LM4755 datasheet(14/23 Pages) NSC

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Typical Performance Characteristics(Note 5) (Continued)

Power Dissipation vs Output Power

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Application Information

The LM4755 contains circuitry to pull down the bias line

internally, effectively shutting down the input stage. An ex-

ternal R-C should be used to adjust the timing of the pull-

down. If the bias line is pulled down too quickly, currents

induced in the internal bias resistors will cause a momentary

DC voltage to appear across the inputs of each amplifier’s

internal differential pair, resulting in an output DC shift to-

wards Vsupply. 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 internal 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 R-C.

TABLE 1. Recommended Values for Mute Circuit

MUTE

12V

18 k

Ω

10 µF

15V

18 k

Ω

10 µF

20V

12 k

Ω

10 µF

24V

12 k

Ω

10 µF

28V

8.2 k

Ω

10 µF

30V

8.2 k

Ω

10 µF

CAPACITOR SELECTION AND FREQUENCY

RESPONSE

With the LM4755, as in all single supply amplifiers, AC

coupling capacitors are used to isolate the DC voltage

present at the inputs (pins 3, 7) and outputs (pins 1, 8). 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 roll-off.

OPERATING IN BRIDGE-MODE

Though designed for use as a single-ended amplifier, the

LM4755 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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