參數(shù)資料
型號(hào)： LM4817MH/NOPB
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻／視頻放大
英文描述： 1.5 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO28
封裝： 4.40 MM, TSSOP-28
文件頁(yè)數(shù)： 5/21頁(yè)
文件大?。?/td> 1119K
代理商： LM4817MH/NOPB
Application Information (Continued)
The LM4817 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
the bridge mode is four times that of a single-ended ampli-
fier. From Equation (3), assuming a 5V power supply and an
8
load, the maximum single channel power dissipation is
0.633W or 1.27W for stereo operation.
P
DMAX =4x(VDD)
2 /(2
π2 R
L) Bridge Mode
(3)
The LM4817’s power dissipation is twice that given by Equa-
tion (2) or Equation (3) when operating in the single-ended
mode or bridge mode, respectively. Twice the maximum
power dissipation point given by Equation (3) must not ex-
ceed the power dissipation given by Equation (4):
P
DMAX’= (TJMAX TA)/
θ
JA
(4)
The LM4817’s T
JMAX = 150C. In the MH package soldered
to a DAP pad that expands to a copper area of 2in
2 on a PCB
, the LM4817’s
θ
JA is 41C/W. At any given ambient tempera-
ture T
J\A, use Equation (4) to find the maximum internal
power dissipation supported by the IC packaging. Rearrang-
ing Equation (4) and substituting P
DMAX for PDMAX’ results in
Equation (5). This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipa-
tion without violating the LM4817’s maximum junction tem-
perature.
T
A =TJMAX 2xPDMAX
θ
JA
(5)
For a typical application with a 5V power supply and an 8
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 98C for the MH
package.
T
JMAX =PDMAX
θ
JA +TA
(6)
Equation (6) gives the maximum junction temperature T
J-
MAX
. If the result violates the LM4817’s 150C, reduce the
maximum junction temperature by reducing the power sup-
ply voltage or increasing the load resistance. Further allow-
ance should be made for increased ambient temperatures.
The above examples assume 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 ambient temperatures are allowed as output
power or duty cycle decreases.
If twice the value given by Equation (3) exceeds the result of
Equation (4), then decrease the supply voltage, increase the
load impedance, or reduce the ambient temperature. If these
measures are insufficient, a heat sink can be added to
reduce
θ
JA. The heat sink can be created using additional
copper area around the package, with connections to the
ground pin(s), supply pin and amplifier output pins. External,
solder attached SMT heatsinks such as the Thermalloy
7106D can also improve power dissipation. When adding a
heat sink, the
θ
JA is the sum of
θ
JC,
θ
CS, and
θ
SA.(
θ
JC is the
junctiontocase thermal impedance,
CS is the casetosink
thermal impedance, and
θ
SAis the sinktoambient thermal
impedance.) Refer to the Typical Performance Characteris-
tics curves for power dissipation information at lower output
power levels.
OUTPUT VOLTAGE LIMITER
The LM4817’s adjustable output voltage limiter can be used
to set a maximum and minimum output voltage swing mag-
nitude. The voltage applied to the V
LIM input (pin 28) controls
the amount voltage limit magnitude.
Without the limiter’s influence (V
LIM = 0V), the LM4817’s
maximum BTL output swing is nominally
2xV
DD
When the limiter input voltage is greater than 0V, the BTL
output voltage swing is
V
OUT-BTL =(2xVDD)-(4xVLIM)
For any given value of V
LIM, the actual output swing will be
limited to within ±200mV.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically
use a 10F in parallel with a 0.1F filter capacitors to stabi-
lize the regulator’s output, reduce noise on the supply line,
and improve the supply’s transient response. However, their
presence does not eliminate the need for a local 1.0F
tantalum bypass capacitance connected between the
LM4817’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation in the output signal. Keep the length of leads and
traces that connect capacitors between the LM4817’s power
supply pin and ground as short as possible. Connecting a
1F capacitor, C
B, between the BYPASS pin and ground
improves the internal bias voltage’s stability and improves
the amplifier’s PSRR. The PSRR improvements increase as
the bypass pin capacitor value increases. Too large, how-
ever, increases turn-on time and can compromise amplifier’s
click and pop performance. The selection of bypass capaci-
tor values, especially C
B, depends on desired PSRR require-
ments, click and pop performance (as explained in the sec-
tion, Proper Selection of External Components), system
cost, and size constraints.
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4817’s shutdown function. Activate micro-power shut-
down by applying V
DD to the SHUTDOWN pin. When active,
the LM4817’s micro-power shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically V
DD/2. The low 0.7A typical
shutdown current is achieved by applying a voltage that is as
near as V
DD as possible to the SHUTDOWN pin. A voltage
thrat is less than V
DD 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 10k
pull-up resistor between the
SHUTDOWN pin and V
DD. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier opera-
tion by closing the switch. Opening the switch connects the
SHUTDOWN pin to V
DD through the pull-up resistor, activat-
ing micro-power shutdown. The switch and resistor guaran-
tee that the SHUTDOWN pin will not float. This prevents
unwanted state changes. In a system with a microprocessor
or a microcontroller, use a digital output to apply the control
voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin
with active circuitry eliminates the pull up resistor.
TABLE 1. LOGIC LEVEL TRUTH TABLE FOR SHUT-
DOWN OPERATION
SHUTDOWN
OPERATIONAL MODE
Low
Full power, stereo BTL
amplifiers
High
Micro-power Shutdown
LM4817
www.national.com
13
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