Cirrus Logic AN168 - Manual (Page 29)

AN168
AN168REV2 29
2. APPENDIX - EXAMPLE SPEAKER
DRIVER CIRCUITS
In the following pages are 5 example speaker driver
circuits. In each case, the maximum gain is listed
that the speaker driver can assume for the
associated power supply voltage and a 4 ohm
speaker. If less gain is needed, R1 can be increased
or R2 can be decreased. As discussed previously,
volume control should be implemented using the
RVol control inside the CS6422. DO NOT allow
the gain of the speaker driver to be changed during
a call.
In each of the circuits, simple bandpass filtering is
implemented by using a single-pole high-pass
corner around 300 Hz and a single-pole low-pass
corner around 4 kHz. In systems that use a
high-quality speaker whose frequency response is
flat above 15 kHz, additional low-pass filtering
may be necessary to reduce the level of D/A noise
from the CS6422.
In these systems, a 3-pole Butterworth filter can be
placed before the speaker driver. An example
3-pole Butterworth filter is shown in Figure 21.
In this filter, the attenuation can be selected by
adjusting the ratio of R2/R1, preferably by
reducing R2.
2.1 Example 1: TDA1519A -- 15 Watts into 4
Ω
The schematic for the TDA1519A speaker driver is
shown in Figure 22. The TDA1519A, from Philips
Semiconductors, is capable of supplying 15 Watts
of RMS power into a 4
Ω load with less than
2% THD. The gain required to achieve 15 Watts is
+17.8 dB, which is implemented with an
attenuation stage of -28.2 dB followed by the
TDA1519A’s inherent +46 dB of gain.
The high-pass corner frequency near 300 Hz is
given by:
The low-pass corner frequency near 4 kHz is given
by:
2.2 Example 2: TDA2003 -- 3 Watts into 4
Ω
The schematic for the TDA2003 speaker driver is
shown in Figure 23. The TDA2003, from ST, is
capable of supplying 3 Watts of RMS power into a
4
Ω load with less than 2% THD. The gain required
to achieve 3 Watts is +10.7 dB, which is
implemented with an attenuation stage of -29.3 dB
followed by the TDA2003 gain implemented as
+40 dB.
The high-pass corner frequency near 300 Hz is
given by:
The low-pass corner frequency near 4 kHz is given
by:
Gain
R2
R1
------- =
C2
1
2
π R2 ( ) 4kHz ( )
------------------------------------- =
Gain 200
R2
R1 R2 +
-------------------- ×
=
Fhp
1
2
πC1 R1 R2
+
( )
----------------------------------------
≈
Flp
1
2
πC2 R1 R2 || ( )
---------------------------------------
≈
Gain
R2
R1 R2 +
--------------------
⎝ ⎠
⎛ ⎞
1
R4
R5
------- + ⎝ ⎠
⎛ ⎞
×
=
Fhp
1
2
πC1 R1 R2
+
( )
----------------------------------------
≈
Flp
1
2
πC2 R1 R2 || ( )
---------------------------------------
≈