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參數(shù)資料
| 型號(hào)： | LM4915 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Pseudo-Differential Mono Headphone Amplifier with Fixed 6dB Gain |
| 中文描述： | 偽差分單聲道耳機(jī)放大器，具有固定6dB增益 |
| 文件頁(yè)數(shù)： | 10/11頁(yè) |
| 文件大小： | 516K |
| 代理商： | LM4915 |
Application Information
(Continued)
then either the supply voltage must be decreased, the load
impedance increased, the ambient temperature reduced, or
the
θ
JA
reduced with heatsinking. In many cases, larger
traces near the output, V
DD
, and GND pins can be used to
lower the
θ
JA
. The larger areas of copper provide a form of
heatsinking allowing higher power dissipation. For the typical
application of a 5V power supply, with a 16
load power
dissipation is not an issue. Recall that internal power dissi-
pation is a function of output power. If typical operation is not
around the maximum power dissipation point, the LM4915
can operate at higher ambient temperatures. Refer to the
Typical Performance Characteristics
curves for power dis-
sipation information.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor location on both the
bypass and power supply pins should be as close to the
device as possible. A larger half-supply bypass capacitor
improves PSRR because it increases half-supply stability.
Typical applications employ a 5V regulator with 10μF and
0.1μF bypass capacitors that increase supply stability. This,
however, does not eliminate the need for bypassing the
supply nodes of the LM4915. A 1μF capacitor is recom-
mended for C
S
. A 4.7μF capacitor is recommended for C
B
.
This value coupled with small input capacitors (0.1μF to
0.47μF) gives virtually zero click and pop with outstanding
PSRR performance.
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4915’s shutdown function. Activate micro-power shut-
down by applying a logic-low voltage to the SHUTDOWN
pin. When active, the LM4915’s micro-power shutdown fea-
ture turns off the amplifier’s bias circuitry, reducing the sup-
ply current. The trigger point is 0.4V for a logic-low level, and
1.8V for a logic-high level. The low 0.1μA (typ) shutdown
current is achieved by applying a voltage that is as near as
ground as possible to the SHUTDOWN pin. A voltage that is
higher than ground may increase the shutdown current.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a
microprocessor, or a microcontroller. When using a switch,
connect an external 100k. pull-up resistor between the
SHUTDOWN pin and V
. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier opera-
tion by opening the switch. Closing the switch connects the
SHUTDOWN pin to ground, activating micro-power shut-
down.
The switch and resistor guarantee that the SHUTDOWN pin
will not float. This prevents unwanted state changes. In a
system with a microprocessor or microcontroller, use a digi-
tal output to apply the control voltage to the SHUTDOWN
pin. Driving the SHUTDOWN pin with active circuitry elimi-
nates the pull-up resistor.
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 LM4915 is tolerant of
external component combinations, and requires minimal ex-
ternal components, consideration to component values must
be used to maximize overall system quality.
The input coupling capacitor, C
, forms a first order high pass
filter which limits low frequency response given by f
=
1/(2
π
R
C
). R
is internally set to 20k
. 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 100Hz to 150Hz. 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 affected by the size of the input coupling capacitor,
C
. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 V
DD
). This
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
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
B
, is the most critical component to minimize
turn-on pops since it determines how fast the LM4915 turns
on. The slower the LM4915’s outputs ramp to their quiescent
DC voltage (nominally 1/2 V
), the smaller the turn-on pop.
Choosing C
equal to 4.7μF along with a small value of C
(in
the range of 0.1μF to 0.47μF), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
C
equal to 1.0μF, the device will be much more susceptible
to turn-on clicks and pops. Thus, a value of C
equal to
4.7μF is recommended in all but the most cost sensitive
designs.
L
www.national.com
10
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