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TDA1085C データシート(PDF) 6 Page - ON Semiconductor |
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TDA1085C データシート(HTML) 6 Page - ON Semiconductor |
6 / 12 page TDA1085C http://onsemi.com 6 Tachogenerator Circuit The tacho signal voltage is proportional to the motor speed. Stability considerations, in addition, require an RC filter, the pole of which must be looked at. The combination of both elements yield a constant amplitude signal on Pin 12 in most of the speed range. It is recommended to verify this maximum amplitude to be within 1.0 V peak in order to have the largest signal/noise ratio without resetting the integrated circuit (which occurs if VPin 12 reaches 5.5 V). It must be also verified that the Pin 12 signal is approximately balanced between “high” (over 300 mV) and “low”. An 8−poles tacho is a minimum for low speed stability and a 16−poles is even better. The RC pole of the tacho circuit should be chosen within 30 Hz in order to be as far as possible from the 150 Hz which corresponds to the AC line 3rd harmonic generated by the motor during starting procedure. In addition, a high value resistor coming from VCC introduces a positive offset at Pin 12, removes noise to be interpreted as a tacho signal. This offset should be designed in order to let Pin 12 reach at least − 200 mV (negative voltage) at the lowest motor speed. We remember the necessity of an individual tacho ground connection. Frequency to Voltage Converter − F V/C CPin 11 has a recommended value of 820 pF for 8−poles tachos and maximum motor rpm of 15000, and RPin 11 must be always 470 K. RPin 4 should be chosen to deliver within 12 V at maximum motor speed in order to maximize signal/noise ratio. As the FV/C ratio as well as the CPin 11 value are dispersed, RPin 4 must be adjustable and should be made of a fixed resistor in service with a trimmer representing 25% of the total. Adjustment would become easier. Once adjusted, for instance at maximum motor speed, the FV/C presents a residual non linearity; the conversion factor (mV per RPM) increases by within 7.7% as speed draws to zero. The guaranteed dispersion of the latter being very narrow, a maximum 1% speed error is guaranteed if during Pin 5 network design the small set values are modified, once forever, according this increase. The following formulas give VPin 4: V Pin 4 + G.0 @ (V CC –Va) @ CPin 11 @ R4 @ f @ (1 ) 120k R Pin11) 1 In volts. G.0 . (VCC − Va) ] 140 Va = 2.0 VBE 120 k = Rint, on Pin 11 Speed Set (Pin 5) Upon designer choice, a set of external resistors apply a series of various voltages corresponding to the various motor speeds. When switching external resistors, verify that no voltage below 80 mV is ever applied to Pin 5. If so, a full circuit reset will occur. Ramps Generator (Pin 6) If only a high acceleration ramp is needed, connect Pin 6 to ground. When a Distribute ramp should occur, preset a voltage on Pin 6 which corresponds to the motor speed starting ramp point. Distribution (or low ramp) will continue up to the moment the motor speed would have reached twice the starting value. The ratio of two is imposed by the IC. Nevertheless, it could be externally changed downwards (Figure 6) or upwards (Figure 7). The distribution ramp can be shortened by an external resistor from VCC charging CPin 7, adding its current to the internal 5.0 mA generator. Power Circuits Triac Triggering pulse amplitude must be determined by Pin 13 resistor according to the needs in Quadrant IV. Trigger pulse duration can be disturbed by noise signals generated by the triac itself, which interfere within Pins 14 and 16, precisely those which determine it. While easily visible, this effect is harmless. The triac must be protected from high AC line dV/dt during external disturbances by 100 nF x 100 W network. Shunt resistor must be as non−inductive as possible. It can be made locally by using constantan alloy wire. When the load is a DC fed universal motor through a rectifier bridge, the triac must be protected from commutating dV/dt by a 1.0 to 2.0 mH coil in series with MT2. Synchronization functions are performed by resistors sensing AC line and triac conduction. 820 k values are normal but could be reduced down to 330 k in order to detect the “zeros” with accuracy and to reduce the residual DC line component below 20 mA. Current Limitation The current limiter starts to discharge Pin 7 capacitor (reference speed) as the motor current reaches the designed threshold level. The loop gain is determined by the resistor connecting Pin 3 to the series shunt. Experience has shown that its optimal value for a 10 Arms limitation is within 2.0 k W. Pin 3 input has a sensitivity in current which is limited to reasonable values and should not react to spikes. If not used, Pin 3 must be connected to a maximum positive voltage of 5.0 V rather than be left open. Loop Stability The Pin 16 network is predominant and must be adjusted experimentally during module development. The values indicated in Figure 4 are typical for washing machine applications but accept large modifications from one model to another. R16 (the sole restriction) should not go below 33 k, otherwise slew rate limitation will cause large transient errors for load steps. |
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