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LTC1697 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1697 Datasheet(HTML) 6 Page - Linear Technology |
6 / 8 page LTC1697 1697f VDIM pin where DC = 0% at VDIM = 1V and DC = 100% at VDIM = 2V. If desired, the internal dimming PWM can be overridden by grounding the CDIM pin and applying the input PWM signal to the VDIM pin. This external PWM signal should be in the range of 150Hz to 500Hz. Lamp Feedback In a typical application, the LAMP pin is connected to the low voltage side of the lamp. The lamp pin is internally connected to ground by a ~50Ω resistor. This resistor will limit the voltage on the LAMP pin to ±0.35V for a 5mARMS lamp current. The lamp feedback circuit removes a current from VC approximating 1/40 of the absolute value of the current through the 50Ω resistor. Current Programming Input (RPROG) The ILAMP current is set with an external resistor con- nected between this pin and ground. ILAMP = 32V/RPROG. VC Compensation The VC node is the point where the lamp feedback current, the programming current, and the control for the switch- ing controller meet. A single capacitor must be connected from the VC pin to ground to compensate the feedback loop. Careful consideration should be given to the value of capacitance used. A large value (1µF) will give excellent stability at high lamp currents but will result in degraded line regulation. On the other hand, a small value (10nF) will result in overshoot and poor load regulation. The value chosen will depend on the maximum load current and dimming range. After these parameters are decided upon, the value of the VC capacitor should be increased until the line regulation becomes unacceptable. A typical value for the VC capacitor is 0.1µF. For further information on compensation please refer to Application Note 65 or consult the factory. OVSEN Operation The OVSEN pin can be used to protect the high voltage transformer from an overvoltage condition that can occur when the lamp is open or not present. Connect this pin through a resistor to the emitters of the drive transistors of the Royer oscillator. The voltage at the OVSEN pin is specified by VOVSEN. The duty cycle of the LTC1697 SW pin will be 0% when the current flowing out of the OVSEN pin reaches IOVSEN (protect). See the manufacturers trans- former specifications for transformer voltage ratings. Thermal Shutdown This IC includes overtemperature protection that is in- tended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous opera- tion above the specified maximum operating junction temperature may result in device degradation or failure. Shutdown Operation There are two ways to place the LTC1697 in shutdown. The SHDN pin can be pulled below VSHDN-1, or the VDIM pin can be pulled below VDIM(SD-I) for more than approximately 50mS. For normal operation, both pins must be pulled high. The SHDN pin must be pulled above VSHDN-H, and the VDIM pin must be pulled above VDIM (SD-H). APPLICATIO S I FOR ATIO |
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