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LM4865 Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM4865
Description  750 mW Audio Power Amplifier with DC Volume Control and Headphone Switch
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM4865 Datasheet(HTML) 6 Page - National Semiconductor (TI)

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Application Information (Continued)
A bridge configuration, such as the one used in LM4865,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, V
O1 and VO2, 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 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 IC power dissipa-
tion as well as permanent loudspeaker damage.
Power dissipation is a major concern when designing a suc-
cessful amplifier, whether the amplifier is bridged or
Equation (1) states the maximum power dissi-
pation point for a single-ended 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 in-
ternal power dissipation point for a bridge amplifier operating
at the same given conditions.
DMAX = 4*(VDD)
Bridge Mode
Since the LM4865 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 LM4865 does not require
heatsinking. From
Equation (1), assuming a 5V power sup-
ply and an 8
Ω load, the maximum power dissipation point is
633 mW. The maximum power dissipation point obtained
Equation (2) must not be greater than the power dissi-
pation that results from
Equation (3):
For package M08A,
= 150˚C/W, and for package
JA = 190˚C/W. TJMAX = 150˚C for the LM4865.
Depending on the ambient temperature, T
A, of the system
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 decreased, the load
impedance increased, or the ambient temperature reduced.
For the typical application of a 5V power supply, with an 8
load, the maximum temperature possible without violating
the maximum junction temperature is approximately 55˚C
provided that device operation is around the maximum
power dissipation point and assuming surface mount pack-
aging. 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 Characteristics curves
for power dissipation information for lower output 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. The
effect of a larger half supply bypass capacitor is improved
PSRR due to increased half-stability. Typical applications
employ a 5V regulator with 10 µF and a 0.1 µF bypass ca-
pacitors which aid in supply stability, but do not eliminate the
need for bypassing the supply nodes of the LM4865. The se-
lection of bypass capacitors, especially C
B, is thus depen-
dent upon desired PSRR requirements, click and pop perfor-
mance as explained in the section, Proper Selection of
External Components, system cost, and size constraints.
In order to reduce power consumption while not in use, the
LM4865 contains a DC Vol/SD pin. The DC Vol/SD pin al-
lows the LM4865 to externally turn off the amplifier’s bias cir-
cuitry. The shutdown feature turns the amplifier off when the
DC Vol/SD pin is brought below 0.3 volts. When the DC
Vol/SD pin is between 0.3V to 0.5V, the LM4865 will be ei-
ther be in shutdown or mute mode. In mute mode the current
drawn will be that of the quiescent supply current. The DC
Vol/SD pin should be tied to GND supply rail for best perfor-
mance if the LM4865 is to go into shutdown mode. As the DC
Vol/SD is increased above 0.5V the amplifier will follow the
attenuation and gain curve in Typical Performance Charac-
The LM4865 possesses a headphone control pin that turns
off the amplifier which drives +Vo2 so that single-ended op-
eration can occur and a bridged connected load is muted.
Quiescent current consumption is reduced when the IC is in
this single-ended mode.
Figure 2 shows the implementation of the LM4865’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-Sense pin (pin 3) to be approximately 50 mV when
there are no headphones plugged into the system. This
logic-low voltage at the HP-Sense pin enables the LM4865
and places it in bridged mode operation. The output coupling
capacitors protect the headphones by blocking the amplifi-
er’s half supply DC voltage.
When there are no headphones plugged into the system and
the IC is in bridged mode configuration, both loads are es-
sentially at a 0V DC potential. Since the HP-Sense threshold
is set at 4V, even in an ideal situation, the output swing can-
not cause a false single-ended trigger.
When a set of headphones are plugged into the system, the
contact pin of the headphone jack is disconnected from the
signal pin, interrupting the voltage divider set up by resistors
R1 and R2. Resistor R1 then pulls up the HP-Sense pin, en-
abling the headphone function. This disables the second
side of the amplifier thus muting the bridged speakers. The
amplifier then drives the headphones, whose impedance is
in parallel with resistor R2. Resistor R2 has negligible effect
on output drive capability since the typical impedance of
headphones are 32
The LM4865 can be used to drive both a bridged 8
Ω speaker
Ω headphone without using the HP-Sense pin. In
this case the HP-Sense would not be connected to the head-
phone jack but to a microprocessor or a switch. By enabling
the HP-Sense pin, the 8
Ω speaker can be muted.
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4865 is tolerant to a
variety of external component combinations, consideration
to component values must be used to maximize overall sys-
tem quality.

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