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TDA7454 Scheda tecnica(PDF) 8 Page - STMicroelectronics |
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TDA7454 Scheda tecnica(HTML) 8 Page - STMicroelectronics |
8 / 13 page OPERATING PRINCIPLE. Thanks to its unique operating principle, the TDA7454 obtains a substantial reduction of power dissipation from traditional class-AB amplifiers without being affected by the massive radiation effects and complex circuitry normally associated with class-D solutions. Its is composed of 8 amplifier blocks, making up 4 bridge-equivalent channels. Half of this struc- ture is drafted in fig 15. These blocks continu- ously change their connections during every sin- gle signal event, according to the instantaneous power demand. This means that at low volumes (output power steadily lower than 2.5 W) the TDA7454 acts as a Single Ended amplifier, condi- tion where block “C” remains disabled and the block “D” behaves like a buffer, which, by furnish- ing the correct DC biasing (half-Vcc) to each pair of speakers, eliminate the needs of otherwise re- quired output-decoupling capacitors. At the same time, SW1 keeps closed. thus ensuring a com- mon biasing point for L-R front / L-R rear speak- ers couples. As a result, the equivalent circuit be- comes that of fig. 16. The internal switches (SW1) are high-speed, dis- sipation-free power MOS types, whose realization has been made possible by the ST- exclusive By- polar-CMOS-DMOS mixed technology process (BCD). From fig. 16 it can be observed that “A” and “B” amplifiers work in phase opposition. Sup- posing their output have the same signal (equal shape/amplitude), the current sourced by “B” will be entirely sunk by “A”, while no current will flow into “D”, causing no power dissipation in the lat- ter. “A” and “B” are practically configured as a bridge whose load is constituted by Ra + Rb (= 8 Ohm, if 4 Ohm speakers are used), with considerable ad- vantages in terms of power dissipation. Designat- ing “A” and “B” for the reproduction of either FRONT or REAR sections of the same channel (LEFT or RIGHT), keeping the fader in centre po- sition (same amplitude for FRONT and REAR sections) and using the same speakers, as it hap- pens during most of the time, will transpose this best-case dissipation condition into practical ap- plications. To fully take advantage of the TDA7454’s low-dis- sipation feature, it is then especially important to adopt some criteria in the channels assignment, using the schematic of fig. 1 as a reference. When the power demand increases to more than 2.5 W, all the blocks will operate as amplifiers, SW1 is opened, leading to the seemingly conven- tional bridge configuration of fig. 17. The efficiency enhancement is based upon the concept that the average output power during the reproduction of normal music/speech programs will stand anywhere between 10 % and 15 % of the rated power (@ THD= 10 %) that the amplifier can deliver. This holds true even at high volumes and frequent clipping occurrence. Applied to the TDA7454 (rated power= 25 W), this will result into an average output level of 2.5 - 3 W in sine-wave operation, region where the dissipated power is about 50 % less than that of a traditional amplifier of equivalent power class (see TDA7454 vs. CLASS-AB characteristics, fig. 18). Equally favourable is the case shown by fig. 19, when gaussian-distributed signal amplitudes, which best simulates the amplifier’s real working conditions, are used. APPLICATION HINTS (ref. to the circuit of fig. 1) STAND-BY and MUTING (pins 4 & 22) Both STAND-BY and MUTING pins are CMOS- compatible. The current sunk by each of them is about 1 µA. For pop prevention it is essential that during TURN ON/OFF sequences the muting be preventively inserted before making stand-by transitions. But, if for any reason, either muting or stand-by are not used, they have to be connected to Vcc through a 100 Kohm (minimum) resis- tance. The R-C networks values in fig. 1 (R1-C6 and R2- C7) are meant to be the minimum-necessary for obtaining the lowest pop levels possible. Any re- ductions (especially for R2-C7) will inevitably im- pair this parameter. SVR (pin 10) The duty of the SVR capacitor (C5) is double: as- suring adequate supply-ripple rejection and con- trolling turn ON/OFF operations. Its indicated value (100 uF) is the minimum-recommended to correctly serve both the purposes. INPUTS (pins 11-12-13-14) The inputs are internally biased at half-Vcc level. The typical input impedance is 15 KOhm, which implies using Cin (C1-C2-C3-C4) = 220 nF for ob- taining a theoretical minimum-reproducible fre- quency of 48 Hz (-3 dB). In any case, Cin val- ues can be enlarged if a lower frequency bound is desired, but, at any Cin enlargement must cor- respond a proportional increase of Csvr (C5), to safeguard the on/off pop aspect. The following table indicates the right values to be used for Cin and Csvr, whose operating voltage can be 10 V. LOW FREQUENCY ROLL-OFF (-3dB) Cin ( µF) Csvr ( µF) 48 0.22 100 22 0.47 220 16 0.68 330 11 1 470 TDA7454 8/13 |
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