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UC3717ANG4 Arkusz danych(PDF) 5 Page - Texas Instruments |
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UC3717ANG4 Arkusz danych(HTML) 5 Page - Texas Instruments |
5 / 12 page PHASE POLARITY INPUT The UC3717A phase polarity input controls current direc- tion in the motor winding. Built-in hysteresis insures im- munity to noise, something frequently present in switched drive environments. A low level phase polarity input enables Q2 and Q3 as shown in Figure 7. During phase reversal, the active transistors are both turned off while winding current delays through the commutating di- odes shown. As winding current decays to zero, the inac- tive transistors Q1 and Q4 turn on and charge the winding with current of the reverse direction. This delay insures noise immunity and freedom from power supply current spikes caused by overlapping drive signals. PHASE INPUT Q1, Q4 Q2, Q3 LOW OFF ON HIGH ON OFF CURRENT CONTROL The voltage divider, comparators, monostable, and two- bit D/A provide a means to sense winding peak current, select winding peak current, and disable the winding sink transistors. The UC3717A switched driver accomplishes current con- trol using an algorithm referred to as "fixed off-time." When a voltage is applied across the motor winding, the current through the winding increases exponentially. The current can be sensed across an external resistor as an analog voltage proportional to instantaneous current. This voltage is normally filtered with a simple RC low- pass network to remove high frequency transients, and then compared to one of the three selectable thresholds. The two bit D/A input signal determines which one of the three thresholds is selected, corresponding to a desired winding peak current level. At the moment the sense volt- age rises above the selected threshold, the UC3717A’s monostable is triggered and disables both output sink drivers for a fixed off-time. The winding current then cir- culates through the source transistor and appropriate di- ode. The reference terminal of the UC3717A provides a means of continuously adjusting the current threshold to allow microstepping. Table 1 presents the relationship between the two-bit D/A input signal and selectable cur- rent level. TABLE 1 IO I1 CURRENT LEVEL 0 0 100% 10 60% 01 19% 1 1 Current Inhibit OVERLOAD PROTECTION The UC3717A is equipped with a new, more reliable ther- mal shutdown circuit which limits the junction tempera- ture to a maximum of 180C by reducing the winding cur- rent. PERFORMANCE CONSIDERATIONS In order to achieve optimum performance from the UC3717A careful attention should be given to the follow- ing items. External Components: The UC3717A requires a mini- mal number of external components to form a complete control and switch drive unit. However, proper selection of external components is necessary for optimum per- formance. The timing pin, (pin 2) is normally connected to an RC network which sets the off-time for the sink power transistor during switched mode. As shown in Fig- ure 8, prior to switched mode, the winding current in- creases exponentially to a peak value. Once peak current is attained the monostable is triggered which turns off the lower sink drivers for a fixed off-time. During off-time winding current decays through the appropriate diode and source transistor. The moment off-time times out, the motor current again rises exponentially produc- ing the ripple waveform shown. The magnitude of wind- ing ripple is a direct function of off-time. For a given off-time TOFF, the values of RT and CT can be calculated from the expression: TOFF = 0.69RTCT with the restriction that RT should be in the range of 10- 100k. As shown in Figure 5, the switch frequency FS is a function of TOFF and TON. Since TON is a function of the reference voltage, sense resistor, motor supply, and winding electrical characteristics, it generally varies dur- ing different modes of operation. Thus, FS may be ap- proximated nominally as: FS = 1⁄1.5 (TOFF). Normally, Switch Frequency Is Selected Greater than Figure 8. A typical winding current waveform. Wind- ing current rises exponentially to a selected peak value. The peak value is limited by switched mode operation producing a ripple in winding current. A phase polarity reversal command is given and wind- ing current decays to zero, then increases exponen- tially. UC3717A FUNCTIONAL DESCRIPTION (cont.) 5 |
Podobny numer części - UC3717ANG4 |
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Podobny opis - UC3717ANG4 |
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