LM4864MMX National Semiconductor, LM4864MMX Datasheet - Page 11

• Manufacturer Part Number: LM4864MMX
• Description: IC AMP AUDIO PWR .725W AB 8MSOP
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4864MMX

• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 725mW x 1 @ 8 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Features: Shutdown, Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: LM4864MMXTR

Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4864 has two operational
amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier’s gain is externally config-
urable, while the second amplifier is internally fixed in a
unity-gain, inverting configuration. The closed-loop gain of
the first amplifier is set by selecting the ratio of R
the second amplifier’s gain is fixed by the two internal 10kΩ
resistors. Figure 1 shows that the output of amplifier one
serves as the input to amplifier two which results in both
amplifiers producing signals identical in magnitude, but out
of phase 180˚. Consequently, the differential gain for the IC
is
By driving the load differentially through outputs V
an amplifier configuration commonly referred to as “bridged
mode” is established. Bridged mode operation is different
from the classical single-ended amplifier configuration where
one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the Audio Power Amplifier
Design section.
A bridge configuration, such as the one used in LM4864,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, V
half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. If an output coupling capacitor is not used in a single-
ended configuration, the half-supply bias across the load
would result in both increased internal lC power dissipation
as well as permanent loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. Equation 1 states the maximum power dissi-
pation point for a bridge amplifier operating at a given supply
voltage and driving a specified output load.
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in
internal power dissipation point for a bridge amplifier oper-
ating at the same conditions.
Since the LM4864 has two operational amplifiers in one
package, the maximum internal power dissipation is 4 times
that of a single-ended amplifier. Even with this substantial
increase in power dissipation, the LM4864 does not require
heatsinking. From Equation 1, assuming a 5V power supply
and an 8Ω load, the maximum power dissipation point is
633 mW. The maximum power dissipation point obtained
from Equation 2 must not be greater than the power dissi-
pation that results from Equation 3:
P
P
DMAX
DMAX
P
= (V
= 4(V
DMAX
DD
DD
= (T
)
2
A
)
/(2π
2
VD
/(2π
JMAX
2
= 2*(R
R
2
R
L
)
− T
L
)
F
A
/R
)/θ
o1
Single-Ended (1)
i
Bridge Mode (2)
)
JA
and V
o2
(3)
, are biased at
F
o1
to R
and V
i
while
o2
,
11
For package MUA08A, θ
θ
package LDA10A, θ
LM4864. Depending on the ambient temperature, T
system surroundings, Equation 3 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 2 is greater than that of
Equation 3, then either the supply voltage must be de-
creased, the load impedance increased, the ambient tem-
perature reduced, or the θ
many cases larger traces near the output, V
pins can be used to lower the θ
provide a form of heatsinking allowing a higher power dissi-
pation. For the typical application of a 5V power supply, with
an 8Ω load, the maximum ambient temperature possible
without violating the maximum junction temperature is ap-
proximately 44˚C provided that device operation is around
the maximum power dissipation point and assuming surface
mount packaging. Internal power dissipation is a function of
output power. If typical operation is not around the maximum
power dissipation point, the ambient temperature can be
increased. Refer to the Typical Performance Characteris-
tics curves for power dissipation information for lower output
powers.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATION
The LM4864’s exposed-dap (die attach paddle) package
(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
layout, fabrication, and mounting an LD (LLP) package is
available from National Semiconductor’s Package Engineer-
ing Group under application note AN1187.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. The capacitor location on both the bypass and
power supply pins should be as close to the device as
possible. The effect of a larger half supply bypass capacitor
is improved PSRR due to increased half-supply stability.
Typical applications employ a 5V regulator with 10 µF and a
0.1 µF bypass capacitors which aid in supply stability, but do
not eliminate the need for bypassing the supply nodes of the
LM4864. The selection of bypass capacitors, especially C
is thus dependent upon desired PSRR requirements, click
and pop performance as explained in the section, Proper
Selection of External Components, system cost, and size
constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4864 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic high is placed on the shutdown pin.
The trigger point between a logic low and logic high level is
typically half supply. It is best to switch between ground and
supply to provide maximum device performance. By switch-
ing the shutdown pin to V
JA
= 170˚C/W, for package N08E, θ
JA
= 63˚C/W. T
JA
JA
DD
= 210˚C/W, for package M08A,
, the LM4864 supply current
JA
reduced with heatsinking. In
. The larger areas of copper
JA
JMAX
= 107˚C/W, and for
= 150˚C for the
DD
www.national.com
, and GND
A
, of the
B
,