lm4867 National Semiconductor Corporation, lm4867 Datasheet - Page 18

• Manufacturer Part Number: lm4867
• Description: Output-transient-free Dual 2.1w Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
INPUT CAPACITOR VALUE SELECTION
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
tor can be expensive and may compromise space efficiency
in portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with this limited frequency response reap
little improvement by using large input capacitor.
Besides effecting system cost and size, C
the LM4867’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quies-
cent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually V
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R
capacitor value that is no higher than necessary to meet the
desired −3dB frequency and is between 0.14C
A shown in Figure 4, the input resistor (R
capacitor, C
that is found using Equation (7).
As an example when using a speaker with a low frequency
limit of 150Hz, C
C
ciency, full range speaker whose response extends below
30Hz.
BYPASS CAPACITOR VALUE SELECTION
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
nected to the BYPASS pin. Since C
the LM4867 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4867’s
outputs ramp to their quiescent DC voltage (nominally 1/2
V
f
i
DD
. Thus, pops can be minimized by selecting an input
shown in Figure 4 allows the LM4867 to drive high effi-
), the smaller the turn-on pop. Choosing C
I
produce a −3dB high pass filter cutoff frequency
i
, using Equation (4) is 0.063µF. The 1.0µF
f
−3dB
= 1/(2πR
i
in Figure 4). A high value capaci-
(Pin numbers in ( ) are for the 20-pin MTE and MT packages.)
IN
C
B
I
B
)
determines how fast
, the capacitor con-
(Continued)
i
has an affect on
I
) and the input
B
FIGURE 8. Headphone Circuit
and 0.20C
B
equal to
DD
(7)
/2)
B
.
18
1.0µF along with a small value of C
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
sary for the desired bandwidth helps minimize clicks and
pops. C
the value of C
eliminated when power is first applied or the LM4867 re-
sumes operation after shutdown.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4867 contains circuitry that eliminates turn-on and
shutdown transients (“clicks and pops“) and transients that
could occur when switching between BTL speakers and
single-ended headphones. For this discussion, turn-on re-
fers to either applying the power supply voltage or when the
shutdown mode is deactivated. While the power supply is
ramping to its final value, the LM4867’s internal amplifiers
are configured as unity gain buffers and are disconnected
from the -OUT and +OUT pins. An internal current source
changes the voltage of the BYPASS pin in a controlled,
linear manner. Ideally, the input and outputs track the voltage
applied to the BYPASS pin. The gain of the internal amplifi-
ers remains unity until the voltage on the bypass pin reaches
1/2 V
the device becomes fully operational and the amplifier out-
puts are reconnected to the -OUT and +OUT pins. Although
the BYPASS pin current cannot be modified, changing the
size of C
relationship between the size of C
Here are some typical turn-on times for various values of C
DD
B
. As soon as the voltage on the bypass pin is stable,
’s value should be in the range of 5 times to 7 times
B
alters the device’s turn-on time. There is a linear
i
. This ensures that output transients are
20001324
C
0.01µF
0.22µF
0.47µF
0.1µF
1.0µF
2.2µF
B
T
i
134ms
300ms
630ms
ON
B
(in the range of 0.1µF to
30ms
63ms
i
3ms
and the turn-on time.
no larger than neces-
B
: