Diodes PAM8008 - Manual (Page 11)

PAM8008
Document number: DSxxxxx Rev. 1 - 0
11 of 14
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October 2012
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PAM8008
A Product Line of
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Application Information
(cont.)
How to Reduce EMI (Electro Magnetic Interference) (cont.)
Most applications required a ferrite bead filter as shown at Figure 1. The ferrite filter reduces EMI around 1MHz and higher. When selecting a
ferrite bead, choose one with high impedance at high frequencies, and low impedance at low frequencies (MH2012HM221-T).
Figure 1: Ferrite Bead Filter to Reduce EMI
PCB Layout Guidelines Grounding
At this stage it is paramount to notice the necessity of separate grounds. Noise currents in the output power stage need to be returned to output
noise ground and nowhere else. Were these currents to circulate elsewhere, they may get into the power supply, the signal ground, etc, worse
yet, they may form a loop and radiate noise. Any of these cases results in degraded amplifier performance. The logical returns for the output
noise currents associated with Class-D switching are the respective GND pins for each channel. The switch state diagram illustrates that GND is
instrumental in nearly every switch state. This is the perfect point to which the output noise ground trace should return. Also note that output
noise ground is channel specific. A two channel amplifier has two seperate channels and consequently must have two seperate output noise
ground traces. The layout of the PAM8008 offers separate GND connections for each channel and in some cases each side of the bridge.
Output noise grounds must be tied to system ground at the power in exclusively. Signal currents for the inputs, reference, etc need to be
returned to quite ground. This ground is only tied to the signal components and the GND pin, and GND then tied to system ground.
Test Setup for Performance Testing (Class D)
1. When the PAM8008 works with LC filters, it should be connected with the speaker before it's powered on, otherwise it will be damaged easily.
2. When the PAM8008 works without LC filters, it's better to add a ferrite chip bead at the outgoing line of speaker for suppressing the possible
electromagnetic interference.
3. The absolute maximum rating of the PAM8008 operation voltage is 6.0V. When the PAM8008 is powered with four battery cells, it should be
noted that the voltage of four new dry or alkaline batteries is over 6V, higher than its maximum operation voltage, which probably make the
device damaged. Therefore, it's recommended to use either four Ni-MH (Nickel Metal Hydride) rechargeable batteries or three dry or alkaline
batteries.
4. The input signal should not be too high, if too high, it will cause the clipping of output signal when increasing the volume. Because the DC
volume control of the PAM8008 has big gain, it will make the device damaged.
5. When testing the PAM8008 without LC filters b y using resistor instead of speaker as the output load, the test results, e.g. THD or efficiency,
will be worse than those using speaker as load.
Notes:
1. The AP AUX-0025 low pass filter is necessary for class-D amplifier measurement with AP analyzer.
2. Two 22μH inductors are used in series with load resistor to emulate the small speaker for efficiency measurement.