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IRAUDAMP4
120 W x 2 Channel Class D Audio Power Amplifier Using IRS20955 and IRF6645
By Johan Strydom, Jun Honda, and Jorge Cerezo
Table of Contents
Page Introduction .......................................................................................... 1 Specifications ....................................................................................... 2 Functional Description.......................................................................... 4 Startup and Shutdown..........................................................................12 Protection .............................................................................................16 Typical Performance ............................................................................21 Design Documents ..............................................................................27
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Introduction
The IRAUDAMP4 reference design is an example of a two-channel 120 W half-bridge Class D audio power amplifier. The reference design will demonstrate how to use the IRS20955, implement protection circuits, and design an optimum PCB layout using the IRF6645 DirectFET MOSFETs. The resulting design requires no heatsink for normal operation (one-eighth of continuous rated power). The reference design contains all the required housekeeping power supplies for ease of use. The two-channel design is scalable, for power and the number of channels.
Applications
AV receivers Home theater systems Mini component stereos Sub-woofers
Features
120 W x two channels, Total Harmonic Distortion (THD) = 1%, 1 kHz Residual Noise: 52 µV, IHF-A weighted, AES-17 filter Distortion: 0.004% THD+N @ 60 W, 4 Efficiency: 96% @ 120 W, 4 , single-channel driven, Class D stage Multiple Protection Features: Over-current protection (OCP), Over-voltage protection (OVP), Under-voltage protection (UVP), DC-protection (DCP), Over-temperature protection (OTP) PWM Modulator: Self-oscillating half-bridge topology with optional clock synchronization Output Power:
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Specifications
General Test Conditions (unless otherwise noted) Supply Voltage ±35 V Load Impedance 4 Self-Oscillating Frequency 400 kHz Gain Setting 26.8 dB Electrical Data IR Devices Used Notes / Conditions
No input signal 1 Vrms input yields rated power
Typical Notes / Conditions IRS20955 gate driver, IRF6645 DirectFET MOSFET Modulator Self-oscillating, second order sigma-delta modulation, analog input Power Supply Range ± 25 - 35 V Output Power CH1-2: (1% THD+N) 120 W 1 kHz Output Power CH1-2: (10% THD+N) 170 W 1 kHz Rated Load Impedance 4 Supply Current 100 mA No input signal Total Idle Power Consumption 7W No input signal Single-channel driven, Channel Efficiency 96% 120 W, Class D stage Audio Performance THD+N, 1 W THD+N, 10 W THD+N, 60 W Dynamic Range Residual Noise, 20Hz - 20 kHz BW, A-Weighted Damping Factor Channel Separation Frequency Response : 20Hz-20 kHz : 20Hz-40 kHz Thermal Performance Idling 2ch x 15 W (1/8 rated power) 2ch x 120 W (rated power) Physical Specifications Dimensions Typical / Class D* 0.005% 0.002% 0.002% 0.001% 0.004% 0.003% 113 dB 70 µV 50 µV 170 95 dB 80 dB ±1 dB ±3 dB Typical TC =30 °C TPCB=37 °C TC =54 °C TPCB=67 °C TC =80 °C TPCB=106 °C 5.8 in (L) x 5.2 in (W) 120 dB 40 µV 20 µV 2000 100 dB 85 dB Notes / Conditions 1 kHz, Single-channel driven A-weighted, AES-17 filter, Single-channel operation Self-oscillating 400 kHz Internal clock 300 kHz 1 kHz, relative to 4 load 100 Hz 10 kHz 1W, 4 - 8 Load Notes / Conditions No signal input, TA=25 °C Continuous, TA=25 °C At OTP shutdown @ 150 s, TA=25 °C
Note: Specifications are typical and not guaranteed *Class D refers to audio performance measurements of the Class D output power stage only, with preamp and output filter bypassed.
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Connection Diagram
35 V, 5 A DC supply 35 V, 5 A DC supply
250 W, Non-inductive Resistors
4
J3 G J7 TP1 TP2
4
J4
CH1 Output
LED
CH2 Output
S1 J9
Protection
J6
J5
J8 S3 Volume
Normal
CH1 Input
CH2 Input
S2 R113
Audio Signal Generator
Figure 1. Typical Test Setup
Pin Description
CH1 IN CH2 IN POWER CH1 OUT CH2 OUT EXT CLK DCP OUT J6 J5 J7 J3 J4 J8 J9 Analog input for CH1 Analog input for CH2 Positive and negative supply (+B / -B) Output for CH1 Output for CH2 External clock sync DC protection relay output
Power-on and Power-off Procedure
Always apply or remove ±35 V bus supplies at the same time.
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Functional Description
Class D Operation
Referring to CH1 as an example, the op-amp U1 forms a front-end second-order integrator with C11, C13 & R25 + R29P. This integrator receives a rectangular feedback waveform from the Class D switching stage and outputs a quadratic oscillatory waveform as a carrier signal. To create the modulated PWM signal, the input signal shifts the average value of this quadratic waveform (through gain relationship between R13 and R31 + R33) so that the duty varies according to the instantaneous value of the analog input signal. The IRS20955 input comparator processes the signal to create the required PWM signal. This PWM signal is internally level-shifted down to the negative supply rail where this signal is split into two signals, with opposite polarity and added deadtime, for high-side and low-side MOSFET gate signals, respectively. The IRS20955 drives two IRF6645 DirectFET MOSFETs in the power stage to provide the amplified PWM waveform. The amplified analog output is re-created by demodulating the amplified PWM. This is done by means of the LC low-pass filter (LPF) formed by L1 and C23, which filters out the Class D switching carrier signal.
Feedback Daughter-board
+B
U1
U1
PWM Modulator and Level Shifter
Integrator
LPF
GND
IRS20955S Gate Driver
IRF6645 Direct-FET
-B
Figure 2. Simplified Block Diagram of Class D Amplifier
Power Supplies
The IRAUDAMP4 has all the necessary housekeeping power supplies onboard and only requires a pair of symmetric power supplies ranging from ±25 V to ±35 V (+B, GND, -B) for operation. The internally-generated housekeeping power supplies include a ±5 V supply for analog signal processing (preamp, etc.), while a +12 V supply (VCC), referenced to B, is included to supply the Class D gate-driver stage. For the externally-applied power, a regulated power supply is preferable for performance measurements, but not always necessary. The bus capacitors, C31 and C32 on the motherboard, along with high-frequency bypass-caps C15-C18 on daughter board, address the high-frequency ripple current that result from switching action. In designs involving unregulated power supplies, the designer should place a set of bus capacitors, having enough capacitance to handle the audio-ripple current, externally. Overall regulation and output voltage ripple for the power supply design are not critical when using the IRAUDAMP4 Class D amplifier as the power supply rejection ratio (PSRR) of the IRAUDAMP4 is excellent (Figure 3).
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+0 -10 -20 -30 -40 d B -50 -60 -70 -80 -90 20
-B +B
50
100
200
500
1k
2k
5k
10k
20k
40k
Hz
Figure 3. Power Supply Rejection Ratio (PSRR) for Negative (-B) and Positive (+B) Supplies
Bus Pumping
Since the IRAUDAMP4 is a half-bridge configuration, bus pumping does occur. Under normal operation during the first half of the cycle, energy flows from one supply through the load and into the other supply, thus causing a voltage imbalance by pumping up the bus voltage of the receiving power supply. In the second half of the cycle, this condition is reversed, resulting in bus pumping of the other supply. These conditions worsen bus pumping: Lower frequencies (bus-pumping duration is longer per half cycle) Higher power output voltage and/or lower load impedance (more energy transfers between supplies) Smaller bus capacitors (the same energy will cause a larger voltage increase) The IRAUDAMP4 has protection features that will shutdown the switching operation if the bus voltage becomes too high (>40 V) or too low (<20 V). One of the easiest countermeasures is to drive both of the channels out of phase so that one channel consumes the energy flow from the other and does not return it to the power supply. Bus voltage detection is only done on the B supply as the effect of the bus pumping on the supplies is assumed to be symmetrical in amplitude (although opposite in phase).
Input
A proper input signal is an analog signal below 20 kHz, up to ±3.5 V peak, having a source impedance of less than 600 . A 30 kHz to 60 kHz input signal can cause LC resonance in the output LPF, resulting in an abnormally large amount of reactive current flowing through the switching stage (especially at 8 or higher impedance towards open load), causing OCP activation. The IRAUDAMP4 has an RC network, or Zobel network, to damp the resonance and protect the board in such event, but is not thermally rated to handle continuous supersonic frequencies. These supersonic input frequencies therefore should be avoided. Separate mono RCA connectors provide input to each of the two channels. Although both channels share a common ground, it is necessary to connect each channel separately to limit noise and crosstalk between channels.
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Output
Both outputs for the IRAUDAMP4 are single-ended and therefore have terminals labeled (+) and (-) with the (-) terminal connected to power ground. Each channel is optimized for a 4 speaker load for a maximum output power of 120 W, but is capable of operating with higher load impedances (at reduced power), at which point the frequency response will have a small peak at the corner frequency of the output LC low pass filter. The IRAUDAMP4 is stable with capacitive-loading; however, it should be realized that the frequency response degrades with heavy capacitive loading of more than 0.1 µF.
Gain Setting / Volume Control
The IRAUDAMP4 has an internal volume control (potentiometer R108 labeled, "VOLUME") for gain adjustment. Gain settings for both channels are tracked and controlled by the volume control IC (U_2) setting the gain from the microcontroller IC (U_1). The maximum volume setting (clockwise rotation) corresponds to a total gain of +37.9 dB (78.8 V/V). The total gain is a product of the power-stage gain, which is constant (+23.2 dB), and the input-stage gain that is directly-controlled by the volume adjustment. The volume range is about 100 dB with minimum volume setting to mute the system with an overall gain of less than -60 dB. For best performance in testing, the internal volume control should be set to a gain of 21.9 V/V, or 1 Vrms input will result in rated output power (120 W into 4 ), allowing for a >11 dB overdrive.
Output Filter Design, Preamplifier and Performance
The audio performance of the IRAUDAMP4 depends on a number of different factors. The section entitled, "Typical Performance" presents performance measurements based on the overall system, including the preamp and output filter. While the preamp and output filter are not part of the Class D power stage, they have a significant effect on the overall performance. Output filter Since the output filter is not included in the control loop of the IRAUDAMP4, the reference design cannot compensate for performance deterioration due to the output filter. Therefore, it is there important to understand what characteristics are preferable when designing the output filter: 1) The DC resistance of the inductor should me minimized to 20 m or less. 2) The linearity of the output inductor and capacitor should be high with respect to load current and voltage. Preamplifier The preamp allows partial gain of the input signal, and in the IRAUDAMP4, controls the volume. The preamp itself will add distortion and noise to the input signal, resulting in a gain through the Class D output stage and appearing at the output. Even a few microvolts of noise can add significantly to the output noise of the overall amplifier. In fact, the output noise from the preamp contributes more than half of the overall noise to the system. It is possible to evaluate the performance without the preamp and volume control, by moving resistors R13 and R14 to R71 and R72, respectively. This effectively bypasses the preamp and connects the RCA inputs directly to the Class D power stage input. Improving the selection of preamp and/or output filter, will improve the overall system performance to approach that of the stand-alone Class D power stage. In the "Typical
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Performance" section, only limited data for the stand-alone Class D power stage is given. For example, results for THD+N vs. Output Power are provided, utilizing a range of different inductors. By changing the inductor and repeating this test, a designer can quickly evaluate a particular inductor.
100 TTTTTTT 10
1
%
0.1
0.01
0.001
0.0001 100m 200m 500m 1 2 5 W 10 20 50 100 200
Figure 4. Results of THD+N vs. Output Power with Different Output Inductors
Self-Oscillating PWM Modulator
The IRAUDAMP4 Class D audio power amplifier features a self-oscillating type PWM modulator for the lowest component count and robust design. This topology represents an analog version of a second-order sigma-delta modulation having a Class D switching stage inside the loop. The benefit of the sigma-delta modulation, in comparison to the carrier-signal based modulation, is that all the error in the audible frequency range is shifted to the inaudible upper-frequency range by nature of its operation. Also, sigmadelta modulation allows a designer to apply a sufficient amount of correction. The self-oscillating frequency is determined by the total delay time inside the control loop of the system. The delay of the logic circuits, the IRS20955 gate-driver propagation delay, the IRF6645 switching speed, the time-constant of front-end integrator (e.g. R25 + R29P, C11 and C13 for CH1) and variations in the supply voltages are critical factors of the self-oscillating frequency. Under nominal conditions, the switching-frequency is around 400 kHz with no audio input signal and a +/-35 V supply.
Adjustments of Self-Oscillating Frequency
The PWM switching frequency in this type of self-oscillating switching scheme greatly impacts the audio performance, both in absolute frequency and frequency relative to the other channels. In absolute terms, at higher frequencies, distortion due to switching-time becomes significant, while at lower frequencies, the bandwidth of the amplifier suffers. In relative terms, interference between channels is most significant if the relative frequency difference is within the audible range. Normally when adjusting the self-oscillating frequency of the different channels, it is best to either match the frequencies accurately,
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or have them separated by at least 25 kHz. With the installed components, it is possible to change the self-oscillating frequency from about 160 kHz up to 600 kHz.
Potentiometers for adjusting self-oscillating frequency R29P Switching frequency for CH1* R30P Switching frequency for CH2* *Adjustments have to be done at an idling condition with no signal input.
Switches and Indicators
There are three different indicators on the reference design: An orange LED, signifying a fault / shutdown condition when lit. A green LED on the motherboard, signifying conditions are normal and no fault condition is present. A green LED on the daughter board, signifying there is power. There are three switches on the reference design: Switch S1 is a trip and reset push-button. Pushing this button has the same effect of a fault condition. The circuit will restart about three seconds after the shutdown button is released. Switch S2 is an internal clock-sync frequency selector. This feature allows the designer to modify the switching frequency in order to avoid AM radio interference. With S3 is set to INT, the two settings "H" and "L" will modify the internal clock frequency by about 20 kHz to 40 kHz, either higher "H" or lower "L." The actual internal frequency is set by potentiometer R113 - "INT FREQ." Switch S3 is an oscillator selector. This three-position switch is selectable for internal self-oscillator (middle position "SELF"), or either internal ("INT") or external ("EXT") clock synchronization.
Switching Frequency Lock / Synchronization Feature
For single-channel operation, the use of the self-oscillating switching scheme will yield the best audio performance. The self-oscillating frequency, however, does change with the duty ratio. This varying frequency can interfere with AM radio broadcasts, where a constant-switching frequency with its harmonics shifted away from the AM carrier frequency, is preferred. In addition to AM broadcasts, multiple channels can also reduce audio performance at low power, and can lead to increased residual noise. Clock frequency locking/synchronization can address these unwanted characteristics. Please note that the switching frequency lock / synchronization feature is not possible for all frequencies and duty ratios, and operates within a limited frequency and duty-ratio range around the self-oscillating frequency (Figure 5).
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600
Locking range
500
Suggested clock frequency for maximum locking range
Operating Frequency (kHz)
400
300
200
Self-oscillating frequency
100
0 10%
20%
30%
40%
50%
60%
70%
80%
90%
Duty Cycle
Figure 5. Typical Lock Frequency Range vs. PWM Duty Ratio (Self-oscillating frequency set to 400 kHz with no input)
As illustrated by the THD+N Ratio vs. Output Power results (Figure 6) , the noise levels increase slightly when all channels are driven (ACD) with the self oscillator, especially below the 5 W range. Residual noise typically increases by a third or more (see "Specifications Audio Performance") compared to a single-channel driven (SCD) configuration. Locking the oscillator frequency results in lowering the residual noise to that of a single-channel-driven system. The output power range, for which the frequencylocking is successful, depends on what the locking frequency is with respect to the selfoscillating frequency. As illustrated in Figure 6, the locking frequency is lowered (from 450 kHz to 400 kHz to 350 kHz and then 300 kHz) as the output power range (where locking is achieved) is extended. Once locking is lost, however, the audio performance degrades, but the increase in THD seems independent from the clock frequency. Therefore, a 300 kHz clock frequency is recommended. It is possible to improve the THD performance by increasing the corner frequency of the high pass filter (HPF) (R17 and C15 for Ch1) that is used to inject the clock signal. This drop in THD, however, comes at the cost of reducing the locking range. Resistor values of up to 100 k and capacitor values down to 10 pF can be used. In the IRAUDAMP4, this switching frequency lock/synchronization feature is achieved with either an internal or external clock input (selectable through S3). If an internal (INT) clock is selected, an internally-generated clock signal will be used, adjusted by setting potentiometer R113 "INT FREQ." If external (EXT) clock signal is selected, a 0 V to 5 V square-wave (~50% duty ratio) logic signal must be applied to BNC connector J17.
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100
10
1
%
0.1 Self
Osc. (ACD)
Int. Clock @ 300 kHz
0.01
Int. Clock @ 450 kHz
0.001 100m 200m 500m 1 2
Self Osc. (Single Channel Driven)
5 10 20 50 100 200
Power (W)
Figure 6. THD+N Ratio vs. Output Power for Different Switching Frequency Lock/Synchronization Conditions
IRS20955 Gate Driver IC
The IRAUDAMP4 uses the IRS20955, which is a high-voltage (up to 200 V), high-speed power MOSFET gate driver with internal deadtime and protection functions specifically designed for Class D audio amplifier applications. These functions include OCP and UVP. A bi-directional current protection feature that protects both the high-side and lowside MOSFETs are internal to the IRS20955, and the trip levels for both MOSFETs can be set independently. In this design, the deadtime can be selected for optimized performance, by minimizing deadtime while limiting shoot-through. As a result, there is no gate-timing adjustment on the board. Selectable deadtime through the DT pin voltage is an easy and reliable function which requires only two external resistors, R11 and R9.
R11 R9
Figure 7. System-level View of Gate Driver IRS20955
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Selectable Deadtime
The IRS20955 determines its deadtime based on the voltage applied to the DT pin. An internal comparator translates which pre-determined deadtime is being used by comparing the DT voltage with internal reference voltages. A resistive voltage divider from VCC sets threshold voltages for each setting, negating the need for a precise absolute voltage to set the mode. The threshold voltages between deadtime settings are set internally, based on different ratios of VCC as indicated in the diagram below. In order to avoid drift from the input bias current of the DT pin, a bias current of greater than 0.5 mA is suggested for the external resistor divider circuit. Suggested values of resistance that are used to set a deadtime are given below. Resistors with up to 5% tolerance can be used.
Deadtime mode DT1 DT2 DT3 DT4 Deadtime ~15 ns ~25 ns ~35 ns ~45 ns R11 <10k 5.6k 8.2k Open R9 Open 4.7k 3.3k <10k DT Voltage VCC 0.46(VCC) 0.29(VCC) COM
Default
Figure 8. Deadtime Settings vs. VDT Voltage
Over-Current Protection (OCP)
In the IRAUDAMP4, the IRS20955 gate driver accomplishes OCP internally, a feature discussed in greater detail in the "Protection" section.
Offset Null (DC Offset)
The IRAUDAMP4 is designed such that no output-offset nullification is required. DC offsets are tested to be less than ±5 mV.
Bridged Output
The IRAUDAMP4 is not intended for BTL operation. However, BTL operation can be achieved by feeding out-of-phase audio input signals to the two input channels. In BTL operation, minimum load impedance is 8 and rated power is 240 W non-clipping. The installed clamping diodes D5 D8 are required for BTL operation, since reactive energy flowing from one output to the other during clipping can force the output voltage beyond the voltage supply rails if not clamped.
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Startup and Shutdown
One of the most important aspects of any audio amplifier is the startup and shutdown procedures. Typically, transients occurring during these intervals can result in audible pop- or click-noise on the output speaker. Traditionally, these transients have been kept away from the speaker through the use of a series relay that connects the speaker to the audio amplifier only after the startup transients have passed and disconnects the speaker prior to shutting down the amplifier. It is interesting to note that the audible noise of the relay opening and closing is not considered "click noise", although in some cases, it can be louder than the click noise of non-relay-based solutions. The IRAUDAMP4 does not use any series relay to disconnect the speaker from the audible transient noise, but rather a shunt-based click noise reduction circuit that yields audible noise levels that are far less that those generated by the relays they replace. This results in a more reliable, superior performance system. For the startup and shutdown procedures, the activation (and deactivation) of the clicknoise reduction circuit, the Class D power stage and the audio input (mute) controls have to be sequenced correctly to achieve the required click noise reduction. The overall startup sequencing, shutdown sequencing and shunt circuit operation are described below.
Click-Noise Reduction Circuit (Solid-State Shunt)
To reduce the turn-on and turn-off click noise, a low impedance shunting circuit is used to minimize the voltage across the speaker during transients. For this purpose, the shunting circuit must include the following characteristics: 1) An impedance significantly lower than that of the speaker being shunted. In this case, the shunt impedance is ~100 m, compared to the nominal 4 speaker impedance. 2) When deactivated, the shunting circuit must be able to block voltage in both directions due to the bi-directional nature of the audio output. 3) The shunt circuit requires some form of OCP. If one of the Class D output MOSFETs fails, or is conducting when the speaker mute (SP MUTE) is activated, the shunting circuit will effectively try to short one of the two supplies (+/-B). The implemented click-noise reduction circuit is shown in Figure 9. Before startup or shutdown of the Class D power stage, the click-noise reduction circuit is activated through the SP MUTE control signal. With SP MUTE signal high, the speaker output is shorted through the back-to-back MOSFETs (U9 for Channel 1) with an equivalent on resistance of about 100 m. The two transistors (U7 for Channel 1) are for the OCP circuit.
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+B
Speaker Mute
Click noise reduction circuit
-B
Transient current paths
Over Current Protection
Figure 9. Class D Output Stage with Click-Noise Reduction Circuit
Startup and Shutdown Sequencing
The IRAUDAMP4 sequencing is achieved through the charging and discharging of the CStart capacitor C117. This, coupled to the charging and discharging of the voltage of CSD (C3 on daughter board for CH1) of the IRS20955, is all that is required for complete sequencing. The conceptual startup and shutdown timing diagrams are show in Figure 10.
CStart Ref1 CStart Ref2 +B
CSD= 2/3VDD CSD +5 V -5 V CStart Time
VCC UVP@-20 V -B CHx_O
SP MUTE Audio MUTE
Class D startup
Music startup
Figure 10. Conceptual Startup Sequencing of Power Supplies and Audio Section Timing
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For startup sequencing, +/-B supplies startup at different intervals. As +/-B supplies reach +5 V and -5 V respectively, the analog supplies (+/-5 V) start charging and, once +B reaches ~16 V, VCC charges. Once B reaches -20 V, the UVP is released and CSD and CStart start charging. Once +/-5 V is established, the click-noise reduction circuit is activated through the SP MUTE control signal. As CSD reaches two-thirds VDD, the Class D stage starts oscillating. Once the startup transient has passed, SP MUTE is released (CStart reaches Ref1). The Class D amplifier is now operational, but the preamp output remains muted until CStart reaches Ref2. At this point, normal operation begins. The entire process takes less than three seconds.
+B CStart Ref2 CStart Ref1 CSD= 2/3VDD
CSD CStart
+5 V Time -5 V
VCC -B UVP@-20 V CHx_O SP MUTE
Audio MUTE Class D shutdown Music shutdown
Figure 11. Conceptual Shutdown Sequencing of Power Supplies and Audio Section Timing
Shutdown sequencing is initiated once UVP is activated. As long as the supplies do not discharge too quickly, the shutdown sequence can be completed before the IRS20955 trips UVP. Once UVP is activated, CSD and CStart are discharged at different rates. In this case, threshold Ref2 is reached first and the preamp audio output is muted. Once CStart reaches threshold Ref1, the click-noise reduction circuit is activated (SP MUTE). It is then possible to shutdown the Class D stage (CSD reaches two-thirds VDD). This process takes less than 200 ms.
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For any external fault condition (OTP, OVP, UVP or DCP see "Protection") that does not lead to power supply shutdown, the system will trip in a similar manner as described above. Once the fault is cleared, the system will reset (similar sequence as startup).
CStart Ref2 CStart Ref1
CStart Ref1
CStart Ref2
CSD= 2/3VDD CSD CStart
Time External trip Reset
CHx_O
SP MUTE
Audio MUTE Music shutdown Class D shutdown Class D startup Music startup
Figure 12. Conceptual Click Noise Reduction Sequencing at Trip and Reset
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Protection
The IRAUDAMP4 has a number of protection circuits to safeguard the system and speaker during operation, which fall into one of two categories, internal faults and external faults, and distinguished by the manner in which a fault condition is treated. Internal faults are only relevant to the particular channel, while external faults affect the whole board. For internal faults, only the offending channel is stopped. The channel will hiccup until the fault is cleared. For external faults, the whole board is stopped using the shutdown sequencing described earlier. Here, the system will also hiccup until the fault is cleared at which time it will restart according to the startup sequencing described earlier.
+B VCC
PWM CSD
20955 OCP OTP
-B -B Green LED Red LED Trip
OVP/ UVP
DCP
To other Channel
Figure 13. Functional Block Diagram of Protection Circuit Implementation
Internal Faults
OCP and OTP are considered internal faults. These internal faults will only shutdown the particular channel by pulling low the relevant CSD pin. The channel will shutdown for about one-half a second and will hiccup until the fault is cleared.
Over-Temperature Protection (OTP)
A separate PTC resistor is placed in close proximity to the high-side IRF6645 DirectFET MOSFET for each of the amplifier channels. If the resistor temperature rises above 100 °C, the OTP is activated. The OTP protection will only shutdown the relevant channel by pulling low the CSD pin and will recover once the temperature at the PTC has dropped sufficiently. This temperature protection limit yields a PCB temperature at the MOSFET of about 100 °C. This setting is limited by the PCB material and not by the operating range of the MOSFET.
Over-Current Protection (OCP)
The OCP internal to the IRS20955 shuts down the IC if an OCP is sensed in either of the output MOSFETs. For a complete description of the OCP circuitry, please refer to the IRS20955 datasheet. Here is a brief description:
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Low-Side Current Sensing
The low-side MOSFET is protected from an overload condition and will shutdown the switching operation if the load current exceeds a preset trip level. The low-side current sensing is based on measurement of MOSFET drain-to-source voltage during the lowside MOSFET on state. The voltage set on the OCSET pin programs the threshold for low-side over-current sensing. Thus, if the VS voltage (during low-side conduction) is higher than the OCSET voltage, the IRS20955 will trip. It is recommended to use VREF to supply a reference voltage to a resistive divider (R5 and R7 for CH1) generating a voltage to OCSET for better variability against VCC fluctuations. For IRAUDAMP4, the low-side over-current trip level is set to 0.65 V. For the IRF6645 DirectFET MOSFETs with a nominal RDS-ON of 28 m at 25 °C, this results in a ~23 A maximum trip level. Since the RDS-ON is a function of temperature, the trip level is reduced to ~15 A at 100 °C.
Figure 14. Simplified Functional Block Diagram of Low-Side Current Sensing (CH1)
High-Side Current Sensing
The high-side MOSFET is protected from an overload condition and will shutdown the switching operation if the load current exceeds a preset trip level. High-side over-current sensing monitors detect an overload condition by measuring drain-to-source voltage (VDS) through the CSH and VS pins. The CSH pin detects the drain voltage with reference to the VS pin, which is the source of the high-side MOSFET. In contrast to the low-side current sensing, the threshold of CSH pin to engage OC protection is internally fixed at 1.2 V. An external resistive divider R23 and R25 (for Ch1) can be used to program a higher threshold. An additional external reverse blocking diode (D5 for Ch1) is required to block high-voltage feeding into the CSH pin during low-side conduction. By subtracting a forward voltage drop of 0.6 V at D5, the minimum threshold which can be set in the high-side is 0.6 V across the drain-to-source. For IRAUDAMP4, the high-side over-current trip level is set to 0.6 V across the high-side MOSFET. For the IRF6645 MOSFETs with a nominal RDS-ON of 28 m at 25 °C, this results in a ~21 A maximum trip level. Since the RDS-ON is a function of temperature, the trip level is reduced to ~14 A at 100 °C.
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Figure 15. Simplified Functional Block Diagram of High-Side Current Sensing (CH1)
For a complete description of calculating and designing the over-current trip limits, please refer to the IRS20955 datasheet.
External Faults
OVP, UVP and DCP are considered external faults. In the event that any external fault condition is detected, the shutdown circuit will activate for about three seconds, during which time the orange "Protection" LED will turn on. If the fault condition has not cleared, the protection circuit will hiccup until fault is removed. Once the fault is cleared, the green "Normal" LED will turn on. There is no manual reset option.
Over-Voltage Protection (OVP)
OVP will shutdown the amplifier if the bus voltage between GND and -B exceeds 40 V. The threshold is determined by the voltages sum of the Zener diode Z105, R140, and VBE of Q109. As a result, it protects the board from bus pumping at very low audio signal frequencies by shutting down the amplifier. OVP will automatically reset after three seconds. Since the +B and B supplies are assumed to be symmetrical (bus pumping, although asymmetrical in time, will pump the bus symmetrically in voltage level). It is sufficient to sense one of the two supply voltages only for OVP. It is therefore up to the user to ensure that the power supplies are symmetrical.
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Under-Voltage Protection (UVP)
UVP will shutdown the amplifier if the bus voltage between GND and -B falls below 20 V. The threshold is determined by the voltages sum of the Zener diode Z107, R145 and VBE of Q110. Same as OVP, UVP will automatically reset after three seconds and only one of the two supply voltages is monitored.
Speaker DC-Voltage Protection (DCP)
DCP is provided to protect against DC current flowing into the speakers. This abnormal condition is rare and is likely caused when the power amplifier fails and one of the highside or low-side IRF6645 DirectFET MOSFETs remain in the ON state. DCP is activated if either of the outputs has more than ±4 V DC offset (typical). Under this fault condition, it is normally required to shutdown the feeding power supplies. Since these are external to the reference design board, an isolated relay is provided (P1) for further systematic evaluation of DC-voltage protection to transmit this condition to the power supply controller and is accessible through connector J9 (pins of J9 are shorted during fault condition).
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Thermal Considerations
The daughter board design can handle one-eighth of the continuous rated power, which is generally considered to be a normal operating condition for safety standards. Without the addition of a heatsink or forced air-cooling, the daughter board cannot handle continuous rated power.
100.0% 90.0% 80.0% 70.0% 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 0.0% 0 20 40 60 80 100 120 140 160 180 Output Power (W)
Figure 16. Efficiency FV. Output Power, 4 Single Channel Driven, ±B supply = ±35 V, 1 kHz Audio Signal
Power Stage Efficiency (%)
67°C 54°C
Figure 17. Thermal image of Daughter Board Two-Channel x 1/8th Rated Power (15 W) in Operation, TC = 54 °C at Steady State ±B supply = ±35 V, 4 Load, 1 kHz audio signal, TA = 25 °C
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Typical Performance
±B supply = ± 35 V, load impedance = 4 , 1 kHz audio signal, Self oscillator @ 400 kHz and internal volume-control set to give required output with 1 Vrms input signal, unless otherwise noted.
+4
+2
8
+0 d B r -4 A -6 -2
4
-8
-10 20
50
100
200
500
1k
Hz
2k
5k
10k
20k
50k
200k
Green Red
+0
CH1 - 4 , 2 V Output CH1 - 8 , 2 V Output
Figure 18. Frequency Characteristics vs. Load Impedance
-20
-40 d B -80
-60
Int.
-100
Self
-120 20 50 100 200 500 Hz 1k 2k 5k 10k 20k
Red Green
CH2 CH1, 60 W, Self Oscillator @ 400 kHz CH2 CH1, 60 W, Internal Clock @ 300 kHz
Figure 19. Channel Separation vs. Frequency
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+0 -10 -20 -30 -40 -50 d B -60 -70 -80 -90 -100 -110 -120 20
Int. Self
50 100 200 500 Hz 1k 2k 5k 10k 20k
Red Green
100 50
CH2 CH1, 60 W, Self Oscillator @ 400 kHz CH2 CH1, 60 W, Internal Clock @ 300 kHz
Figure 20. Stand-alone Class D Power Stage: Channel Separation vs. Frequency
10
1
% 0.1
0.01
0.001
0.0001 100m
200m
500m
1
2 W
5
10
20
50
100
200
Green Red
CH1 - ACD, ±B = ± 35 V, Volume gain 21.9 V/V AUX-25 filter CH1 - ACD, ±B = ± 35 V, Volume gain 21.9 V/V 3rd order RC filter
Figure 21. Stand-alone Class D Power Stage: THD+N Ratio vs. Output Power
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100
T
10
±30 V
1 % 0.1
±25 V
±35 V
0.01
0.001 100m
200m
500m
1
2 W
5
10
20
50
100
200
Green Blue Red
100 50
CH1, ±B = ±35 V, Volume gain 21.9 V/V CH1, ±B = ±30 V, Volume gain 21.9 V/V CH1, ±B = ±25 V, Volume gain 21.9 V/V
Figure 22. THD+N Ratio vs. Output Power
±30 V
10 5
1 0.5 % 0.2 0.1
±25 V
±35 V
0.02 0.01
0.002 0.001 100m 200m 500m 1 2 W 5 10 20 50 100 200
Green Blue Red
CH1 - ACD, ±B = ±35 V, Volume gain 21.9 V/V CH1 - ACD, ±B = ±30 V, Volume gain 21.9 V/V CH1 - ACD, ±B = ±25 V, Volume gain 21.9 V/V
Figure 23. THD+N Ratio vs. Output Power (ACD)
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100
10
1
%
0.1
100 W
0.01
1W
0.001
10 W
0.0001 20 50 100 200 500 Hz 1k 2k 5k 10k 20k
Green Blue Red
100
CH1, 1 W Output CH1, 10 W Output CH1, 100 W Output
Figure 24. THD+N Ratio vs. Frequency
10
1
%
0.1
100 W
0.01
1W 10 W
0.001
0.0001 20 50 100 200 500 Hz 1k 2k 5k 10k 20k
Green Yellow Red
CH1 - ACD, 1 W Output CH1 - ACD, 10 W Output CH1 - ACD, 100 W Output
Figure 25. THD+N Ratio vs. Frequency (ACD)
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+0
-20
-40
d B
-60
V -80
Int.
-100
Self
-120
-140 10 20 50 100 200 500 Hz 1k 2k 5k 10k 20k
Green Red
+0
CH1 - ACD, 1 V, 1 kHz, Self Oscillator @ 400 kHz CH1 - ACD, 1 V, 1 kHz, Internal Clock @ 300 kHz
Figure 26. Frequency Spectrum (ACD)
-20
-40
d -60 B V -80
-100
Self Int.
-120
-140 10
20
50
100
200
500 Hz
1k
2k
5k
10k
20k
Green Red
CH1 - ACD, No signal, Self Oscillator @ 400 kHz CH1 - ACD, No signal, Internal Clock @ 300 kHz
Figure 27. Residual Noise (ACD)
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Red Trace: Total Distortion + Noise Voltage Green Trace: Output Voltage
60 W / 4 , 1 kHz, THD+N=0.004%
174 W / 4 , 1 kHz, THD+N=10%
Figure 28. Measured Output and Distortion Waveforms
CSD pin VS pin Load current VS pin
CSD pin
Load current
Figure 29. Typical OCP Waveforms Showing Load Current and Switch Node Voltage (VS)
CSD pin VS pin VS pin
CSD pin
Load current
Load current
Figure 30. Typical OCP Waveforms Showing CSD Trip and Hiccup
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IRAUDAMP4 Design Documents
Motherboard Schematics:
Figure 31. Housekeeping and Protection Circuits
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Figure 32. Audio Channels 1 and 2
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Daughter Board Schematics:
Figure 33. Daughter Board Schematic with Class D Stage for Two Audio Channels
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IRAUDAMP4 Bill of Materials
Motherboard:
IRAUDAMP4 MOTHERBOARD BILL OF MATERIAL NO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Designator C1, C5, C6, C101, C102, C103, C104, C105, C106, C115 C2, C3 C7, C8, C9, C10 C11, C12, C13, C14 C15, C16 C17, C18 C19, C20, C119 C23, C24 C23_1, C24_1 C25, C26 C27, C28, C29, C30, C40, C41, C42, C43, C44, C45, C46, C47 R29, R30, R55, R56, R60, R61, R62, R63, R64, R65, R66, R67, R71, R72 C31, C32 C33, C34, C48, C49 C107, C109 C108, C114 C110 C111, C113 C112 C116, C117 D1, D2, D3, D4, D11, D12, D103, D104, D105, D106, D107 D5, D6, D7, D8 D101, D102 HS1 J1A, J1B J2A, J2B J3, J4 J5, J6 J7 J8 J9 L1, L2 NORMAL P1 PROTECTION Q101 Q102, Q104, Q106, Q111 Q103, Q105, Q107, Q108, Q109, Q110 R1, R2, R51, R52, R57, R58, R110, R126 R3, R4, R114 R5, R6 R7, R8, R10, R11, R27, R28, R115, R116, R117 R9, R105 R13, R14 R17, R18 R53, R54, R106, R121, R122, R130, R131, R132, R133, R137, R139, R141, R145, R146, R147, R149 R19, R20 R21, R22, R23, R24, R39, R40 R25, R26, R120 R29P, R30P R31, R32 R33, R34 R35, R36, R37, R38, R109, R118, R119, R123 R47, R48 # 10 2 4 4 2 2 3 2 2 2 1 2 1 4 2 4 2 2 1 2 1 2 1 1 4 2 1 2 2 2 2 1 1 1 2 1 1 1 1 4 6 8 3 2 9 2 2 2 1 6 2 6 3 2 2 2 8 2 Footprint RB2/5 RB2/5 CR3225-1210 1206 0805 AXIAL0.19R 1206 CAP MKP AXIAL0.2R CAP MKPs 0805 0805 RB5/12_5 AXIAL0.1R 0805 0805 0805 0805 0805 rb2/5 SOD-123 SMA SOD-123 Heat_S6in1 CON EISA-31 CON_POWER MKDS5/2-9.5 cp1418 J HEADER3 BNC_RA CON ED1567 Inductor Led rb2/5 DIP-6 Led rb2/5 SOT89 SOT23-BCE SOT23-BCE 0805 0805 1206 0805 0805 0805 0805 0805 0805 0805 1206 POT_SRM 2512 1206 0805 2512 PartType 10 µF, 50 V 2.2 µF, 50 V 3.3 µF, 50 V 1 nF, 200 V 33 pF 150 pF, 500 V 0.1 µF, 50 V 0.47 µF, 630 V OPEN 0.1 µF, 400 V OPEN OPEN 1000 µF,50 V OPEN 4.7 µF, 16 V 10 nF, 50 V 1 nF, 50 V 100 pF, 50 V 1200 pF, 50 V 100 µF, 16 V 1N4148 MURA120 MA2YD2300 HEAT SINK CON EISA31 CON_POWER 277-1022 1418-ND 277-1272 BNC ED1567 22 µH 404-1106-ND PVT412 404-1109-ND FX941 MMBT5401 MMBT5551 100 k 100 4.7 47 10 3.3 k, 1% 22 k 47 k 0 4.7 100 1 k 47 k, 1% 1 k 1 k 10, 1 W Part No 565-1106-ND 565-1103-ND 445-1432-1-ND PCC2009CT-ND 478-1281-1-ND 338-1052-ND PCC104BCT-ND 495-1315-ND 495-1311-ND 565-1114-ND PCC2323CT-ND PCC103BNCT-ND PCC102CGCT-ND PCC101CGCT-ND 478-1372-1-ND 565-1037-ND 1N4148WDICT-ND MURA120T3OSCT-ND MA2YD2300LCT-ND 294-1086-ND A26453-ND A26454-ND 277-1271-ND or 651-1714971 CP-1418-ND 277-1272-ND or 651-1714984 A32248-ND ED1567 ETQA21ZA220 or ETQA17B220 160-1143-ND PVT412-ND 160-1140-ND FCX491CT-ND MMBT5401DICT-ND MMBT5551-7DICT-ND P100KACT-ND P100ACT-ND P4.7ECT-ND P47ACT-ND P10ACT-ND P3.3KZCT-ND P22KACT-ND P47KACT-ND P0.0ACT-ND P4.7ACT-ND P100ECT-ND 3361P-102GCT-ND PT47KAFCT-ND P1.0KECT-ND P1.0KACT-ND PT10XCT Vender Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey or Mouser Digikey Digikey or Mouser Digikey Digikey Panasonic Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey
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55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88
R49, R50 R68, R69 R101, R102, R103, R104 R107, R138 R108 R111, R124, R125, R134, R140, R143, R144, R148 R112 R113 R127, R128, R129 R135 R136, R142 S1 S2 S3 U1, U2 U3, U4 U7, U8 U9, U10 U_1 U_2 U_3 U_4 U_5 U_6 Z1, Z2, Z103 Z101, Z102 Z104 Z105 Z106, Z107 Z108 Volume Knob Thermalloy TO-220 mounting kit with screw 1/2" Standoffs 4-40 4-40 Nut
2 2 4 2 1 8 1 1 3 1 2 1 1 1 2 2 2 2 1 1 1 1 1 1 3 2 1 1 2 1 1 3 5 5
1206 AXIAL-0.3 2512 0805 V_Control 0805 0805 POTs 1206 0805 0805 Switch SW-EG1908-ND SW-EG1944-ND SO-8 SOT25 MINI5 SO-8 SOIC16 N8A M14A TO-220 Fullpak TO-220 Fullpak TO-220 Fullpak SOD-123 DL-41 SOD-123 SOD-123 SOD-123 SOD-123
2.2 k OPEN 47 , 1 W 4.7 k CT2265 10 k 820 5 k POT 6.8 k 82 k 68 k SW-PB SW_H-L SW-3WAY TLC081 74AHC1G04 XN01215 IRF7341 CS3310 3310S06S 74HC14 MC78M05 MC79M05 MC78M12 15 V 4.7 V 24 V 39 V 18 V 8.2 V
P2.2KECT-ND PT47XCT-ND P4.7KACT-ND CT2265-ND P10KACT-ND P820ACT-ND 3362H-502-ND P6.8KECT-ND P82KACT-ND P68KACT-ND P8010S-ND EG1908-ND EG1944-ND 296-7264-1-ND 296-1089-1-ND XN0121500LCT-ND IRF7341 73C8016 or 72J5420 3310-IR01 296-1194-1-ND MC78M05BTOS-ND MC79M05CTOS-ND MC78M12CTOS-ND BZT52C15-FDICT-ND 22H3923 BZT52C24-FDICT-ND BZT52C39-13-FDICTND BZT52C18-FDICT-ND BZT52C8V2-FDICT-ND MCCPMB1 46F4081 8401K-ND H724-ND
Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey IR Newark *Tachyonix Digikey Digikey Digikey Digikey Digikey Newark Digikey Digikey Digikey Digikey Newark Newark Digikey Digikey
100 per bag
*Tachyonix Corporation, 14 Gonaka Jimokuji Jimokuji-cho, Ama-gun Aichi, JAPAN 490-1111 http://www.tachyonix.co.jp [email protected]
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Voltage regulator mounting:
Item Description 1 2 3 4
7
Insulator Thermalfilm Shoulder Washer Flat Washer #4 No. 4-40 UNC-2B Hex Nut No. 4-40 UNC-2A X 1/2 Long Phillips Pan Head Screw Lockwasher, No.4 Heatsink PCB
5 6 7
8
8
Daughter Board:
IRAUDAMP4 DAUTHER-BOARD BILL OF MATERIAL NO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Designator C1, C2, C9, C10 C3, C4 C5, C6 C7, C8 C11, C12 C13, C14 C15, C16, C17, C18 C21, C22, C23, C24 D1, D2 D3, D4 D5, D6 D7 D8, D9 DS1 J1 J2 Q1, Q2 Q3, Q4, Q5, Q6 Q7, Q8 R1, R2 R3, R4, R25, R26 R5, R6, R9, R10 R7, R8 R11, R12 R13, R14 R15, R16, R27, R28, R29, R30 R19, R20 R21, R22, R41, R42 R23, R24 R31, R32 R33, R34 R35, R36, R37, R38, R39, R40 R43 Rp1, Rp2 U1, U2 # 4 2 2 2 2 2 4 4 2 2 2 1 2 1 1 1 2 4 2 2 4 4 2 2 2 6 2 4 2 2 2 6 1 2 2 Footprint 1206 T491 0805 1812 0805 0805 1206 0805 SOD-123 SMA SOD-123 SMB SOD-123 LED CON EISA-31 CON_POWER SOT23-BCE DirectFET MOSFET6645 SOT23-BCE 0805 1206 0805 0805 0805 0805 0805 0805 0805 0805 0805 0805 0805 0805 0805 M16A Part Type 2.2 µF, 25 V 10 µF, 16 V OPEN 22 µF, 25 V 47 pF 0.1 µF 0.1 µF,100 V 47 nF 1N4148 MURA120T3OSCT-ND BAV19WDICT-ND S1DB-FDICT-ND MA2YD2300 160-1414-1-ND CON EISA31 CON_POWER MMBT5401 IRF6645 MMBT3904 100 10 k 8.2 k 1.2 k 3.3 k 4.7 10 33 k 10 k 0 1 1 k 100 k 4.7 k 100 C IRS20955SPBF Part No 490-3368-1-ND 399-3706-1-ND 445-1607-1-ND PCC470CGCT-ND PCC1840CT-ND PCC2239CT-ND PCC1836CT-ND 1N4148WDICT-ND MURA120T3OSCT-ND BAV19W-FDICT-ND RS1DB-FDICT-ND MA2YD2300LCT-ND 160-1414-1-ND 2011-03-ND 2011-04-ND MMBT5401DICT-ND IRF6645 MMBT3904-FDICT-ND P100ACT-ND P10KECT-ND P8.2KACT-ND P1.2KACT-ND P3.3KACT-ND P4.7ACT-ND P10ACT-ND P33KACT-ND P10KACT-ND P0.0ACT-ND P1.0ACT-ND P1.0KACT-ND P100KACT-ND P4.7KACT-ND 594-2322-675-21007 IRS20955SPBF Vendor Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey IR Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Mouser IR
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IRAUDAMP4 PCB Specifications
Figure 34. Motherboard and Daughter Board Layer Stack
Motherboard:
Material: Layer stack: Dimensions: Solder mask: Plating: Silkscreen: FR4, UL 125 °C 2 layers, 1 oz. Cu 5.2 in x 5.8 in x 0.062 in LPI solder mask, SMOBC on top and bottom layers Open copper solder finish On top and bottom layers
Daughter board:
Material: Layer stack: Dimensions: Solder mask: Plating: Silkscreen: FR4, UL 125 °C 2 Layers, 1 oz. Cu each, through-hole plated 3.125 in x 1.52 in x 0.062 in LPI solder mask, SMOBC on top and bottom layers Open copper solder finish On top and bottom layers
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IRAUDAMP4 PCB Layers
Motherboard:
Figure 35. Top Layer and Pads
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Figure 36. Bottom Layer and Pads
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Figure 37. Bottom-Side Solder-Mask and Silkscreen
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4.0
Figure 38. Top-Side Solder-Mask and Silkscreen
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Daughter Board:
Figure 39. PCB Layout Top Layer and Pads
Figure 40. PCB Layout Top-Side Solder-Mask and Silkscreen
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Figure 41. PCB Layout Bottom Layer and Pads
Figure 42. PCB Layout Bottom-Side Solder-Mask and Silkscreen
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IRAUDAMP4 Mechanical Construction
Motherboard
Figure 43. Top Side of Motherboard Showing Component Locations
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Figure 44. Bottom Side of Motherboard Showing Component Locations
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Daughter Board
Figure 45. Top Side Showing Component Locations
Figure 46. Bottom Side Showing Connector Locations
02/15/2007
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