Datasheet

Single-Ended Output Capacitor, C
O
c
O i
1
f =
2 C Zp
(4)
Bleeder Resistor for Single-Ended Output Capacitor
Output Filter and Frequency Response
TPA3125D2
www.ti.com ........................................................................................................................................................................................... SLOS611 – DECEMBER 2008
in high-gain applications. For this reason, a low-leakage tantalum or ceramic capacitor is the best choice. When
polarized capacitors are used, the positive side of the capacitor should face the amplifier input in most
applications as the dc level there is held at AV
CC
/8 volts, which is likely higher than the source dc level. Note that
it is important to confirm the capacitor polarity in the application. Additionally, lead-free solder can create dc
offset voltages due to leakage, so it is important to ensure that boards are cleaned properly.
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 20 dB/decade. The cutoff frequency is
determined by
Table 3 shows some common component values and the associated cutoff frequencies:
Table 3. Common Filter Responses
C
o
- DC Blocking Capacitor ( µ F)
Speaker Impedance ( Ω )
f
c
= 60 Hz ( – 3 dB) f
c
= 40 Hz ( – 3 dB) f
c
= 20 Hz ( – 3 dB)
4 680 1000 2200
6 470 680 1500
8 330 470 1000
The single-ended application schematic shows a 4.7 k Ω resistor from the IC side of the single-ended output
capacitor to ground. This resistor is used to bleed the charge off of the capacitor when the amp is powered
down, preventing pop if the amp is power back up quickly. The value of the resistor can be adjusted to control
the time required to discharge the capacitor. The discharge time is proportional to the RC time constant of the
resistor and capacitor.
For the best frequency response, a flat-passband 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 4 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 and voltage
rating of 50V is suggested.
Table 4. Recommended Filter Output Components
Output Configuration Speaker Impedance ( Ω ) Filter Inductor ( µ H) Filter Capacitor (nF)
4 22 680
Single - Ended (SE)
8 33 220
Bridge - Tied Load (BTL) 8 22 680
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