LM4818M National Semiconductor, LM4818M Datasheet - Page 10

• Manufacturer Part Number: LM4818M
• Description: 350mW Audio Power Amplifier with Shutdown Mode
• Manufacturer: National Semiconductor

www.national.com
Application Information
operation by closing the switch. Opening the switch con-
nects the shutdown pin to V
activating micro-power shutdown. The switch and resistor
guarantee that the SHUTDOWN pin will not float. This pre-
vents unwanted state changes. In a system with a micropro-
cessor or a microcontroller, use a digital output to apply the
control voltage to the SHUTDOWN pin. Driving the SHUT-
DOWN pin with active circuitry eliminates the pull-up resistor
PROPER SELECTION OF EXTERNAL COMPONENTS
Optimizing the LM4818’s performance requires properly se-
lecting external components. Though the LM4818 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4818 is unity gain stable, giving the designer maxi-
mum design flexibility. The gain should be set to no more
than a given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio CO-
DECs have outputs of 1V
Audio Power Amplifier Design section for more informa-
tion on selecting the proper gain.
Another important consideration is the amplifier’s close-loop
bandwidth. To a large extent, the bandwidth is dictated by
the choice of external components shown in Figure 1 . The
input coupling capacitor, C
that limits low frequency response. This value should be
chosen based on needed frequency response for a few
distinct reasons discussed below
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor (C
capacitor can be expensive and may compromise space
efficiency in portable designs. In many cases the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with limited frequency response reap little
improvement by using a large input capacitor.
Besides affecting system cost and size, C
the LM4818’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 value. Higher value capacitors need
more time to reach a quiescent DC voltage (usually 1/2 V
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R
than necessary to meet the desired -3dB frequency can
minimize pops.
As shown in Figure 1 , the input resistor (R
capacitor, C
that is found using Equation (5).
As an example when using a speaker with a low frequency
limit of 150Hz, C
F
. Thus, selecting an input capacitor value that is no higher
i
produce a -3dB high pass filter cutoff frequency
i
, using Equation (5) is 0.063µF. The 0.39µF
f
-3dB
= 1/(2 R
RMS
i
, forms a first order high pass filter
DD
(2.83V
through the pull-up resistor,
i
i
C
in Figure 1 ). A high value
i
) (Hz)
P-P
). Please refer to the
(Continued)
i
has an effect on
i
) and the input
DD
(5)
)
10
C
efficiency, full range speaker whose response extends down
to 20Hz.
Besides optimizing the input capacitor value, the bypass
capacitor value, C
pass capacitor’s value is the most critical to minimizing
turn-on pops because it determines how fast the LM4818
turns on. The slower the LM4818’s outputs ramp to their
quiescent DC voltage (nominally 1/2V
turn-on pop. While the device will function properly (no os-
cillations or motorboating), with C
device will be much more susceptible to turn-on clicks and
pops. Thus, a value of C
recommended in all but the most cost sensitive designs.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the value of C
connected to the BYPASS pin. Since C
fast the LM4818 settles to quiescent operation, its value is
critical when minimizing turn-on pops. The slower the
LM4818’s outputs ramp to their quiescent DC voltage (nomi-
nally 1/2V
equal to 1.0µF along with a small value of C
0.1µF to 0.39µF) produces a click-less and pop-less shut-
down function. As discussed above, choosing C
than necessary for the desired bandwidth helps minimize
clicks and pops. If using the optional capacitor, C
capacitance see at the BYPASS pin is C
the values shown in Figure 1, Typical Audio Amplifier
Application Circuit, for C
capacitance seen by the BYPASS pin is not significant rela-
tive to capacitor value tolerances.
Optimizing Click and Pop Reduction Performance
The LM4818 contains circuitry that minimizes turn-on and
shutdown transients or ’clicks and pops’. For this discussion,
turn on refers to either applying the power or supply voltage
or when the shutdown mode is deactivated. While the power
supply is ramping to it’s final value, the LM4818’s internal
amplifiers are configured as unity gain buffers. An internal
current source charges the voltage of the bypass capacitor,
C
manner. Ideally, the input and outputs track the voltage
charging on the bypass capacitor. The gain of the internal
amplifiers remains unity until the bypass capacitor is fully
charged to 1/2V
capacitor is stable, the device becomes fully operational.
Although the BYPASS pin current cannot be modified,
changing the size of the bypass capacitor, C
device’s turn-on time and magnitude of ’clicks and pops’.
Increasing the value of C
pops. However, this presents a tradeoff: as the size of C
increases, the turn-on time (Ton) increases. There is a linear
relationship between the size of C
using the optional capacitor, C
at the BYPASS pin is C
at the BYPASS pin must be considered for the table below
and when optimizing click and pop performance. Below are
some typical turn-on times for various values of C
i
B
, connected to the BYPASS pin in a controlled, linear
shown in Figure 1 allows the LM4818 to drive a high
DD
), the smaller the turn-on pop. Choosing C
DD
B
. As soon as the voltage on the bypass
requires careful consideration. The by-
0.01µF
0.22µF
C
0.1µF
B
B
B
and C
B
reduces the magnitude of turn-on
equal to or greater than 1.0µF is
B
and C
B2
B2
, the total capacitance see
. The total capacitance see
B
T
200ms
440ms
B
ON
20ms
and the turn on time. If
B2
less than 1.0µF, the
DD
B
the change in the
B
+ C
), the smaller the
B
determines how
i
, the capacitor
(in the range of
B2
. When using
B
B2
, alters the
i
B
no larger
, the total
:
B
B