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LTC1693-1CS8 Datasheet(PDF) 10 Page - Linear Technology |
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LTC1693-1CS8 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LTC1693 corresponding to MOSFET’s VGS value (VCC in this case) can be readily obtained from the manafacturer’s QGS vs VGS curves: Load Capacitive Power (MOS) = (VCC)(QG)(f) Transition state power losses are due to both AC currents required to charge and discharge the drivers’ internal nodal capacitances and cross-conduction currents in the internal gates. UVLO and Thermal Shutdown The LTC1693’s UVLO detector disables the input buffer and pulls the output pin to ground if VCC < 4V. The output remains off from VCC = 1V to VCC = 4V. This ensures that during start-up or improper supply voltage values, the LTC1693 will keep the output power MOSFET off. The LTC1693 also has a thermal detector that similarly disables the input buffer and grounds the output pin if junction temperature exceeds 145 °C. The thermal shut- down circuit has 20 °C of hysteresis. This thermal limit helps to shut down the system should a fault condition occur. Input Voltage Range LTC1693’s input pin is a high impedance node and essen- tially draws neligible input current. This simplifies the input drive circuitry required for the input. The LTC1693 typically has 1.2V of hysteresis between its low and high input thresholds. This increases the driver’s robustness against any ground bounce noises. However, care should still be taken to keep this pin from any noise pickup, especially in high frequency switching applications. In applications where the input signal swings below the GND pin potential, the input pin voltage must be clamped to prevent the LTC1693’s parastic substrate diode from turning on. This can be accomplished by connecting a series current limiting resistor R1 and a shunting Schottky diode D1 to the input pin (Figure 4). R1 ranges from 100 Ω to 470 Ω while D1 can be a BAT54 or 1N5818/9. VCC IN R1 D1 GND LTC1693 INPUT SIGNAL GOING BEL0W GND PIN POTENTIAL PARASITIC SUBSTRATE DIODE 1693 F04 Bypassing and Grounding LTC1693 requires proper VCCbypassingandgroundingdue to its high speed switching (ns) and large AC currents (A). Careless component placement and PCB trace routing may cause excessive ringing and under/overshoot. To obtain the optimum performance from the LTC1693: A. Mount the bypass capacitors as close as possible to the VCC and GND pins. The leads should be shortened as much as possible to reduce lead inductance. It is recommended to have a 0.1 µF ceramic in parallel with a low ESR 4.7 µF bypass capacitor. For high voltage switching in an inductive environment, ensure that the bypass capacitors’ VMAX ratings are high enough to prevent breakdown. This is especially important for floating driver applications. B. Use a low inductance, low impedance ground plane to reduce any ground drop and stray capacitance. Re- member that the LTC1693 switches 1.5A peak currents and any significant ground drop will degrade signal integrity. C. Plan the ground routing carefully. Know where the large load switching current is coming from and going to. Maintain separate ground return paths for the input pin and output pin. Terminate these two ground traces only at the GND pin of the driver (STAR network). D. Keep the copper trace between the driver output pin and the load short and wide. Figure 4 APPLICATIONS INFORMATION |
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