LM4846 Datasheet, PDF (15/19 Pages) National Semiconductor (TI) – Output Capacitor-less Audio Subsystem with Programmable National 3D

Application Information (Continued)
LM4846’s supply pins and ground. Keep the length of leads
and traces that connect capacitors between the LM4846’s
power supply pin and ground as short as possible. Connect-
ing a 2.2µF capacitor, C_B, between the BYPASS pin and
ground improves the internal bias voltage’s stability and
improves the amplifier’s PSRR. The PSRR improvements
increase as the bypass pin capacitor value increases. Too
large, however, increases turn-on time and can compromise
the amplifier’s click and pop performance. The selection of
bypass capacitor values, especially C_B, depends on desired
PSRR requirements, click and pop performance (as ex-
plained in the section, Proper Selection of External Compo-
nents), system cost, and size constraints.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C_i in Figures 1 & 2). A high value
capacitor can be expensive and may compromise space
efficiency in portable designs. In many cases, however, the
speakers used in portable systems, whether internal or ex-
ternal, have little ability to reproduce signals below 150Hz.
Applications using speakers with this limited frequency re-
sponse reap little improvement by using large input capaci-
tor.
The internal input resistor (R_i), nominal 20kΩ, and the input
capacitor (C_i) produce a high pass filter cutoff frequency that
is found using Equation (15).
f_c = 1 / (2πR_iC_i)
(15)
As an example when using a speaker with a low frequency
limit of 150Hz, C_i, using Equation (15) is 0.053µF. The
0.22µF C_i shown in Figure 1 allows the LM4846 to drive high
efficiency, full range speaker whose response extends below
40Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C_B, the capacitor con-
nected to the BYPASS bump. Since C_B determines how fast
the LM4846 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4846’s
outputs ramp to their quiescent DC voltage (nominally V_DD/
2), the smaller the turn-on pop. Choosing C_B equal to 1.0µF
along with a small value of C_i (in the range of 0.1µF to
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C_i no larger than neces-
sary for the desired bandwidth helps minimize clicks and
pops. C_B’s value should be in the range of 5 times to 7 times
the value of C_i. This ensures that output transients are
eliminated when power is first applied or the LM4846 re-
sumes operation after shutdown.
15
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