Datasheet

LM4765
www.ti.com SNAS030C –AUGUST 1998–REVISED APRIL 2013
BRIDGED AMPLIFIER APPLICATION
The LM4765 has two operational amplifiers internally, allowing for a few different amplifier configurations. One of
these configurations is referred to as “bridged mode” and involves driving the load differentially through the
LM4765's outputs. This configuration is shown in Figure 5. Bridged mode operation is different from the classical
single-ended amplifier configuration where one side of its load is connected to ground.
A bridge amplifier design has a distinct advantage over the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified supply voltage. Consequently, theoretically 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.
A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal
power dissipation. For each operational amplifier in a bridge configuration, the internal power dissipation will
increase by a factor of two over the single ended dissipation. Thus, for an audio power amplifier such as the
LM4765, which has two operational amplifiers in one package, the package dissipation will increase by a factor
of four. To calculate the LM4765's maximum power dissipation point for a bridged load, multiply Equation 1 by a
factor of four.
This value of P
DMAX
can be used to calculate the correct size heat sink for a bridged amplifier application. Since
the internal dissipation for a given power supply and load is increased by using bridged-mode, the heatsink's θ
SA
will have to decrease accordingly as shown by Equation 3. Refer to the section, DETERMINING THE CORRECT
HEAT SINK, for a more detailed discussion of proper heat sinking for a given application.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM4765 is a split supply amplifier. But as shown in Figure 6, the LM4765 can also
be used in a single power supply configuration. This involves using some external components to create a half-
supply bias which is used as the reference for the inputs and outputs. Thus, the signal will swing around half-
supply much like it swings around ground in a split-supply application. Along with proper circuit biasing, a few
other considerations must be accounted for to take advantage of all of the LM4765 functions.
The LM4765 possesses a mute and standby function with internal logic gates that are half-supply referenced.
Thus, to enable either the Mute or Standby function, the voltage at these pins must be a minimum of 2.5V above
half-supply. In single-supply systems, devices such as microprocessors and simple logic circuits used to control
the mute and standby functions, are usually referenced to ground, not half-supply. Thus, to use these devices to
control the logic circuitry of the LM4765, a “level shifter,” like the one shown in Figure 42, must be employed. A
level shifter is not needed in a split-supply configuration since ground is also half-supply.
Figure 42. Level Shift Circuit
When the voltage at the Logic Input node is 0V, the 2N3904 is “off” and thus resistor R
c
pulls up mute or standby
input to the supply. This enables the mute or standby function. When the Logic Input is 5V, the 2N3904 is “on”
and consequently, the voltage at the collector is essentially 0V. This will disable the mute or standby function,
and thus the amplifier will be in its normal mode of operation. R
shift
, along with C
shift
, creates an RC time constant
that reduces transients when the mute or standby functions are enabled or disabled. Additionally, R
shift
limits the
current supplied by the internal logic gates of the LM4765 which insures device reliability. Refer to the MUTE
MODE and STANDBY MODE sections in the APPLICATION INFORMATION section for a more detailed
description of these functions.
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