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UC1625 Scheda tecnica(PDF) 9 Page - Texas Instruments |
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UC1625 Scheda tecnica(HTML) 9 Page - Texas Instruments |
9 / 15 page 9 UC1625 UC2625 UC3625 Figure 6. Soft start discharge current vs. temperature. TYPICAL CHARACTERISTICS (cont.) Figure 7. Current sense amplifier transfer function. Power Stage Design The UC3625 is useful in a wide variety of applications, including high-power in robotics and machinery. The power output stages used in such equipment can take a number of forms, according to the intended perfor- mance and purpose of the system. Below are four differ- ent power stages with the advantages and disadvantages of each shown. For high-frequency chopping, fast recovery circulating diodes are essential. Six are required to clamp the wind- ings. These diodes should have a continuous current rating at least equal to the operating motor current, since diode conduction duty-cycle can be high. For low-voltage systems, Schottky diodes are preferred. In higher voltage systems, diodes such as Microsemi UHVP high voltage platinum rectifiers are recom- mended. In a pulse-by-pulse current control arrangement, current sensing is done by resistor RS, through which the tran- sistor's currents are passed (Fig. A, B, and C). In these cases, RD is not needed. The low-side circulating di- odes go to ground and the current sense terminals of the UC3625 (ISENSE1 and ISENSE2) are connected to RS through a differential RC filter. The input bias current of the current sense amplifier will cause a common mode offset voltage to appear at both inputs, so for best accu- racy, keep the filter resistors below 2k and matched. The current that flows through RS is discontinuous be- cause of chopping. It flows during the on time of the power stage and is zero during the off time. Conse- quently, the voltage across RS consists of a series of pulses, occurring at the PWM frequency, with a peak value indicative of the peak motor current. To sense average motor current instead of peak cur- rent, add another current sense resistor (RD in Fig. D) to measure current in the low-side circulating diodes, and operate in four quadrant mode (pin 22 high). The nega- tive voltage across RD is corrected by the absolute value current sense amplifier. Within the limitations im- posed by Table 1, the circuit of Fig. B can also sense average current. APPLICATION INFORMATION (cont.) |
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