lm4911qmm National Semiconductor Corporation, lm4911qmm Datasheet - Page 18

• Manufacturer Part Number: lm4911qmm
• Description: Stereo Headphone Amplifier For Automotive
• Manufacturer: National Semiconductor Corporation

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POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a suc-
cessful design. When operating in capacitor-coupled mode,
Equation 1 states the maximum power dissipation point for a
single-ended amplifier operating at a given supply voltage
and driving a specified output load.
Since the LM4911QMM has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number which results from Equation 1. From
Equation 1, assuming a 3V power supply and an 32Ω load,
the maximum power dissipation point is 14mW per amplifier.
Thus the maximum package dissipation point is 28mW.
When operating in OCL mode, the maximum power dissipa-
tion increases due to the use of the third amplifier as a buffer
and is given in Equation 2:
The maximum power dissipation point obtained from either
Equation 1 or 2 must not be greater than the power dissipation
that results from Equation 3:
For package MUB10A, θ
θ
ing on the ambient temperature, T
ings, Equation 3 can be used to find the maximum internal
power dissipation supported by the IC packaging. If the result
of Equation 1 or 2 is greater than that of Equation 3, then
either the supply voltage must be decreased, the load
impedance increased or T
tion of a 3V power supply, with a 32Ω load, the maximum
ambient temperature possible without violating the maximum
junction temperature is approximately 144°C provided that
device operation is around the maximum power dissipation
point. Thus, for typical applications, power dissipation is not
an issue. Power dissipation is a function of output power and
thus, if typical operation is not around the maximum power
dissipation point, the ambient temperature may be increased
accordingly. Refer to the Typical Performance Characteristics
curves for power dissipation information for lower output pow-
ers.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4911QMM's exposed-DAP (die attach paddle) pack-
age (LD) provides a low thermal resistance between the die
and the PCB to which the part is mounted and soldered. This
allows rapid heat transfer from the die to the surrounding PCB
copper traces, ground plane, and surrounding air.
The LD package should have its DAP soldered to a copper
pad on the PCB. The DAP's PCB copper pad may be con-
nected to a large plane of continuous unbroken copper. This
plane forms a thermal mass, heat sink, and radiation area.
Further detailed and specific information concerning PCB lay-
out, fabrication, and mounting an LD (LLP) package is avail-
JA
= 63°C/W. T
P
P
P
DMAX
DMAX
DMAX
JMAX
= 150°C for the LM4911QMM. Depend-
= (V
= 4(V
= (T
JA
JMAX
DD
A
DD
= 190°C/W; for package LDA10A,
)
reduced. For the typical applica-
2
)
2
- T
/ (2
/ (
A
π
π
) / θ
A
, of the system surround-
2
2
R
R
JA
L
L
)
)
(1)
(2)
(3)
18
able from National Semiconductor's Package Engineering
Group under application note AN1187.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important
for low noise performance and high power supply rejection.
The capacitor location on the power supply pins should be as
close to the device as possible.
Typical applications employ a 3V regulator with 10mF tanta-
lum or electrolytic capacitor and a ceramic bypass capacitor
which aid in supply stability. This does not eliminate the need
for bypassing the supply nodes of the LM4911QMM. A by-
pass capacitor value in the range of 0.1µF to 1µF is recom-
mended for C
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4911QMM's shutdown function. Activate micro-power
shutdown by applying a logic-low voltage to the SHUTDOWN
pin. When active, the LM4911QMM's micro-power shutdown
feature turns off the amplifier's bias circuitry, reducing the
supply current. The trigger point varies depending on supply
voltage and is shown in the Shutdown Hysteresis Voltage
graphs in the Typical Performance Characteristics section.
The low 0.1µA(typ) shutdown current is achieved by applying
a voltage that is as near as ground as possible to the SHUT-
DOWN 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 100kΩ pull-up re-
sistor between the SHUTDOWN pin and V
switch between the SHUTDOWN pin and ground. Select nor-
mal amplifier operation by opening the switch. Closing the
switch connects the SHUTDOWN pin to ground, activating
micro-power shutdown.
The switch and resistor guarantee that the SHUTDOWN pin
will not float. This prevents unwanted state changes. In a sys-
tem with a microprocessor or 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.
Shutdown enable/disable times are controlled by a combina-
tion of C
on/off times from Shutdown. Smaller V
turn on/off time for a given value of C
times also improve the LM4911QMM's resistance to click and
pop upon entering or returning from shutdown. For a 2.4V
supply and C
seconds to enter or return from shutdown. This longer shut-
down time enables the LM4911QMM to have virtually zero
pop and click transients upon entering or release from shut-
down.
Smaller values of C
of increased pop and click and reduced PSRR. Since shut-
down enable/disable times increase dramatically as supply
voltage gets below 2.2V, this reduced turn-on time may be
desirable if extreme low supply voltage levels are used as this
would offset increases in turn-on time caused by the lower
supply voltage. This technique is not recommended for OCL
mode since shutdown enable/disable times are very fast
(0.5s) independent of supply voltage.
When in cap-coupled mode, some restrictions on the usage
of Mute are in effect when entering or returning from shut-
down. These restrictions require Mute not be toggled imme-
diately following a return or entrance to shutdown for a brief
B
and V
B
S
.
= 4.7µF, the LM4911QMM requires about 2
DD
. Larger values of C
B
will decrease turn-on time, but at the cost
DD
B
results in longer turn
B
values also increase
. Longer shutdown
DD
. Connect the