10 kHz to 100 kHz, these spikes can contain frequency
components that extend into the hundreds of megahertz. The
power supply input needs to be decoupled with a good quality,
low ESL, low ESR capacitor, with a minimum value of 4.7 μF.
This capacitor bypasses low frequency noises to the ground
plane. For high frequency transient noises, use a 0.1 μF capacitor
as close as possible to the VDD pin of the device. Placing the
decoupling capacitor as close as possible to the SSM2335 helps
to maintain efficient performance.