LM4766TF National Semiconductor, LM4766TF Datasheet - Page 16

• Manufacturer Part Number: LM4766TF
• Description: IC,Audio Amplifier,DUAL,BIPOLAR,ZIP,15PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM4766TF

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Information
Again it must be noted that the value of θ
upon the system designer’s amplifier requirements. If the
ambient temperature that the audio amplifier is to be working
under is higher than 25˚C, then the thermal resistance for the
heat sink, given all other things are equal, will need to be
smaller.
SUPPLY BYPASSING
The LM4766 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM4766 should have its supply leads bypassed with low-
inductance capacitors having short leads that are located
close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation
known as “motorboating” or by high frequency instabilities.
These instabilities can be eliminated through multiple by-
passing utilizing a large tantalum or electrolytic capacitor
(10µF or larger) which is used to absorb low frequency
variations and a small ceramic capacitor (0.1µF) to prevent
any high frequency feedback through the power supply lines.
If adequate bypassing is not provided, the current in the
supply leads which is a rectified component of the load
current may be fed back into internal circuitry. This signal
causes distortion at high frequencies requiring that the sup-
plies be bypassed at the package terminals with an electro-
lytic capacitor of 470µF or more.
BRIDGED AMPLIFIER APPLICATION
The LM4766 has two operational amplifiers internally, allow-
ing 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 2. Bridged mode op-
eration 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 con-
figuration, 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 dissipa-
tion will increase by a factor of four. To calculate the
LM4766’s maximum power dissipation point for a bridged
load, multiply Equation (1) by a factor of four.
(Continued)
SA
is dependent
16
This value of P
heat sink for a bridged amplifier application. Since the inter-
nal dissipation for a given power supply and load is in-
creased by using bridged-mode, the heatsink’s θ
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 appli-
cation.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM4766 is a split supply am-
plifier. But as shown in Figure 3, 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 elimi-
nates 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 applica-
tion, the half-supply needs to charge up just like the supply
rail, V
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 3, the resistor R
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 perfor-
mance.
As shown in Figure 3, the resistors labeled R
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
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.
CC
. This makes the task of attaining a clickless and
DMAX
can be used to calculate the correct size
BI
, namely 10kΩ and 200kΩ. These
BI
help bias up
SA
will have
INP