PAM8004 Datasheet, PDF (8/11 Pages) Power Analog Micoelectronics – Filterless 3W Class-D Stereo Audio Amplifier

PAM8004
Filterless 3W Class-D Stereo Audio Amplifier
Application Information
Maximum Gain
As shown in block diagram (page 2), the
PAM8403 has two internal amplifier stages. The
first stage's gain is externally configurable, while
the second stage's is internally fixed. The
closed-loop gain of the first stage is set by
selecting the ratio of R_f to R_i while the second
stage's gain is fixed at 2x.The output of amplifier
1 serves as the input to amplifier 2, thus the two
amplifiers produce signals identical in
magnitude, but different in phase by 180°.
Consequently, the differential gain for the IC is
A_VD=20*log [2*(R_f/R_i)]
The PAM8403 sets maximum R_f=142kΩ,
minimum R_i=18kΩ, so the maximum closed-gain
is 24dB.
Power supply decoupling
The PAM8004 is a high performance CMOS
audio amplifier that requires adequate power
supply decoupling to ensure the output THD and
PSRR as low as possible. Power supply
decoupling affects low frequency response.
Optimum decoupling is achieved by using two
capacitors of different types targeting to 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
1.0µF, works best, placing it as close as possible
to the device V_DD terminal. For filtering lower-
frequency noise signals, a large capacitor of
20µF (ceramic) or greater is recommended,
placing it near the audio power amplifier.
Input Capacitor (C_i)
Large input capacitors are both expensive and
space hungry for portable designs. Clearly, a
certain sized capacitor is needed to couple in low
frequencies without severe attenuation. But in
many cases the speakers used in portable
systems, whether internal or external, have little
ability to reproduce signals below 100Hz to
150Hz. Thus, using a large input capacitor may
not increase actual system performance. In this
case, input capacitor (C_i) and input resistance
(R_i) of the amplifier form a high-pass filter with
the corner frequency determined by equation
below.
1
fC=
2πRiCi
In addition to system cost and size, click and pop
performance is affected by the size of the input
coupling capacitor, C_i. A larger input coupling
capacitor requires more charge to reach its
quiescent DC voltage (nominally 1/2 V_DD). This
charge comes from the internal circuit via the
feedback and is apt to create pops upon device
enable. Thus, by minimizing the capacitor size
based on necessary low frequency response,
turn-on pops can be minimized.
Analog Reference Bypass Capacitor (C_BYP)
The Analog Reference Bypass Capacitor (C ) _BYP is
the most critical capacitor and serves several
important functions. DuAring start-up or recovery
from shutdown mode, C_BYP determines the rate at
which the amplifier starts up. The second
function is to reduce noise caused by the power
supply coupling into the output drive signal. This
noise is from the internal analog reference to the
amplifier, which appears as degraded PSRR and
THD+N.
A ceramic bypass capacitor (C ) _BYP with values of
0.1µF to 1.0µF is recommended for the best THD
and noise performance. Increasing the bypass
capacitor reduces clicking and popping noise
from power on/off and entering and leaving
shutdown.
Under Voltage Lock-out (UVLO)
The PAM8004 incorporates circuitry designed to
detect low supply voltage. When the supply
voltage drops to 2.0V or below, the PAM8004
outputs are disabled, and the device comes out
of this state and starts to normal function when
V_DD ≥2.2V.
Short Circuit Protection (SCP)
The PAM8004has short circuit protection
circuitry on the outputs to prevent damage to the
device when output-to-output or output-to-GND
short occurs. When a short circuit is detected on
the outputs, the outputs are disabled
immediately. If the short was removed, the
device activates again.
Power Analog Microelectronics,Inc
www.poweranalog.com
8
08/2010 Rev1.0