C
(DM)
C
(CM)
C
(CM)
Load (R
L
)
L
L
2
2
2
2
1
1
1
1
L
& &
¦
¦
ª
º ª
º
«
»
«
»
u u S
«
» «
»
¬
¼
¬
¼
TAS54x4C Hardware Design Guidelines
Change C
x
to a value that is about 1.5 to 2 times the previous value. Keep R
x
the same. Again, measure
the frequency of the ringing on the waveform. If no ringing is available to measure, change C
x
to a slightly
smaller value. The value of C
x
should be labeled as C2 and the ringing frequency is ƒ2.
Use
to calculate the value for L.
where
•
L is the value of the stray inductance that should be snubbed
(3)
L is a bulk inductance and is not any individual inductance.
Find the appropriate values of C
x
and R
x
. Use
to calculate the appropriate R
x
.
R
x
= 2 ×
π
× ƒ
x
× L
where
•
ƒ
x
corresponds with the selected value of C
x
(4)
If C1 is used then use ƒ1, and if C2 is used, then use ƒ2.
To account for tolerances and differences in production units, use a value that is 0.7 to 0.8 of the
calculated R
x
. Too high a value for the R
x
could allow for a spike, but too low of a value for R
x
could
cause the snubber to draw excessive current and overheat. Use
to calculate the power loss in
the resistor.
P = C
x
× V
2
× ƒ
S
where
•
V is the voltage at PVDD
•
ƒ
S
is the switching frequency
(5)
1.4.2
Demodulation Filter Design
An output LC-demodulation filter is required to reconstruct the audio signal and protect the amplifier. The
components must be carefully selected for proper operation of the Class-D amplifier. The inductor is
especially of concern because an improper inductor can cause a class-D amplifier to shutdown or fail.
The circuit consists of two inductors and three capacitors as shown in
. The main demodulation
filter consists of the two inductors, L, and the difference mode capacitor, CDM. The two common mode
capacitors, CCM, filter any common mode signals from the switching waveform.
Figure 4. Typical Output Demodulation Filter
9
SLOA196 – June 2014
TAS54x4C Design Guide
Copyright © 2014, Texas Instruments Incorporated
