LM4867MT National Semiconductor, LM4867MT Datasheet - Page 13

• Manufacturer Part Number: LM4867MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

Application Information
the bridge mode is four times that of a single-ended ampli-
fier. From Equation (3), assuming a 5V power supply and an
4
1.27W or 2.54W for stereo operation.
The LM4867’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):
The LM4867’s TJMAX = 150˚C. In the LQ package soldered
to a DAP pad that expands to a copper area of 5in
PCB, the LM4867’s
soldered to a DAP pad that expands to a copper area of 2in
on a PCB, the LM4867’s
temperature T
nal power dissipation supported by the IC packaging. Rear-
ranging Equation (4) and substituting P
sults in Equation (5). This equation gives the maximum
ambient temperature that still allows maximum stereo power
dissipation without violating the LM4867’s maximum junction
temperature.
For a typical application with a 5V power supply and an 4
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 99˚C for the LQ
package and 45˚C for the MTE package.
Equation (6) gives the maximum junction temperature
T
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 the result of Equation (2) is greater than that of Equation
(3), 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
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
JMAX
load, the maximum single channel power dissipation is
. If the result violates the LM4867’s 150˚C, reduce the
JA
P
. The heat sink can be created using additional
DMAX
A
JA
, use Equation (4) to find the maximum inter-
T
= 4
is the sum of
P
A
T
DMAX
= T
JMAX
*
JA
(V
JMAX
’ = (T
JA
DD
is 20˚C/W. In the MTE package
= P
is 41˚C/W. At any given ambient
)
2
− 2 X P
/(2
DMAX
JMAX
JC
2
,
R
− T
L
CS
JA
): Bridge Mode
DMAX
, and
A
DMAX
+ T
)/
(Continued)
JA
A
JA
for P
SA
. (
DMAX
JC
2
is the
on a
’ re-
(3)
(4)
(5)
(6)
2
13
junction−to−case
case−to−sink
sink−to−ambient thermal impedance.) Refer to the Typical
Performance Characteristics curves for power dissipation
information at lower output power levels.
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 10µF in parallel with a 0.1µF 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 1.0µF
tantalum bypass capacitance connected between the
LM4867’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation. Keep the length of leads and traces that connect
capacitors between the LM4867’s power supply pin and
ground as short as possible. Connecting a 1µF capacitor,
C
internal bias voltage’s stability and improves the amplifier’s
PSRR. The PSRR improvements increase as the bypass pin
capacitor value increases. Too large, however, increases
turn−on time and can compromise the amplifier’s click and
pop performance. The selection of bypass capacitor values,
especially C
click and pop performance (as explained in the section,
Proper Selection of External Components), system cost,
and size constraints.
MICRO−POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4867’s shutdown function. Activate micro−power shut-
down by applying V
the LM4867’s micro−power shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically V
shutdown current is achieved by applying a voltage that is as
near as V
that is less than V
Table 1 shows the logic signal levels that activate and deac-
tivate micro−power shutdown and headphone amplifier op-
eration. To ensure that the output signal remains
transient−free, do not cycle the shutdown function
faster than 1Hz.
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
SHUTDOWN pin and V
SHUTDOWN pin and ground. Select normal amplifier opera-
tion by closing the switch. Opening the switch connects the
SHUTDOWN pin to V
vating micro−power shutdown. The switch and resistor guar-
antee 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.
B
, between the BYPASS pin and ground improves the
DD
B
as possible to the SHUTDOWN pin. A voltage
, depends on desired PSRR requirements,
thermal
DD
DD
thermal
DD
to the SHUTDOWN pin. When active,
may increase the shutdown current.
DD
impedance,
through the pull−up resistor, acti-
. Connect the switch between the
pull−up resistor between the
DD
impedance,
/2. The low 0.7µA typical
and
CS
SA
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