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LM4755T Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM4755T Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 18 page  Typical Performance Characteristics(Note 5) (Continued) Application Information The LM4755 contains circuitry to pull down the bias line in- ternally, effectively shutting down the input stage. An exter- nal 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 volt- age to appear across the inputs of each amplifier’s internal differential pair, resulting in an output DC shift towards Vsup- ply. 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 val- ues for the external R-C. TABLE 1. Recommended Values for Mute Circuit V MUTE V CC Rm Cm 5V 12V 18 k Ω 10 µF 5V 15V 18 k Ω 10 µF 5V 20V 12 k Ω 10 µF 5V 24V 12 k Ω 10 µF 5V 28V 8.2 k Ω 10 µF 5V 30V 8.2 k Ω 10 µF CAPACITOR SELECTION AND FREQUENCY RESPONSE With the LM4755, as in all single supply amplifiers, AC cou- pling 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 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 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 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. Power Dissipation vs Output Power DS100059-60 Power Dissipation vs Output Power DS100059-61 www.national.com 9 |
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