Rx
R
(SNUBBER)
Cx
C
(SNUBBER)
C
(BYPASS)
0.1 µF
PVDD
High-side FET current-loop
Low-side FET
current-loop
Q2
Low-side FET
Q1
High-side FET
TAS54x4C Hardware Design Guidelines
1.4.1
Snubbers
A snubber is an RC network placed at the output of the amplifier to perform several tasks. The snubber
will damp any ringing or overshoot on the PWM output waveform. Overshoot can over voltage stress the
MOSFETs. The overshoot and ringing are also potential sources of EMI. The overshoot and ringing are
caused by the stray inductance in the IC leads, IC bondwires, and PCB traces. The current stored in these
inductances must have a low impedance drainage path to ground. If a path is not provided the stored
current will find a path through parasitic capacitance on the PCB and cause the overshoot and ringing.
Another property of the snubber is the reduction of the total harmonic distortion (THD) of the amplifier. The
overshoot and ringing at the output is present in the feedback signal to the amplifier. The amplifier must
then try to eliminate this overshoot and ringing from the signal. The amplifier cannot completely remove
this signal which is then present on the output as distortion.
To calculate the proper output snubber, measure the voltage spike at the output pin. Use section 4 in
Voltage Spike Measurement Technique and Specification
,
, as a reference on performing this
measurement.
shows the basic output circuit. A bypass capacitor, C
(BYPASS)
, must be included in the design
because it is part of the current path for snubbing the inductance of the high-side FET. The terminals of
C
(BYPASS)
must be close to the PVDD pins and the PGND pins of the IC. R
x
and C
x
should be close to the
output pin and the PGND pins of the IC. This necessary to reduce the series inductance of the PCB
traces. The current loops that are formed by R
x
and C
x
are labeled in
as
High-side FET current-
loop
and
Low-side FET current-loop
. If R
x
and C
x
are not present, the current stored in the drain, source,
and lead inductances have no place to sink to during dead time. This current then flows through parasitic
capacitances on the PCB and appears on the waveform as ringing. Good control of the high-side and low-
side current loops is necessary for good over-voltage protection because of spikes and for good EMI
results.
Figure 3. One-Half of the Typical Output Stage With Snubbers
Use a value of R
x
that is a little higher than the estimate value. A good starting R
x
value for the
TAS5414C and TAS5424C device is 10
Ω
. Use surface mounted devices (SMD) because the series
inductance (ESL) is low.
For C
x
select a small value of 470 pF to 1000 pF. Also use SMD parts. Use the techniques listed in this
design guide to measure the spike and the associated ringing. Measure the frequency of the ringing. If
there is no ringing, use a higher value resistor for R
x
or a smaller capacitor for C
x
. The final C
x
should be
labeled as C1 and the ringing frequency is ƒ1.
8
TAS54x4C Design Guide
SLOA196 – June 2014
Copyright © 2014, Texas Instruments Incorporated