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LM4861 Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM4861
Description  1.1W Audio Power Amplifier with Shutdown Mode
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM4861 Datasheet(HTML) 6 Page - National Semiconductor (TI)

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Application Information
As shown in
Figure 1 , the LM4861 has two operational am-
plifiers internally, allowing for a few different amplifier con-
figurations. 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
f to Ri while
the second amplifier’s gain is fixed by the two internal 40 k
Figure 1 shows that the output of amplifier one
serves as the input to amplifier two which results in both am-
plifiers producing signals identical in magnitude, but out of
phase 180˚. Consequently, the differential gain for the IC is:
vd =2*(Rf/Ri)
By driving the load differentially through outputs V
O1 and
O2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion 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. Consequently, four times the output power is
possible as compared to a single-ended amplifier under the
same conditions. 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 caus-
ing excessive clipping which will damage high frequency
transducers used in loudspeaker systems, please refer to
the Audio Power Amplifier Design section.
A bridge configuration, such as the one used in Boomer Au-
dio Power Amplifiers, also creates a second advantage over
single-ended amplifiers. Since the differential outputs, V
and V
O2, are biased at half-supply, no net DC voltage exists
across the load. This eliminates the need for an output cou-
pling capacitor which is required in a single supply, single-
ended amplifier configuration. Without an output coupling ca-
pacitor in a single supply, single-ended amplifier, the half-
supply bias across the load would result in both increased
internal IC power dissipation and also permanent loud-
speaker damage. An output coupling capacitor forms a high
pass filter with the load requiring that a large value such as
470 µF be used with an 8
Ω load to preserve low frequency
response. This combination does not produce a flat re-
sponse down to 20 Hz, but does offer a compromise be-
tween printed circuit board size and system cost, versus low
frequency response.
Power dissipation is a major concern when designing a suc-
cessful amplifier, whether the amplifier is bridged or single-
ended. A direct consequence of the increased power deliv-
ered to the load by a bridge amplifier is an increase in
internal power dissipation. Equation 1 states the maximum
power dissipation point for a bridge amplifier operating at a
given supply voltage and driving a specified output load.
DMAX = 4*(VDD)
Since the LM4861 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 in-
crease in power dissipation, the LM4861 does not require
heatsinking. From Equation 1, assuming a 5V power supply
and an 8
Ω load, the maximum power dissipation point is
625 mW.The maximum power dissipation point obtained
from Equation 1 must not be greater than the power dissipa-
tion that results from Equation 2:
For the LM4861 surface mount package,
JA = 140˚C/W and
JMAX = 150˚C. Depending on the ambient temperature, TA,
of the system surroundings, Equation 2 can be used to find
the maximum internal power dissipation supported by the IC
packaging. If the result of Equation 1 is greater than that of
Equation 2, then either the supply voltage must be de-
creased or the load impedance increased. For the typical ap-
plication of a 5V power supply, with an 8
Ω load, the maxi-
mum ambient temperature possible without violating the
maximum junction temperature is approximately 62.5˚C pro-
vided that device operation is around the maximum power
dissipation point. Power dissipation is a function of output
power and thus, if typical operation is not around the maxi-
mum power dissipation point, the ambient temperature can
be increased. Refer to the Typical Performance Character-
istics curves for power dissipation information for lower out-
put powers.
As with any power amplifier, proper supply bypassing is criti-
cal for low noise performance and high power supply rejec-
tion. The capacitor location on both the bypass and power
supply pins should be as close to the device as possible. As
displayed in the Typical Performance Characteristics sec-
tion, the effect of a larger half supply bypass capacitor is im-
proved low frequency THD + N 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 sup-
ply nodes of the LM4861. The selection of bypass capaci-
tors, especially C
B, is thus dependant upon desired low fre-
quency THD + N, system cost, and size constraints.
In order to reduce power consumption while not in use, the
LM4861 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. The shutdown feature turns the am-
plifier off when a logic high is placed on the shutdown pin.
Upon going into shutdown, the output is immediately discon-
nected from the speaker. A typical quiescent current of
0.6 µA results when the supply voltage is applied to the shut-
down pin. In many applications, a microcontroller or micro-
processor output is used to control the shutdown circuitry
which provides a quick, smooth transition into shutdown. An-
other solution is to use a single-pole, single-throw switch that
when closed, is connected to ground and enables the ampli-
fier. If the switch is open, then a soft pull-up resistor of 47 k
will disable the LM4861. There are no soft pull-down resis-
tors inside the LM4861, so a definite shutdown pin voltage
must be applied externally, or the internal logic gate will be
left floating which could disable the amplifier unexpectedly.
The LM4861 is unity-gain stable and requires no external
components besides gain-setting resistors, an input coupling
capacitor, and proper supply bypassing in the typical appli-
cation. However, if a closed-loop differential gain of greater
than 10 is required, a feedback capacitor may be needed, as
shown in
Figure 2, to bandwidth limit the amplifier. This feed-
back capacitor creates a low pass filter that eliminates pos-
sible high frequency oscillations. Care should be taken when
calculating the −3 dB frequency in that an incorrect combina-

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