TAS54x4C Hardware Design Guidelines
The Q factor is defined as the ratio of the total energy in a system to the energy lost per cycle. Therefore,
depending on the Q factor, inductance will vary with current in different ways. This variation gives both
high-Q and low-Q inductors certain advantages that ultimately become trade-offs or constraints in a
design. As an example,
shows three inductance curves for three inductors with different Q
factors.
A high-Q inductor has little change in inductance before the inductor saturates. While a high-Q inductor
provides flat inductance versus current response and good idle current, it can saturate too quickly. As a
result, the inductance value may drop too low and cause the switching current to increase rapidly which
possibly triggers the current protection circuitry prematurely.
A low-Q inductor does not saturate quickly. Instead, a low-Q inductor stores more energy but becomes
hotter than a high-Q inductor. The inductance value begins dropping immediately with current. The filter
characteristics change with current, the switching frequency is not attenuated as well, and the idle current
will be higher. A low Q is found to have an adverse effect on the THD of an amplifier.
A good Q value is often somewhere between a high-Q and low-Q inductor, so that the inductance is stable
up to a high current level and does not have the THD effects of a low-Q device. The core saturation
occurs but not quickly, allowing for proper filtering and amplifier functionality during high current peaks.
The current rating is typically defined as the current where the inductance falls to 20% of the initial
inductance. Ensure that the inductance does not drop below a minimum value (approximately 20% of the
value) at the maximum current of the amplifier. If the current-rating value is not tested by the
manufacturer, testing is required to determine if the inductor can be used.
The DC resistance (RDC) is the series resistance of the copper wire at room temperature. This resistance
increases with temperature. As this resistance increases, less power is delivered to the loudspeakers and
more power is dissipated in the voice coil creating more heat in the voice coil, increasing RDC further. The
RDC is determined by the amplifier power and nominal speaker impedance. A 25-W rated amplifier into a
4-
Ω
speaker should use an inductor with a maximum RDC of 25 m
Ω
. When either amplifier power
increases or speaker impedance decreases, the RDC of the inductor must decrease.
Other losses occur that decrease the power that is delivered to the speaker. The ferrite or powdered iron
core loses a small amount of power in the core to generate the flux or B-field which is called core loss.
The core loss is constant over the current rating of the inductor. Typically, the core loss is the dominating
loss at idle and is also temperature dependent.
Parallel impedance can provide an indication of the idle power losses. Parallel impedance is a
measurement that can indicate the lumped losses in the inductor which includes the core loss. A high
resistance indicates a better inductor. An inductor with a parallel resistance of 50
Ω
is not a good inductor
for Class-D amplifier use, whereas an inductor with 250
Ω
is considered a good inductor. This parameter
is typically measured with an impedance bridge set in the parallel mode.
Now that the inductor can be specified, the specifications over temperature must be understood by the
user. At high current, the inductance typically decreases dramatically as temperature increases.
Temperature rise is because of self-heating from copper wire loss and core loss. The inductor can also be
heated from external heat sources. Therefore, evaluating the inductor for the specifications over
temperature is necessary.
1.5.2
Capacitors
The capacitor material for the LC filters must create a stable capacitance over voltage and temperature.
The best materials are the film and foil, and metalized film. These types of capacitors offer the best
stability, but unfortunately can be large and costly. Ceramic capacitors with X7R materials function just as
well as film capacitors over voltage and temperature and have a much better cost and size.
11
SLOA196 – June 2014
TAS54x4C Design Guide
Copyright © 2014, Texas Instruments Incorporated
