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參數(shù)資料
| 型號： | LM4884MH/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 2.6 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO20 |
| 封裝： | TSSOP-20 |
| 文件頁數(shù)： | 8/22頁 |
| 文件大小： | 1189K |
| 代理商： | LM4884MH/NOPB |
Application Information (Continued)
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful bridged or single-ended amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified output load.
P
DMAX =(VDD)
2/(2
π2R
L)
Single-Ended
(2)
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in the
internal power dissipation point for a bridge amplifier oper-
ating at the same given conditions.
P
DMAX =4*(VDD)
2/(2
π2R
L)
Bridge Mode
(3)
The LM4884 has four operational amplifiers in one package
and the maximum internal power dissipation is four times
that of a single-ended amplifier. From Equation (3), assum-
ing a 5V power supply and an 8
load, the maximum power
dissipation point is 2W. The maximum power dissipation
point obtained from Equation (3) must not exceed the power
dissipation predicted by Equation (4):
P
DMAX =(TJMAX TA)/
θ
JA
(4)
For the exposed DAP TSSOP package,
θ
JA=
41C/W.
T
JAMAX = 150C for the LM4884. For a given ambient tem-
perature T
A, Equation (4) can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation (3) is greater than that of Equation (4),
decrease the supply voltage, increase the load impedance,
or reduce the ambient temperature. For a typical application
with a 5V power supply and an 8
load, the maximum
ambient temperature that does not violate the maximum
junction temperature is approximately 68C. This further as-
sumes that a device is a surface mount part operating
around the maximum power dissipation point. Since internal
power dissipation is a function of output power, higher am-
bient temperatures are allowed as output power decreases.
Refer to the Typical Performance Characteristics curves for
power dissipation information at lower output power levels.
BTL GAIN SELECTION
The LM4884 features four fixed, internally set, BTL voltage
gains: 6dB, 10dB, 15.6dB, and 21.6dB. Select one of the
four gains by applying a logic level signal to the GAIN0
(MSB) and GAIN1 (LSB) digital inputs.
The closed-loop gain of the first amplifier is adjustable, hav-
ing four different gains, whereas two internal 20k
resistors
set the second amplifier’s gain at -1. Table 1 below, shows
the state of the two logic inputs required to select one of the
four gain values.
Table 1. Gain Settings and Input Resistance
GAIN 0 GAIN 1
Selected Gain
(dB)
Input Resistance
(R
i)
0
6
90k
0
1
10
70k
1
0
15.6
45k
1
21.6
25k
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. The capacitors connected to the bypass and power
supply pins should be placed as close to the LM4884 as
possible. The capacitor connected between the bypass pin
and ground improves the internal bias voltage’s stability,
producing improved PSRR. The improvements to PSRR
increase as the bypass pin capacitor value increases.
Typical applications employ a 5V regulator with 10F and a
0.1F filter capacitors that aid in supply stability. Their pres-
ence, however, does not eliminate the need for bypassing
the LM4884’s supply pins. The selection of bypass capacitor
values, especially C
B, depends on desired PSRR require-
ments, click and pop performance (as explained in theSe-
lecting External Components section), system cost, and
size constraints.
MICRO-POWER SHUTDOWN
The LM4884 features an active-low micro-power shutdown
mode. The voltage applied to the SHUTDOWN pin controls
the LM4884’s shutdown function. Activate micro-power shut-
down by applying 0V to the SHUTDOWN pin. The logic
threshold is typically 0.4V for a logic low and 1.5V for a logic
high. When active, the LM4884’s micro-power shutdown
feature turns off the amplifier’s bias circuitry, disables the
internal V
DD/2 generator, and forces the amplifier outputs
into a high impedance state. The result is greatly reduced
power supply current. The low 0.1A typical shutdown cur-
rent is achieved by applying a voltage to the SHUTDOWN
pin that is as near to GND as possible. A voltage that is
greater than GND may increase the shutdown current.
There are a few methods to control the micro-power shut-
down. These include using a single-pole, single-throw switch
(SPST), a microprocessor, or a microcontroller. When using
a switch, connect a 100k
pull-down resistor between the
SHUTDOWN pin and GND and the SPST switch between
the SHUTDOWN pin and V
DD. Select normal amplifier op-
eration by closing the switch. Opening the switch applies
GND to the SHUTDOWN pin, activating micro-power shut-
down. The switch and resistor guarantee that the SHUT-
DOWN pin will not float. This prevents unwanted state
changes. In a system with a microprocessor or a microcon-
troller, use a digital output to apply the active-state voltage to
the SHUTDOWN pin. Driving the SHUTDOWN pin with ac-
tive circuitry eliminates the pull-down resistor.
LM4884
www.national.com
16
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