Silicon Laboratories SI824XCLASSD, SI8410, SI8241 User Manual
Si824xClassD-KIT
CLASS D AUDIO AMPLIFIER REFERENCE DESIGN USER ’S GUIDE
1. Kit Contents
The Si824x Class D Audio Amplifier Reference Design Kit contains the following items:
Audio Class D Amplifier reference design board featuring:
Si8241 ISOdriver
Si8410 Digital Isolator
2. Introduction
This User's Guide discusses the Silicon Labs Audio Class D amplifier reference design, a stereo, two-state, halfbridge Class D amplifier leveraging the performance advantages of the Si8241 ISOdriver. This two-channel Class
D Audio Amplifier delivers 120 W per channel into 8 , while enabling < 0.02% THD at 60 W and > 95 dB SNR.
High-power audio systems are adopting digital audio technology and evolving toward lower-power, “green”
products that meet Energy Star™ guidelines. Class-D audio systems have started penetrating the high-end
markets where low noise, extremely good THD performance, and very high ou tput power are requir ed in consumer
and industrial applications. The Si824x audio driver family consists of single-chip, isolated, Class-D gate drivers
powering audio systems delivering from 30 W up to 1000 W of audio power.
Occasionally, a new IC is introduced that challenges the current technological hegemony. With features that make
these products the perfect drivers for Class D amplification, the Silicon Labs Si8241/44 Audio Gate Drivers
represent a new standard for the Class D amplifier industry. Based on Silicon Labs' proprietary isolation
technology, the Si824x audio drivers incorporate input-to-output isolation that enables level-translation of signals
without additional external circuits. The Si824x audio drivers feature adjustable dead-time control for achieving
optimal THD, overlap protection that safeguards against shoot-through current damage, robust immunity to latchup and high-voltage transients, and lower BOM costs and PCB footprints compared to non-isolated gate drive
solutions. A typical audio driver application using the Si824x is shown in Figure 1. For more information on the
Si824x audio driver, refer to the Si824x datasheet at www.silabs.com/audio.
Figure 1. Si824x-Based Class D Audio Driver Block Diagram
Rev. 0.2 11/10 Copyright © 2010 by Silicon Laboratories Si824xClassD-KIT
Si824xClassD-KIT
3. Hardware Overview and Demo
3.1. Reference Design Board Architecture
The Silicon Labs Class D reference design architecture uses a phase-shift, self-oscillating modulation approach
that is capable of achieving far greater signal-to-noise ratio than clock driven amplifiers (see Figure 2). This selfoscillating implementation eliminates the circuitry necessary to generate the triangle waveform. To keep the circuit
as simple as possible, a two-state, half-bridge is implemented and exemplifies the benefits of using the Si8241
Audio Gate Driver.
Figure 2. Block Diagram of Si8241-Based Class D Amplifier
3.2. Setting up the Class D Amplifier
Figure 3 illustrates connections necessary to power the Class D amplifier. The user will need the following items:
MP3 player (or equivalent line out audio source)
Audio Y-Cable (stereo male to Dual RCA Male)
±50 V supply capable of sourcing 3 Amps
±12 V supply capable of sourcing 500 mA
Two speakers (up to 150 W)
Silicon Labs Class D reference design board
Warning:
1. To protect the amplifier and speakers, before turning on the main power (the ±50 V supplies) to the amplifier, ensure that
the audio source from the MP3 player has its volume set to the lowest possible level.
2. The L-bracket can heat up to 80 °C and should be handled with care.
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Si824xClassD-KIT
Speaker B
(up to 150 W)
_
_
Speaker A
(up to 150 W)
Audio Source/
MP3 Player
+12 V (500 mA)
-12 V (500 mA)
+50 V (3 A)
-50 V (3 A)
PGND
GND
3.3. Powering up the Class D Amplifier and Playing Music
After the audio source, audio Y-cable, supplies, and speakers have been properly connected as shown in Figure 3.
1. Turn on the ±12 V supplies.
2. Then, turn on the ±50 V supplies. Notice that LED1 should turn on for abou t 1 sec and then turn of f. This on and
off sequence clears any overcurrent protection faults and indicates to the user that the amplifier is ready to
amplify the audio input signal. If the LED does not turn off after about 1 sec, cycle on and off the ±12 V supplies.
Repeat this step until the LED turns off. If the LED never turns off, the board has probably been damaged.
3. With the audio source's volume turned down to its lowest output level, start the audio source.
4. Adjust the volume to the desired listening level.
5. Enjoy your music!
Note: If the power being delivered to the speakers is too great, the overcurrent protection circuitry will trip and shut
off audio to the speakers, protecting the amplifier. LED1 will turn on in this condition. See "4.4. Overcurrent
Protection" on page 4 for more details on clearing this fault condition.
Figure 3. Si824xClassD-KIT (Top V iew)
Figure 4. Si824xClassD-KIT (Bottom View)
Rev. 0.2 3
Si824xClassD-KIT
Si8241
ISOdriver
-38V
PWM
NC
VDDI
GNDI
DISABLE
DT
NC
VDDI
VDDA
VOA
GNDA
NC
NC
VDDB
VOB
GNDB
-50V
+50V
C1
C3
PWM
+5V
C2
SHDN_HI
R3
C4
D1
R1
R2
-38V
4. Additional Features and Architectural Considerations
4.1. Gate Drive Structure
Figure 5 illustrates the ease with which the Si8241 can drive a two-state, half-bridge class D amplifier. The boot
supply tied to D1 must be 12 V higher than the –50 V reference (–38 V) so that the MOSFETs each have a 12 V
drive signal. The closed loop gain of the Silicon Labs Class D reference design is implemented such that
approximately 1 Vpp input will yield full output power into an 8 load.
Figure 5. Si8241 Audio Gate Driver Gate Drive Circuit
4.2. Self Oscillation
The amplifier is self-oscillating, enabling its signal-to-noise ratio to far exceed that of a clock driven system. The
main mechanism for this is the delta-sigma effect of shifting in-band noise to a much higher out-of-band frequency.
The amplifier is a basic, phase-shift type, which has significant advantages over an amplifier running as a
hysteretic oscillator. There is a pole in the forward path G(s) and a pole in the feedback path H(s). The 180 ° phase
shift, coupled with the transport delay, yields an oscillation frequency of nearly 500 kHz. The frequency of
oscillation is set by capacitors in each audio channel where reducing capacitance value increases oscillation
frequency. Tight tolerance capacitors are used to keep the channel frequencies as close to each other as possible.
4.3. Heat Sink L-Bracket
The amplifier design includes an L-bracket to sink excess heat from the power transistors. At full power, the Lbracket's temperature should increase to no higher than 80 degrees Celsius.
4.4. Overcurrent Protection
The Silicon Labs Class D reference design has an overcurrent protection circuit consisting of a low-power
comparator floating off the upper and lower bus voltages. The upper rail circuit is shown in Figure 6 and is
duplicated on the lower rail. It monitors the current flowing through the 0.005 resistor (RSENSE). Zener diode D1
and resistor R4 supply power to the comparator and the Silicon Labs Si8410 digital isolator. The Si8410 performs
the necessary level shifting to interface to the shutdown circuitry. Th e circuit is set to trip at roughly a 20 A fau lt,
usually caused by a short-circuit acro ss the spe aker term inals or a lar ge over drive sign al at the a udio inpu ts. Note
the upper and lower overcurrent circuits are ORed together through a pair of diodes and sent to the reset control
circuit. The normally low Si8410 A1 input is driven high upon detection of an overcurrent cond ition a nd ass erts the
SHUTDOWN signal, forcing the reset controller to assert a reset signal, momentarily halting amplifier operation.
The reset control circuit attempts restart after one second, and, if the fault is still present, again cycles reset in
“hiccup” mode with a frequency of one second. This process continues until the fault is removed. Overcurrent
protection can be removed by uninstalling JP1 and JP2.
4 Rev. 0.2
Si824xClassD-KIT
+
-
C1
R2
R4
R3
Si8410
Digital Isolator
R5
R1
RSENSE
GND2
GND2
GND1
A1
VDD1
VDD1
To High Side
MOSFET
VDD2
B1
5V
SHUTDOWN
D1
50V
Figure 6. Over Current Protection Circuit
4.5. Undervoltage Protection
The undervoltage protection comparator monitors the positive bus voltage and releases the undervoltage lockout
when the voltage is above 37 V . The amplifier starts up after a one-second delay. Note that the red LED remains lit
when the amplifier is in shutdown mode and turns off when the amplifier is enabled.
4.6. Other Features
A protection circuit jumper (JP 3) option is included that allows the amplifier to be manually shut down. This jumper
can be replaced with a switch or other control circuit, allowing the amplifier to be muted. The one-second
undervoltage lockout delay allows the op-amps and comparator to settle before the shut-down circuit is released,
thereby preventing speaker p ops. To aid in system performance evaluation, there are individual jumper options
(JP1 and JP2) on each channel that allow the user to enable or disable each channel independently of the other.
4.7. Performance
The Silicon Labs Class D reference design board was tested for THD + N, SNR, DFD, and IFD with an Audio
Precision analyzer. During these tests, the main power (the ±50 V supplies) used a TR 180 fr om Hy pex e lec troni cs
(a low-noise supply) to maximize the amplifier's measured performance. Refer to “AN542: Performance
Improvements in Class D Audio Amplifiers using Si824x Audio Drivers” for additional information concerning the
amplifier's noise and efficiency performance and a more in-depth discussion concerning the amplifier's
architectural design. Please visit www.silabs.com/audio.
Rev. 0.2 5
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