Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4860 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
f
to R
i
while
the second amplifier’s gain is fixed by the two internal 40 kΩ
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:
A
vd
=2*(R
f
/R
i
)
By driving the load differentially through outputs V
O1
and
V
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 Deslgn 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
O1
and V
O2
, are biased at 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 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 470 µF be used with an 8Ωload to preserve low fre-
quency response. This combination does not produce a flat
response down to 20 Hz, but does offer a compromise
between printed circuit board size and system cost, versus
low frequency 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.
P
DMAX
=4*(V
DD
)
2
/(2π
2
R
L)
(1)
Since the LM4860 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 LM4860 does not require
heatsinking. From Equation 1, assuming a 5V power supply
and an 8Ωload, the maximum power dissipation point is 625
mW. The maximum power dissipation point obtained from
Equation 1 must not be greater than the power dissipation
that results from Equation 2:
P
DMAX
=(T
JMAX
−T
A
)/θ
JA
(2)
For the LM4860 surface mount package, θ
JA
= 100˚C/W and
T
JMAX
= 150˚C. Depending on the ambient temperature, T
A
,
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 8Ωload, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 88˚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 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 LM4860. The selection of
bypass capacitors, especially C
B
, is thus dependant upon
desired low frequency THD+N, system cost, and size con-
straints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4860 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. There is a built-in threshold which
produces a drop in quiescent current to 500 µA typically. For
a 5V power supply, this threshold occurs when 2V–3V is
applied to the shutdown pin. A typical quiescent current of
0.6 µA results when the supply voltage is applied to the
shutdown pin. In many applications, a microcontroller or
microprocessor output is used to control the shutdown cir-
cuitry which provides a quick, smooth transition into shut-
down. Another solution is to use a single-pole, single-throw
switch that when closed, is connected to ground and enables
the amplifier. If the switch is open, then a soft pull-up resistor
of 47 kΩwill disable the LM4860. There are no soft pull-
down resistors inside the LM4860, so a definite shutdown
pin voltage must be appliied externally, or the internal logic
gate will be left floating which could disable the amplifier
unexpectedly.
HEADPHONE CONTROL INPUTS
The LM4860 possesses two headphone control inputs that
disable the amplifier and reduce I
DD
to less than 1 mA when
either one or both of these inputs have a logic-high voltage
placed on their pins.
LM4860
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