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tda7284 record/playback circuit with alc wide operating supply voltage (3v to 12v) very low input noise (v i = 1.2 m v) internal compensation for high gain application (double speed re- cording) built-in alc circuitry good svr dc controlled switches for mute or equalization switching func- tions description the tda7284 is a monolithic integrated circuit in a dip/so-14designed for 6v, 9v and 12v ac/dc portable cassette equipment application. may 1997 dip14 so14 ordering number: tda7284 TDA7284D block diagram 1/14
symbol parameter value unit v s supply voltage 14 v t op operating temperature range -20 to 70 c t stg ,t j storage and junction temperature range -40 to 150 c thermal data symbol description s014 dip14 unit r th j-amb thermal resistance junction-ambient max 200 120 c/w dc characteristics (t amb =25 ;v s = 6v; v i = 0v; r i = 10k w ; alc = off) terminal no. 1 2 3 4567891011121314 terminal voltage (v) 0 0 0 0 2.6 0 1.3 1.3 0 2.6 6 4.6 0 0 absolute maximum ratings pin connection (top view) tda7284 2/14
figure 1: test and application circuit tda7284 3/14
figure 2: p.c. board and component layout of the circuit of fig. 1 (1:1 scale). tda7284 4/14
electrical characteristics (v s = 6v, t amb =25 c unless otherwise specified refer to test cir- cuit) symbol parameter test condition min. typ. max. unit v s supply voltage 3 12 v i d quiescent current 4.5 8 ma e n input noise r g = 2.2k w bw = 22hz to 22khz 1.2 m v r i input resistance 30 50 70 k w g o open loop gain 65 78 db v o output voltage thd < 1% alc off alc on 1.2 0.7 1.8 0.9 1.1 v rms v rms thd total harmonic distortion v o =1v rms alc = on v i = 100mv 0.1 0.3 0.5 1 % % alc range d v o = 3db 47 db cb channel balance alc on 0 2 db svr supply voltage rejection f = 120hz, c svr =33 m f v r = 100mv, r g = 10k w alc = off 50 db cs cross-talk alc off 70 db pin 3 turn off threshold i o =<1 m a 0.8 1.3 v pin 3 turn on threshold 1.7 2.25 v pin 3 turn on saturation r l = 10k w 0.1 0.2 v figure 3: drain current vs. supply voltage figure 4: recording closed loop gain vs. frequency tda7284 5/14
figure 7: output voltage vs. input voltage figure 8: output voltage vs. input voltage figure 5: playback closed loop gain vs frequency figure 6: normalized output voltage vs. supply voltage figure 9: output voltage vs. input voltage figure 10: distortion vs. input voltage tda7284 6/14
figure 13: crosstalk vs. frequency (alc = off) figure 14: crosstalk vs. frequency (alc = off) figure 11: distortion vs. input voltage figure 12: svr vs. frequency (alc = off) tda7284 7/14
circuit description operational amplifier the operational amplifier consists essentially of a very low noise input stage decoupled from the single-ended output stage by means of an emitter follower (fig. 15 ). the compensations provided in order to have high gain bandwith product allowing the use for double speed recording application. automatic level control system (alc) this system maintains the level of the signal to be recorded at a value which prevents saturation of the tape and which optimizes the signal to noise ratio even there are notable variations in the input signal. before presenting the alc circuit of tda7284 it is worth describing the operation of the automatic level control as a system.a diagram showing the basis of operation is given in fig.16. this consists of an amplifier (op-amp) having con- stant gain (g v = 1+r4/r3),which in feedback transforms output signal level information (usually by means of a peak-to-peak detector) into a con- tinuous voltage which drives the networks indi- cated by t and rd. the element t transforms the continuous voltage level into a signal capable of modifying the circuit conditions symbolized by variable resistor rd. the value assumed by the resistor rd is a func- tion of the output signal level vo and is such that the voltage vc at the input of the op-amp is con- stant,even variations of vi are present.obviously if vo is less than a certain value the system is not controlled. in this case : v i =v c =v o /g v (g v is the gain of the op-amp) for the tda7284 the value of v o below which the system is not controlled is around 1 vrms. let us now consider the speed of response of the system (when controlled) to positive and negative changes of the input signal i.e. the limiting time,the time for return to nominal level (1 vrms) and the recovery time. limiting time, and time for return to nominal level. let us suppose that at certain moment t o , the in- put signal increases by + d vi as shown in fig. 17. figure 15 figure 16: basic diagram of the alc stage tda7284 8/14
usually such an increase drives the op-amp into saturation and the time for which it remains in this condition is called the limiting time(t1). t1 depends on the relationship between the ex- ternal capacitances, the time constant t=r1 ? c1, the supply voltage and the signal variation. the criteria for choosing the length of t1 are the result of several compromises. in particular if t1 is too long, there will be audible distortion during playback (during t1 the output is a square wave),and if it is too short, the sensation of in- creased level will be lost while dynamic compres- sion phenomena and instability may occur. the time for return to nominal level is defined as the total time between the instant to and the in- stant in which the output reassumes the nominal value. this time (ts) is roughly equal to 5 ? t1. on the basis of tests carried out it has been found that a musical signal with high dynamic range ( d v i =+40 db) is to be recorded, the best value of ts is between 200 and 300ms. recovery time . let us now suppose that at the instant to the input signal decreases of d vi (fig. 18). the recovery time (trec) is defined as the time between the instant to and the instant in which the output signal returns to the nominal level. this time depends essentially on the discharge time constant of r2 ? c2 ( see fig. 16) and on the size of the step - d vi. in this case too, if this time is too long the signal to noise ratio on the tape de- teriorates. if it is too short the sensation of the low signal level is lost during playback. the alc system of the tda7284 fig. 16 becomes the following (fig. 19) where the figure 17: limiting and level setting time figure 18: recovery time figure 19 tda7284 9/14
peak-to-peak detector of fig. 16 is now inside the broken line 1 while the system which allows a di- namic resistance varying with the dc voltage level (i.e. inversely proportional to the op-amp output signal),is inside the broken line 2. it should be noted that the generator resistance ri has no influence on the controlled voltage value vc, although its value should be between 1 and 47 kohm. the lower limit is determined by the minimum dy- namic resistance of 10 ohm and therefore to have a control range of 40 db for the input signal, ri must be greather than 1.5 kohm. the upper limit results from the necessity to limit the attenuation of the signal by the input imped- ance of the op-amp. switches two dc-controlled switches are also included in the chip (fig. 20 ) fig. 19 shows the typical application circuit of the tda7284 utilizing the equalization switch for nor- mal or chrome tape playback equalization.the advantage is the components can be placed near to the ic, while the tape selector switch can be at a remote location, hence reduce the chances of noise and oscillation due to components layout. another advantage is that only one pole is needed for the tape selector switch as compared to the two poles needed by conventional circuits (one separate pole for each channel). fig. 22 shows the use of the switches to obtain the mute function. figure 20 figure 21: application circuit with dc switching of normal/chrome tape equalization tda7284 10/14
svr a refernce circuit is enclosed to provide a stable voltage and to supply a stable current to all cur- rent mirrors. svr capacitor is also connected to this block for good ripple rejection. figure 22: application circuit with output muting tda7284 11/14
dip14 package mechanical data dim. mm inch min. typ. max. min. typ. max. a1 0.51 0.020 b 1.39 1.65 0.055 0.065 b 0.5 0.020 b1 0.25 0.010 d 20 0.787 e 8.5 0.335 e 2.54 0.100 e3 15.24 0.600 f 7.1 0.280 i 5.1 0.201 l 3.3 0.130 z 1.27 2.54 0.050 0.100 tda7284 12/14
so14 package mechanical data dim. mm inch min. typ. max. min. typ. max. a 1.75 0.069 a1 0.1 0.2 0.004 0.008 a2 1.6 0.063 b 0.35 0.46 0.014 0.018 b1 0.19 0.25 0.007 0.010 c 0.5 0.020 c1 45 (typ.) d 8.55 8.75 0.336 0.344 e 5.8 6.2 0.228 0.244 e 1.27 0.050 e3 7.62 0.300 f 3.8 4.0 0.15 0.157 l 0.5 1.27 0.020 0.050 m 0.68 0.027 s8 (max.) tda7284 13/14
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specification mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. sgs- thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1997 sgs-thomson microelectronics printed in italy all rights reserved sgs-thomson microelectronics group of companies australia - brazil - canada - china - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. tda7284 14/14

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