In
d
u
c
ta
n
c
e
(µ
H
)
Current and Q factor (A)
10 µH
5 µH
4 A
8 A
Q = 35
Q = 15
Q = 60
Causes High THD
Core Saturation
Initial Inductance
2 A
0 A
6 A
10 A
11 µH
9 µH
8 µH
7 µH
6 µH
4 µH
3 µH
2 µH
1 µH
High Q
Moderate Q
Low Q
L
co
R
Q
¦
/
u S u
u
2
L
2
co
L
R
2
Q
L
C
L
¦
4
R
u
u
u S u
u
TAS54x4C Hardware Design Guidelines
The calculation of these components is based on the nominal load impedance R
L
and the desired
frequency response. The frequency response is dependent on two values, the cutoff frequency and a
characteristic of the roll-off.
where
•
Q = a characteristic of the ƒ
CO
roll-off
•
ƒ
CO
= the cutoff frequency
(6)
For a typical amplifier response the value of Q should be 0.7071
The unloaded condition of the TAS54xx family of devices must be understood. The Q of the filter is
directly proportional to the load. In an unloaded condition the load resistance is infinity and therefore the
value of Q would be infinity also. See
(7)
The peak of the Q value is located at ƒ
CO
. The signal at this frequency is greatly amplified and can be
measured on an oscilloscope. This signal is seen as a sine wave and can be mistaken as an oscillating
amplifier.
1.5
Component Selection
1.5.1
Inductors
When the inductance value has been determined, three additional specifications must still be considered
when specifying an inductor. Aside from the inductance, the Q factor, current rating, and DC resistance
(RDC) values must be considered. Do not confuse the inductor Q with filter Q; these are two different
parameters.
The inductance value is usually specified at 0 A which is typically called the initial inductance. A typical
tolerance is 20%. As current increases through the inductor, the inductance eventually decrease because
of the Q Factor.
Figure 5. Inductance Versus Current and Q Factor
10
TAS54x4C Design Guide
SLOA196 – June 2014
Copyright © 2014, Texas Instruments Incorporated
