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5962-9232501VXA Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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5962-9232501VXA Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 14 page Application Information (Continued) USING THE CURRENT SENSE OUTPUT The CURRENT SENSE output (pin 8) has a sensitivity of 377 µA per ampere of output current. For optimal accuracy and linearity of this signal, the value of voltage generating resistor between pin 8 and ground should be chosen to limit the maximum voltage developed at pin 8 to 5V, or less. The maximum voltage compliance is 12V. It should be noted that the recirculating currents (free wheel- ing currents) are ignored by the current sense circuitry. Therefore, only the currents in the upper sourcing outputs are sensed. USING THE THERMAL WARNING FLAG The THERMAL FLAG output (pin 9) is an open collector transistor. This permits a wired OR connection of thermal warning flag outputs from multiple LMD18200’s, and allows the user to set the logic high level of the output signal swing to match system requirements. This output typically drives the interrupt input of a system controller. The interrupt ser- vice routine would then be designed to take appropriate steps, such as reducing load currents or initiating an orderly system shutdown. The maximum voltage compliance on the flag pin is 12V. SUPPLY BYPASSING During switching transitions the levels of fast current changes experienced may cause troublesome voltage tran- sients across system stray inductance. It is normally necessary to bypass the supply rail with a high quality capacitor(s) connected as close as possible to the V S Power Supply (Pin 6) and GROUND (Pin 7). A 1 µF high- frequency ceramic capacitor is recommended. Care should be taken to limit the transients on the supply pin below the Absolute Maximum Rating of the device. When operating the chip at supply voltages above 40V a voltage suppressor (transorb) such as P6KE62A is recommended from supply to ground. Typically the ceramic capacitor can be eliminated in the presence of the voltage suppressor. Note that when driving high load currents a greater amount of supply bypass capacitance (in general at least 100 µF per Amp of load current) is required to absorb the recirculating currents of the inductive loads. CURRENT LIMITING Current limiting protection circuitry has been incorporated into the design of the LMD18200. With any power device it is important to consider the effects of the substantial surge currents through the device that may occur as a result of shorted loads. The protection circuitry monitors this increase in current (the threshold is set to approximately 10 Amps) and shuts off the power device as quickly as possible in the event of an overload condition. In a typical motor driving application the most common overload faults are caused by shorted motor windings and locked rotors. Under these con- ditions the inductance of the motor (as well as any series inductance in the V CC supply line) serves to reduce the magnitude of a current surge to a safe level for the LMD18200. Once the device is shut down, the control cir- cuitry will periodically try to turn the power device back on. This feature allows the immediate return to normal operation in the event that the fault condition has been removed. While the fault remains however, the device will cycle in and out of thermal shutdown. This can create voltage transients on the V CC supply line and therefore proper supply bypassing tech- niques are required. The most severe condition for any power device is a direct, hard-wired (“screwdriver”) long term short from an output to ground. This condition can generate a surge of current through the power device on the order of 15 Amps and require the die and package to dissipate up to 500 Watts of power for the short time required for the protection circuitry 01056824 FIGURE 4. Transitions in Brake, Direction, or PWM Must Be Separated By At Least 1 µsec www.national.com 8 |
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