LM4752T National Semiconductor, LM4752T Datasheet - Page 10

• Manufacturer Part Number: LM4752T
• Description: Stereo 11W Audio Power Amplifier
• Manufacturer: National Semiconductor

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Typical Performance Characteristics
Power Dissipation vs Output Power
Application Information
CAPACITOR SELECTION AND FREQUENCY
RESPONSE
With the LM4752, as in all single supply amplifiers, AC cou-
pling capacitors are used to isolate the DC voltage present at
the inputs (pins 2,6) and outputs (pins 1,7). As mentioned
earlier in the External Components section these capaci-
tors create high-pass filters with their corresponding input/
output impedances. The Typical Application Circuit shown
in Figure 1 shows input and output capacitors of 0.1 µF and
1,000 µF respectively. At the input, with an 83 k
put resistance, the result is a high pass 3 dB point occurring
at 19 Hz. There is another high pass filter at 39.8 Hz created
with the output load resistance of 4 . Careful selection of
these components is necessary to ensure that the desired
frequency response is obtained. The Frequency Response
curves in the Typical Performance Characteristics section
show how different output coupling capacitors affect the low
frequency rolloff.
APPLICATION CIRCUIT WITH MUTE
With the addition of a few external components, a simple
mute circuit can be implemented, such as the one shown in
Figure 3 . This circuit works by externally pulling down the
half supply bias line (pin 5), effectively shutting down the in-
put stage.
When using an external circuit to pull down the bias, care
must be taken to ensure that this line is not pulled down too
quickly, or output “pops” or signal feedthrough may result. If
the bias line is pulled down too quickly, currents induced in
the internal bias resistors will cause a momentary DC volt-
age to appear across the inputs of each amplifier’s internal
DS100039-51
Power Dissipation vs Output Power
typical in-
10
(Continued)
differential pair, resulting in an output DC shift towards
V
pull-down time such that output “pops” and signal
feedthroughs will be minimized. The pull-down timing is a
function of a number of factors, including the external mute
circuitry, the voltage used to activate the mute, the bias ca-
pacitor, the half-supply voltage, and internal resistances
used in the half-supply generator. Table 1 shows a list of rec-
ommended values for the external mute circuitry.
OPERATING IN BRIDGE-MODE
Though designed for use as a single-ended amplifier, the
LM4752 can be used to drive a load differentially (bridge-
mode). Due to the low pin count of the package, only the
non-inverting inputs are available. An inverted signal must
be provided to one of the inputs. This can easily be done with
the use of an inexpensive op-amp configured as a standard
inverting amplifier. An LF353 is a good low-cost choice. Care
must be taken, however, for a bridge-mode amplifier must
theoretically dissipate four times the power of a single-ended
type. The load seen by each amplifier is effectively half that
of the actual load being used, thus an amplifier designed to
drive a 4
load when operating in bridge-mode.
V
MUTE
SUPPLY
5V
V
V
S
S
DS100039-52
10 k
20 k
20 k
. An R-C timing circuit should be used to limit the
R1
load in single-ended mode should drive an 8
TABLE 1. Values for Mute Circuit
1.2 k
10 k
910
R2
4.7 µF 360
4.7 µF 180
4.7 µF 180
C1
R3
100 µF 21V–32V
100 µF 15V–32V
47 µF
C
B
22V–32V
V
CC