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LTC1155CN8 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1155CN8 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page 8 LTC1155 S APPLICATI I FOR ATIO If the MOSFET is turned ON and the power supply (battery) removed, the inductor current is delivered by the supply capacitor. The supply capacitor must be large enough to deliver the energy demanded by the discharging inductor. If the storage capacitor is too small, the supply lead of the LTC1155 may be pulled below ground, permanently destroying the device. Consider the case of a load inductance of 1mH which is supporting 3A when the 6V power supply connection is interrupted. A supply capacitor of at least 250 µF is required to prevent the supply lead of the LTC1155 from being pulled below ground (along with any other circuitry tied to the supply). Any wire between the power MOSFET source and the load will add a small amount of parasitic inductance in series with the load (approximately 0.4 µH/foot). Bypass the power supply lead of the LTC1155 with a minimum of 10 µF to ensure that this parasitic load inductance is discharged safely, even if the load is otherwise resistive. Large Inductive Loads Large inductive loads (>0.1mH) may require diodes con- nected directly across the inductor to safely divert the stored energy to ground. Many inductive loads have these diodes included. If not, a diode of the proper current rating should be connected across the load to safely divert the stored energy. Reverse-Battery Protection The LTC1155 can be protected against reverse-battery conditions by connecting a resistor in series with the ground lead as shown in Figure 5. The resistor limits the supply current to less than 50mA with –12V applied. Since the LTC1155 draws very little current while in normal operation, the drop across the ground resistor is minimal. The TTL or CMOS driving logic is protected against reverse-battery conditions by the 100k input current lim- iting resistor. The addition of 100k resistance in series with the input pin will not affect the turn ON and turn OFF times which are dominated by the controlled gate charge and discharge periods. Figure 4. Switched Supply 1155 F04 IRLZ34 L LOAD LTC1155 GND GND G1 DS1 VS IN1 CDLY RSEN 0.025 Ω RDLY CS + + sense pin and dramatically reducing the amount of time the MOSFET is in an overload condition. The drain sense resistor value is selected to limit the maximum DC current to 4A. Above 28A, the delay time drops to 10 µs. Switched Supply Applications Large inductive loads, such as solenoids, relays and motors store energy which must be directed back to either the power supply or to ground when the supply voltage is interrupted (see Figure 4). In normal operation, when the switch is turned OFF, the energy stored in the inductor is harmlessly absorbed by the MOSFET; i.e., the current flows out of the supply through the MOSFET until the inductor current falls to zero. 1155 F03 IRLZ34 LOAD LTC1155 GND GND G1 DS1 VS IN1 VS = 5.0V CDLY 0.22 µF RSEN 0.025 Ω RDLY 270k D1 1N4148 Figure 3. Using a Speed-Up Diode |
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