Texas Instruments TPA005D02 - Manual (Page 25)

The TPA005D02 Audio Power Amplifier Evaluation Module
3-11 Details
3.2.3.3 Power Filtering
Power supply considerations include power supply decoupling and high
frequency bypass loops. Electrolytic capacitors are used for decoupling and
ceramic or mica capacitors are used for high frequency bypass applications.
Decoupling capacitors serve to smooth the input voltage and assist the
amplifier by providing current when needed. These capacitors may shunt
relatively large ripple currents to ground and must have a low equivalent series
resistance (ESR) to reduce power and heat dissipation in the device. The ESR
combines all losses, both series and parallel, in a capacitor at a given
frequency in order to reduce the equivalent circuit to a simple RC series
connection, valid only for low frequencies (less than 1 MHz).
Other considerations are the voltage rating, capacitance, physical size, and
the specific type of capacitor. The voltage rating should exceed the maximum
supply voltage expected in order to handle voltage surges and spikes without
being damaged. The capacitance is then important, as it specifies the amount
of energy that can be stored in the capacitor. Once the voltage rating and
capacitance are known, the size can be determined.
Since the focus was to get the largest capacitance possible yet keep the size
to a minimum, tantalum capacitors, instead of aluminum electrolytic
capacitors, were chosen. Tantalum capacitors provide a higher capacitance
value in a smaller package and have lower ESR values than aluminum
electrolytic capacitors. SMT packages further reduce the inductance
associated with lead lengths. All of these considerations led to the selection
of a 220
µ
F SMT tantalum capacitor as the primary decoupling capacitor.
The high frequency bypass capacitors are usually small in size, limited by the
size of the capacitance to approximately 10
µ
F or less. Ceramic capacitors
have extremely low ESR and dissipate very little power. Lower ESR means a
lower net impedance at higher frequencies, which is more suitable for filtering
the higher frequency components of the power supply, especially voltage
spikes. Bypass capacitors should be placed as close as possible to the IC
power input pins and also as close to the IC power ground pins as possible.
The idea is to form the smallest possible loop, or path, over which the high
frequency signals can travel, and minimize the impedance. A short path with
a high impedance defeats the purpose.
The power pins (VDD) were placed at the top and bottom of the IC package,
and the power traces and filtering capacitors were arranged to balance the left
and right channels of the IC. A trace along the bottom of the board links the
V
DD
pins for EVM stand-alone operation and supplies power to the right and
left channels in parallel. This places the capacitors and their ESR in parallel,
increasing the overall capacitance seen by the power source while greatly
reducing the ESR.