Texas Instruments | TAS5421-Q1 Class-D Output Snubber Design | Application notes | Texas Instruments TAS5421-Q1 Class-D Output Snubber Design Application notes

Texas Instruments TAS5421-Q1 Class-D Output Snubber Design Application notes
Developer's Guide
SLOA201 – March 2015
Class-D Output Snubber Design Guide
The design for a class-D audio system sometimes requires a snubber circuit on the output. This design
guide includes what a snubber circuit does and how to design one if needed.
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Contents
What Is an Output Snubber? ............................................................................................... 1
System-Level Impact of Ringing and Overshoot ........................................................................ 1
Designing an Output Snubber.............................................................................................. 1
List of Figures
1
1
One-Half of the Typical H-Bridge Output Stage With Snubbers ....................................................... 2
What Is an Output Snubber?
An output snubber is an RC network placed at the output of a switching audio amplifier. The snubber
dampens any ringing or overshoot on the PWM output waveform. The stray inductance in the IC leads, IC
bond wires, and PCB traces causes the overshoot and ringing. Having an output snubber provides a lowimpedance drainage path to ground for the energy stored in these inductances. Without a provided path,
the stored current finds a path through parasitic capacitance on the PCB and causes the overshoot and
ringing.
2
System-Level Impact of Ringing and Overshoot
Overshoot can stress the output MOSFETs of a class-D device by overvoltage. The overshoot and ringing
are also potential sources of EMI. The snubber also improves the total harmonic distortion (THD) of the
amplifier. The overshoot and ringing at the output are 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.
3
Designing an Output Snubber
To design the proper output snubber, measure the voltage spike at the output pin. Use Section 4 in
Voltage Spike Measurement Technique and Specification, SLEA025, as a reference in performing this
measurement.
Figure 1 shows the basic output circuit. Inclusion of a bypass capacitor, C(BYPASS), is necessary 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 power pins and the ground pins of the IC. R(x) and C(x) should be
close to the output pin and the ground pins of the IC. This is necessary to reduce the series inductance of
the PCB traces. Figure 1 has labels of High-side FET current-loop and Low-side FET current-loop for the
current loops formed by R(x) and C(x). If R(x) and C(x) are not present, the current stored in the drain, source,
and lead inductances has no place to sink 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 lowside current loops is necessary for good protection from overvoltage spikes and for good EMI results.
SLOA201 – March 2015
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Class-D Output Snubber Design Guide
Copyright © 2015, Texas Instruments Incorporated
1
Designing an Output Snubber
www.ti.com
Supply
High-side FET current-loop
Q1
High-side FET
R(x)
Low-side FET
current-loop
Q2
Low-side FET
R(SNUBBER)
C(BYPASS)
0.1 µF
C(x)
C(SNUBBER)
Figure 1. One-Half of the Typical H-Bridge Output Stage With Snubbers
For R(x) start with a value of 10 Ω and use a surface-mounted device (SMD) to keep the series inductance
(ESL) low. For C(x) select a small value of 470 pF to 1000 pF, and also an SMD. Use the techniques listed
in SLEA025 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 C(1) and the ringing frequency is f(1).
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. Label the value of C(x) as C(2) and the ringing frequency as f(2).
Use Equation 1 to calculate the value for L.
é
ùé 1
1
1 ù
úê
ú
L=ê
ê (C(2) - C(1) ) ´ 4 ´ p 2 ú ê f(2) 2 f(1) 2 ú
ë
ûë
û
(1)
where L is the value of the stray inductance that requires snubbing.
L is a bulk inductance and is not any individual inductance.
Find the appropriate values of C(x) and R(x). Use Equation 2 to calculate the appropriate R(x).
R ( x ) = 2 ´ p ´ f( x ) ´ L
(2)
The ringing frequency f(x) is for a given C(x). If the application is to use C(1) for the snubber capacitor then
use f(1) in the equation to calculate the proper R(x) (or use C(2) and f(2)).
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 R(x) could allow for a spike, but too low a value for R(x) could cause the
snubber to draw excessive current and overheat. Use Equation 3 to calculate the power loss in the
resistor.
P = C( x ) ´ V 2 ´ f(S)
(3)
where:
• V is the supply voltage
• f(S) is the switching frequency
2
Class-D Output Snubber Design Guide
SLOA201 – March 2015
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