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LMD18245 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LMD18245 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 18 page  Applications Information POWER SUPPLY BYPASSING Step changes in current drawn from the power supply occur repeatedly during normal operation and may cause large voltage spikes across inductance in the power supply line. Care must be taken to limit voltage spikes at V CC to less than the 60V Absolute Maximum Rating. At a change in the direc- tion of the load current, the initial load current tends to raise the voltage at the power supply rail ( Figure 3) again. Current transients caused by the reverse recovery of the clamp di- odes tend to pull down the voltage at the power supply rail. Bypassing the power supply line at V CC is required to protect the device and minimize the adverse effects of normal op- eration on the power supply rail. Using botha1µF high fre- quency ceramic capacitor and a large-value aluminum elec- trolytic capacitor is highly recommended. A value of 100 µF per ampere of load current usually suffices for the aluminum electrolytic capacitor. Both capacitors should have short leads and be located within one half inch of V CC. OVERCURRENT PROTECTION If the forward current in either source switch exceeds a 12A threshold, internal circuitry disables both source switches, forcing a rapid decay of the fault current ( Figure 5). Approxi- mately 3 µs after the fault current reaches zero, the device restarts. Automatic restart allows an immediate return to nor- mal operation once the fault condition has been removed. If the fault persists, the device will begin cycling into and out of thermal shutdown. Switching large fault currents may cause potentially destructive voltage spikes across inductance in the power supply line; therefore, the power supply line must be properly bypassed at V CC for the motor driver to survive an extended overcurrent fault. In the case of a locked rotor, the inductance of the winding tends to limit the rate of change of the fault current to a value easily handled by the protection circuitry. In the case of a low inductance short from either output to ground or between outputs, the fault current could surge past the 12A shutdown threshold, forcing the device to dissipate a substantial amount of power for the brief period required to disable the source switches. Because the fault power must be dissi- pated by only one source switch, a short from output to ground represents the worst case fault. Any overcurrent fault is potentially destructive, especially while operating with high supply voltages ( ≥30V), so precautions are in order. Sinking V CC for heat with 1 square inch of 1 ounce copper on the printed circuit board is highly recommended. The sink switches are not internally protected against shorts to V CC. THERMAL SHUTDOWN Internal circuitry senses the junction temperature near the power bridge and disables the bridge if the junction tempera- ture exceeds about 155˚C. When the junction temperature cools past the shutdown threshold (lowered by a slight hys- teresis), the device automatically restarts. UNDERVOLTAGE LOCKOUT Internal circuitry disables the power bridge if the power sup- ply voltage drops below a rough threshold between 8V and 5V. Should the power supply voltage then exceed the thresh- old, the device automatically restarts. The Typical Application Figure 6 shows the typical application, the power stage of a chopper drive for bipolar stepper motors. The 20 k Ω resistor and 2.2 nF capacitor connected between RC and ground set the off-time at about 48 µs, and the 20 k Ω resistor connected between CS OUT and ground sets the gain at about 200 mA per volt of the threshold for chopping. Digital signals control the thresholds for chopping, the directions of the winding currents, and, by extension, the drive type (full step, half step, etc.). A µprocessor or µcontroller usually provides the digital control signals. DS011878-15 Trace: Fault Current at 5A/div Horizontal: 20 µs/div FIGURE 5. Fault Current with V CC = 30V, OUT 1 Shorted to OUT 2, and CS OUT Grounded 9 www.national.com |
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