Diodes PAM8124 - Manual (Page 8)

PAM8124
Document number: DS36627 Rev. 1 - 2
8 of 15
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October 2013
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PAM8124
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Application Information
(cont.)
Input Resistance
Changing the gain setting can vary the input resistance of the amplifier from its smallest value (6.67kΩ ±20%) to the largest value (40kΩ ±20%).
As a result, if a single capacitor is used in the input high-pass filter, the –3dB cutoff frequency may change when changing gain steps.
The –3dB frequency can be calculated using the following Equation. Use the Ri values given in Table 1.
( ) C
1
f
2 RiCi π
=
Single-Ended Output Capacitor
In single-ended (SE) applications, the dc blocking capacitor forms a high-pass filter with the speaker impedance. The frequency response rolls off
with decreasing frequency at a rate of 20dB/decade. The cutoff frequency is determined by:
SE L
coh C R
f
π 2
1
=
Table 2 shows some common component values and the associated cutoff frequencies:
Table 2: Common Filter Responses
R
L
-Speaker Impedance (Ω)
CSE-DC Blocking Capacitor (µF)
fc = 60Hz (-3dB) fc = 40Hz (-3dB) fc = 20Hz (-3dB)
4 680 1000 2200
6 470 680 1500
8 330 470 1000
Output Filter and Frequency Response
For the best frequency response, a flat pass band output filter (second-order Butterworth) may be used. The output filter components consist of
the series inductor and capacitor to ground at the LOUT and ROUT pins. There are several possible configurations, depending on the speaker
impedance and whether the output configuration is single-ended (SE) or bridge-tied load (BTL). Table 3 lists the recommended values for the filter
components. It is important to use a high-quality capacitor in this application. A rating of at least X7R is required.
Table 3: Recommended Filter Output Components
Output Configuration Speaker Impedance(Ω) Filter Inductor(µH) Filter Capacitor(nF)
Single Ended (SE)
4 22 680
8 33 220
Bridge Tied Load (BTL) 8 22 680