LM4752 datasheet(14/22 Pages) NSC

14 / 22 page

Typical Performance Characteristics (Continued)

Power Dissipation vs Output Power

10003951

10003952

Application Information

CAPACITOR SELECTION AND FREQUENCY

RESPONSE

With the LM4752, as in all single supply amplifiers, AC

coupling 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 capacitors create high-pass filters with their corre-

sponding input/output impedances. The Typical Applica-

tion 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

Ω typical input 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

Ω. 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

input 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

differential pair, resulting in an output DC shift towards

SUPPLY. An R-C timing circuit should be used to limit the

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

capacitor, the half-supply voltage, and internal resistances

used in the half-supply generator. Table 1 shows a list of

recommended values for the external mute circuitry.

TABLE 1. Values for Mute Circuit

MUTE

Ω

10 k

Ω 4.7 µF 360Ω 100 µF 21V–32V

Ω

1.2

Ω

4.7 µF

180

Ω 100 µF 15V–32V

Ω

910

Ω 4.7 µF 180Ω 47 µF 22V–32V

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

Ω load in single-ended mode should drive an 8Ω

load when operating in bridge-mode.

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