lm4891mm National Semiconductor Corporation, lm4891mm Datasheet - Page 10

• Manufacturer Part Number: lm4891mm
• Description: 1 Watt Audio Power Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4891 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic high is placed on the shutdown pin.
By switching the shutdown pin to V
current draw will be minimized in idle mode. While the device
will be disabled with shutdown pin voltages more than
1.0V
value of 0.1µA. (Idle current is measured with the shutdown
pin tied to V
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry to provide a
quick, smooth transition into shutdown. Another solution is to
use a single-pole, single-throw switch in conjunction with an
external pull-up resistor. When the switch is closed, the
shutdown pin is connected to ground which enables the
amplifier. If the switch is open, then the external pull-up
resistor to V
antees that the shutdown pin will not float thus preventing
unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4891 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4891 is unity-gain stable which gives the designer
maximum system flexibility. The LM4891 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the
section, Audio Power Amplifier Design, for a more com-
plete explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in Figure 1 . The input coupling capacitor, C
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed
frequency response for a few distinct reasons.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenu-
ation. But in many cases the speakers used in portable
systems, whether internal or external, have little ability to
reproduce signals below 100 Hz to 150 Hz. Thus, using a
large input capacitor may not increase actual system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
reach its quiescent DC voltage (nominally 1/2 V
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the
capacitor size based on necessary low frequency response,
turn-on pops can be minimized.
i.
A larger input coupling capacitor requires more charge to
DC
, the idle current may be greater than the typical
DD
DD
).
will disable the LM4891. This scheme guar-
DD
, the LM4891 supply
(Continued)
i
DD
, forms a
). This
10
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
turn-on pops since it determines how fast the LM4891 turns
on. The slower the LM4891’s outputs ramp to their quiescent
DC voltage (nominally 1/2 V
Choosing C
(in the range of 0.1 µF to 0.39 µF), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
C
to turn-on clicks and pops. Thus, a value of C
1.0 µF is recommended in all but the most cost sensitive
designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω AUDIO AMPLIFIER
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Per-
formance Characteristics section, the supply rail can be
easily found. A second way to determine the minimum sup-
ply rail is to calculate the required V
and add the output voltage. Using this method, the minimum
supply voltage would be (V
V
age vs Supply Voltage curve in the Typical Performance
Characteristics section.
5V is a standard voltage, in most applications, chosen for the
supply rail. Extra supply voltage creates headroom that al-
lows the LM4891 to reproduce peaks in excess of 1W with-
out producing audible distortion. At this time, the designer
must make sure that the power supply choice along with the
output impedance does not violate the conditions explained
in the Power Dissipation section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 3.
From Equation 3, the minimum A
Since the desired input impedance was 20 kΩ, and with a
A
R
address the bandwidth requirements which must be stated
as a pair of −3 dB frequency points. Five times away from a
−3 dB point is 0.17 dB down from passband response which
is better than the required
OD BOT
VD
B
Given:
i
= 20 kΩ and R
equal to 0.1 µF, the device will be much more susceptible
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
of 3, a ratio of 1.5:1 of R
f
f
L
H
= 100 Hz/5 = 20 Hz
= 20 kHz * 5 = 100 kHz
and V
B
B
, is the most critical component to minimize
equal to 1.0 µF along with a small value of C
OD TOP
f
A
are extrapolated from the Dropout Volt-
= 30 kΩ. The final design step is to
VD
= (R
opeak
±
DD
0.25 dB specified.
f
f
/R
to R
), the smaller the turn-on pop.
100 Hz–20 kHz
+ (V
i
) 2
VD
i
results in an allocation of
OD TOP
is 2.83; use A
opeak
+ V
using Equation 2
OD BOT
±
B
1 Wrms
0.25 dB
VD
1 Vrms
)), where
equal to
20 kΩ
= 3.
8Ω
(2)
(3)
i