PAM8303D
Ultra Low EMI, 3W Filterless
Mono Class-D Audio Power Amplifier
Application Information
Input Resistance (Ri)
The input resistors (Ri) set the gain of the
amplifier according to Equation 1.
2
´
150k
W æ
V
ö
Gain
=
ç ÷
Ri
è
V
ø
Resistor matching is very important in fully
differential amplifiers. The balance of the output
on the reference voltage depends on matched
ratios of the resistors. CMRR, PSRR, and
cancellation of the second harmonic distortion
diminish if resistor mismatch occurs. Therefore,
it is recommended to use 1% tolerance resistors
or better to keep the performance optimized.
Matching is more important than overall
tolerance. Resistor arrays with 1% matching can
be used with a tolerance greater than 1%.
Place the input resistors very close to the
PAM8303D to limit noise injection on the high-
impedance nodes.
For optimal performance the gain should be set
to 2X(Ri=150k) or lower. Lower gain allows the
PAM8303D to operate at its best, and keeps a
high voltage at the input making the inputs less
susceptible to noise. In addition to these
features, higher value of Ri minimizes pop noise.
Input Capacitors (Ci )
In the typical application, an input capacitor, Ci,
is required to allow the amplifier to bias the input
signal to the proper DC level for optimum
operation. In this case, Ci and the minimum input
impedance Ri form is a high-pass filter with the
corner frequency determined in the follow
equation:
1
f
C
=
(
2
p
RiCi
)
It is important to consider the value of Ci as it
directly affects the low frequency performance of
the circuit. For example, when Ri is 150k Ω and
the specification calls for a flat bass response
are down to 150Hz. Equation is reconfigured as
followed:
1
Ci
=
(
2
p
R
i
f
c
)
When input resistance variation is considered,
the Ci is 7nF, so one would likely choose a value
of 10nF. A further consideration for this capacitor
is the leakage path from the input source through
the input network ( Ci , Ri + Rf ) to the load. This
leakage current creates a DC offset voltage at
the input to the amplifier that reduces useful
headroom, especially in high gain applications.
For this reason, a low-leakage tantalum or
ceramic capacitor is the best choice. When
polarized capacitors are used, the positive side
of the capacitor should face the amplifier input in
most applications as the DC level is held at V
DD
/2,
which is likely higher than the source DC level.
Please note that it is important to confirm the
capacitor polarity in the application.
Decoupling Capacitor (C
S
)
The PAM8303D is a high-performance CMOS
audio amplifier that requires adequate power
supply decoupling to ensure the output total
harmonic distortion (THD) as low as possible.
Power supply decoupling also prevents the
oscillations causing by long lead length between
the amplifier and the speaker.
The optimum decoupling is achieved by using
two different types of capacitors that target on
different types of noise on the power supply
leads. For higher frequency transients, spikes, or
digital hash on the line, a good low equivalent-
series-resistance (ESR) ceramic capacitor,
typically 1 μ F, is placed as close as possible to
the device each VDD and PVDD pin for the best
operation. For filtering lower frequency noise
signals, a large ceramic capacitor of 10 μ F or
greater placed near the audio power amplifier is
recommended.
How to Reduce EMI
Most applications require a ferrite bead filter for
EMI elimination shown at Figure 1. The ferrite
filter reduces EMI around 1MHz and higher.
When selecting a ferrite bead, choose one with
high impedance at high frequencies, but low
impedance at low frequencies.
Ferrite Bead
OUT+
200pF
Ferrite Bead
OUT-
200pF
Figure 1: Ferrite Bead Filter to Reduce EMI
Shutdown operation
In order to reduce power consumption while not
in use, the PAM8303D contains shutdown
circuitry that is used to turn off the amplifier’s
bias circuitry. This shutdown feature turns the
Power Analog Microelectronics
,
Inc
www.poweranalog.com
09/2008 Rev 1.4
12
PAM [ POWER ANALOG MICOELECTRONICS ]
