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LM4860 Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM4860
Description  Series 1W Audio Power Amplifier with Shutdown Mode
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM4860 Datasheet(HTML) 6 Page - National Semiconductor (TI)

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Application Information (Continued)
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 Au-
dio 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 cou-
pling 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 8
Ω load to preserve low fre-
quency response. This combination does not produce a flat
response down to 20 Hz, but does offer a compromise be-
tween printed circuit board size and system cost, versus low
frequency response.
POWER DISSIPATION
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
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 * (VDD)
2/(2
π2R
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 in-
crease in power dissipation, the LM4860 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 dissipation
that results from Equation 2:
P
DMAX = (TJMAX −TA)/θJA
(2)
For the LM4860 surface mount package,
θ
JA = 100˚C/W and
T
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 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 Character-
istics curves for power dissipation information for lower out-
put powers.
POWER SUPPLY BYPASSING
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 LM4860. The selection of bypass capaci-
tors, especially C
B, is thus dependant upon desired low fre-
quency THD+N, system cost, and size constraints.
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 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. 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 ap-
plied to the shutdown pin. 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 LM4860. There are no soft pull-down resis-
tors 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.
Unlike the shutdown function, the headphone control func-
tion does not provide the level of current conservation that is
required for battery powered systems. Since the quiescent
current resulting from the headphone control function is
1000 times more than the shutdown function, the residual
currents in the device may create a pop at the output when
coming out of the headphone control mode. The pop effect
may be eliminated by connecting the headphone sensing
output to the shutdown pin input as shown in
Figure 4. This
solution will not only eliminate the output pop, but will also
utilize the full current conservation of the shutdown function
by reducing I
DD to 0.6 µA. The amplifier will then be fully
shutdown. This configuration also allows the designer to use
the control inputs as either two headphone control pins or a
headphone control pin and a shutdown pin where the lowest
level of current consumption is obtained from either function.
Figure 5 shows the implementation of the LM4860’s head-
phone control function using a single-supply headphone am-
plifier. The voltage divider of R1 and R2 sets the voltage at
the HP-IN1 pin to be approximately 50 mV when there are
no headphones plugged into the system. This logic-low volt-
age at the HP-IN1 pin enables the LM4860 to amplify AC sig-
nals. Resistor R3 limits the amount of current flowing out of
the HP-IN1 pin when the voltage at that pin goes below
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