Datasheet

LM4766
SNAS031F –SEPTEMBER 1998–REVISED MARCH 2013 www.ti.com
BRIDGED AMPLIFIER APPLICATION
The LM4766 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
LM4766'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
LM4766, which has two operational amplifiers in one package, the package dissipation will increase by a factor
of four. To calculate the LM4766's maximum power dissipation point for a bridged load, multiply Equation 2 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 4. 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 LM4766 is a split supply amplifier. But as shown in Figure 6, the LM4766 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 LM4766 functions, like the mute
function.
CLICKS AND POPS
In the typical application of the LM4766 as a split-supply audio power amplifier, the IC exhibits excellent “click”
and “pop” performance when utilizing the mute mode. In addition, the device employs Under-Voltage Protection,
which eliminates unwanted power-up and power-down transients. The basis for these functions are a stable and
constant half-supply potential. In a split-supply application, ground is the stable half-supply potential. But in a
single-supply application, the half-supply needs to charge up just like the supply rail, V
CC
. This makes the task of
attaining a clickless and popless turn-on more challenging. Any uneven charging of the amplifier inputs will result
in output clicks and pops due to the differential input topology of the LM4766.
To achieve a transient free power-up and power-down, the voltage seen at the input terminals should be ideally
the same. Such a signal will be common-mode in nature, and will be rejected by the LM4766. In Figure 6, the
resistor R
INP
serves to keep the inputs at the same potential by limiting the voltage difference possible between
the two nodes. This should significantly reduce any type of turn-on pop, due to an uneven charging of the
amplifier inputs. This charging is based on a specific application loading and thus, the system designer may need
to adjust these values for optimal performance.
As shown in Figure 6, the resistors labeled R
BI
help bias up the LM4766 off the half-supply node at the emitter of
the 2N3904. But due to the input and output coupling capacitors in the circuit, along with the negative feedback,
there are two different values of R
BI
, namely 10kΩ and 200kΩ. These resistors bring up the inputs at the same
rate resulting in a popless turn-on. Adjusting these resistors values slightly may reduce pops resulting from
power supplies that ramp extremely quick or exhibit overshoot during system turn-on.
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