Datasheet

LM4755

SNAS010E –FEBRUARY 1999–REVISED APRIL 2013

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APPLICATION INFORMATION

The LM4755 contains circuitry to pull down the bias line internally, effectively shutting down the input stage. An

external 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 towards 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

Rm Cm

5V 12V 18 kΩ 10 µF

5V 15V 18 kΩ 10 µF

5V 20V 12 kΩ 10 µF

5V 24V 12 kΩ 10 µF

5V 28V 8.2 kΩ 10 µF

5V 30V 8.2 kΩ 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

DESCRIPTION section these capacitors 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 kΩ 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 4Ω.

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

4Ω load in single-ended mode should drive an 8Ω load when operating in bridge-mode.

Product Folder Links: LM4755