LM4752TS datasheet(10/18 Pages) NSC

10 / 18 page

Typical Performance Characteristics (Continued)

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

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 in-

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

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 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

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 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.

TABLE 1. Values for Mute Circuit

MUTE

10 k

Ω 10 kΩ 4.7 µF 360Ω 100 µF 21V–32V

20 k

Ω 1.2 kΩ 4.7 µF 180Ω 100 µF 15V–32V

20 k

Ω 910Ω 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.

Power Dissipation vs Output Power

DS100039-51

Power Dissipation vs Output Power

DS100039-52

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