NXP Semiconductors TDA8932B 14.5 Gain reduction

TDA8932B_3 © NXP B.V. 21 June 2007. All rights reserved.
Product data sheet Rev. 03— 21 June 2007 25 of 48
NXP Semiconductors TDA8932B
Class-D audio amplifier
The 3 dB cut-off frequency is equal to:
(7)
Where:
f-3dB = 3 dB cut-off frequency (Hz)
RL= load impedance (Ω)
Cse = single-ended capacitance (F); see Figure 36
Table 15 shows an overview of the required SE capacitor values in case of 60 Hz, 40 Hz
or 20 Hz 3 dB cut-off frequency.
14.5 Gain reduction
The gain of the TDA8932B is internally fixed at 30 dB for SE (or 36 dB for BTL). The gain
can be reduced by a resistive voltage divider at the input (see Figure 10).
When applying a resistive divider, the total closed-loop gain Gv(tot) can be calculated by
Equation 8 and Equation 9:
(8)
Where:
Gv(tot) = total closed-loop voltage gain (dB)
Gv(cl) = closed-loop voltage gain, fixed at 30 dB for SE (dB)
REQ = equivalent resistance, R3 and Zi (Ω)
R1 = series resistor (Ω)
R2 = series resistor (Ω)
Table 15. SE capacitor values
Impedance (Ω) Cse (µF)
f-3dB =60Hz f-3dB =40Hz f-3dB =20Hz
4 680 1000 2200
6 470 680 1500
8 330 470 1000
f3dB–1
2πRL ×Cse×
-----------------------------------
=
Fig 10. Input configuration for reducing gain
001aad762
100
kΩ
470 nF
470 nF
audio in R3
R1
R2
Gvtot() Gvcl() 20 REQ
REQ R1 R2+()+
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log+=