LM4861MX/NOPB National Semiconductor, LM4861MX/NOPB Datasheet - Page 8

• Manufacturer Part Number: LM4861MX/NOPB
• Description: Audio Amp Speaker 1-CH Mono 1.1W Class-AB 8-Pin SOIC N T/R
• Manufacturer: National Semiconductor

Specifications of LM4861MX/NOPB

• Package: 8SOIC N
• Function: Speaker
• Amplifier Type: Class-AB
• Total Harmonic Distortion Noise: 0.72@8Ohm@1W %
• Typical Psrr: 65 dB
• Output Signal Type: Differential
• Maximum Load Resistance: 8 Ohm
• Output Type: 1-Channel Mono
• Amplifier Class: AB
• No. Of Channels: 1
• Output Power: 1.1W
• Supply Voltage Range: 2V To 5.5V
• Load Impedance: 8ohm
• Operating Temperature Range: -40°C To +85°C
• Amplifier Case Style: SOIC
• Rohs Compliant: Yes

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Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1 , the LM4861 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 40kΩ
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
V
“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
Audio Power Amplifiers, also creates a second advantage
over single-ended amplifiers. Since the differential outputs,
V
exists across the load. This eliminates the need for an output
coupling capacitor which is required in a single supply,
single-ended amplifier configuration. Without an output cou-
pling capacitor in a single supply, single-ended amplifier, the
half-supply bias across the load would result in both in-
creased internal IC power dissipation and also permanent
loudspeaker damage. An output coupling capacitor forms a
high pass filter with the load requiring that a large value such
as 470µF be used with an 8Ω load to preserve low frequency
response. This combination does not produce a flat re-
sponse down to 20Hz, but does offer a compromise between
printed circuit board size and system cost, versus low fre-
quency response.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered 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.
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
increase in power dissipation, the LM4861 does not require
heatsinking. From Equation 1, assuming a 5V power supply
and an 8Ω load, the maximum power dissipation point is
O2
O1
, an amplifier configuration commonly referred to as
and V
O2
, are biased at half-supply, no net DC voltage
P
DMAX
A
= 4*(V
vd
= 2 * (R
DD
)
2
f
/ (2π
/ R
i
)
2
R
L
)
f
to R
O1
i
while
and
(1)
8
625mW.The maximum power dissipation point obtained from
Equation 1 must not be greater than the power dissipation
that results from Equation 2:
For the LM4861 surface mount package, θ
T
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
application of a 5V power supply, with an 8Ω load, the
maximum 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 Charac-
teristics curves for power dissipation information for lower
output powers.
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. As displayed in the Typical Performance Charac-
teristics section, the effect of a larger half supply bypass
capacitor is improved low frequency THD+N due to in-
creased half-supply stability. Typical applications employ a
5V regulator with 10µF and a 0.1µF bypass capacitors which
aid in supply stability, but do not eliminate the need for
bypassing the supply nodes of the LM4861. The selection of
bypass capacitors, especially C
desired low frequency THD+N, system cost, and size con-
straints.
SHUTDOWN FUNCTION
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
amplifier 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 0.6µA
results when the supply voltage is applied to the shutdown
pin. In many applications, a microcontroller or microproces-
sor output is used to control the shutdown circuitry which
provides a quick, smooth transition into shutdown. Another
solution is to use a single-pole, single-throw switch that
when closed, is connected to ground and enables the am-
plifier. If the switch is open, then a soft pull-up resistor of
47kΩ will disable the LM4861. There are no soft pull-down
resistors 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 unex-
pectedly.
HIGHER GAIN AUDIO AMPLIFIER
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
feedback capacitor creates a low pass filter that eliminates
JMAX
= 150˚C. Depending on the ambient temperature, T
P
DMAX
= (T
JMAX
− T
B
, is thus dependant upon
A
) / θ
JA
JA
= 140˚C/W and
(2)
A
,