Diodes PAM8004 - Manual (Page 7)

PAM8004
Document number: DSxxxxx Rev. 1 - 0
7 of 11
www.diodes.com
November 2012
© Diodes Incorporated
PAM8004
A Product Line of
Diodes Incorporated
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
secind stage’s is internally fixed. The closed-loop gain of the first stage is set by selecting the ratio of RF to RI 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
)] R / R ( * 2 log[ * 20 A
I F VD

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 of noise on the power supply leads. For higher frequency transients, spikes, or digital hash on the line, a good low equivalent-
series-resisitance (ESR) ceramic capacitor, typically 1.0µF, works best, placing it as close as possible to the device VDD 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 attenu ation. 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 (CI) and input resistance (RI) of the amplifier form a high-pass filter with the corner frequency determined by equation below.
C R 2
1
f
I I
C 

In addition to system cost and size, click and pop perfor mance is affected by the size of the input the coupling capacitor, C
I
. A larger input
coupling capacitor requires more charge to reach its quiescent DC voltage (nominally ½ 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. During start-up or recovery
from shutdown mode, CBYP 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 (CBYP) 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 leavingshutdown.
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 out puts are disabled immediately. If the short was removed, the device
activates again.
Over Temperature Protection
Thermal protection on the PAM8004 prevents the device from damage when the internal die temperature exceeds +140°C. There is a 15 degree
tolerance on this trip point from device to device. Once the die temperature exceeds the thermal set point, the device outputs are disabled. This
is not a latched fault. The thermal fault is cleared once the temperature of the die is reduced by +30°C. This large hysteresis will prevent motor
boating sound well and the device begins normal operation at this point without external system intervention.