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LT1103CT7 Datasheet(PDF) 11 Page - Linear Technology |
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LT1103CT7 Datasheet(HTML) 11 Page - Linear Technology |
11 / 32 page 11 LT1103/LT1105 PIN FUNCTIONS LT1103 FB: The Feedback pin is the inverting input to the sampling error amplifier. The noninverting input is tied to a 4.5V reference. The FB pin is used for output voltage sensing. The input bias current is a function of the control pin VC voltage and can be used for load regulation compensation by including a resistor in series with the FB pin. The sampling error amplifier has a typical gm of 0.012 mhos and the output of the sampling error amplifier has asym- metrical slew rate to reduce overshoot during start-up conditions or following the release of an output overload. VC: The VC control pin is used for frequency compensa- tion, current limiting and shutdown. It is the high imped- ance output of the sampling error amplifier and the input of the current limit comparator. GND: The Ground pin acts as both the negative sense point for the internal sampling error amplifier feedback signal and as the high current path for the 2A switch. Also, the case of the 7-lead TO-220 is connected to ground. Proper connections to ground for signal paths and high current paths must be made in order to insure good load regulation. OSC: The Oscillator pin sets the operating frequency of the regulator with one external capacitor to ground. Maximum duty cycle can also be adjusted by using an external resistor to alter the charge/discharge ratio. VIN: The Input Supply pin is designed to operate with voltages of 12V to 30V. The supply current is typically 200 µA up to the start-up threshold of 16V. Normal oper- ating supply current is fairly flat at 18mA down to the shutdown threshold of 7V. Switching is inhibited for VIN less than 12V due to the gate drive detection circuit. 15V: A 15V reference is used to bias the gate of an external power FET. The voltage temperature coefficient is typically 3mV/ °C and the output can source 30mA. Typical dropout voltage is 1.5V for VIN less than 17V and 30mA of load current. VSW: The Switch Output pin is the collector of the internal NPN power switch. This pin has a typical ON resistance of 0.4 Ω and a minimum breakdown voltage of 50V. This pin also ties to the FET gate drive detection circuit. LT1105 All functions on the LT1105 are equivalent to the LT1103 with the exception of the VSW pin and the ILIM pin and the availability of the OVLO, 5V, and SS functions. OVLO: The Overvoltage Lockout pin inhibits switching when the pin is pulled above its threshold voltage of 2.5V. OVLO is implemented with a resistor divider network from the rectified DC line and is used to protect the external FET from an overvoltage condition in the off state. This func- tion is only available on the 14-lead PDIP. 5V: A 5V reference is available to power primary-side circuitry. The temperature coefficient is typically 50ppm/ °C and the output can source 25mA. This func- tion is only available on the 14-lead PDIP. SS: The Soft-Start pin is used to either program start-up time with a capacitor to ground or to set external current limit with a resistor divider. The SS pin has a 40 µA pull-up current and is reset to 0V by a 1mA pull-down current during start-up and shutdown. This function is only avail- able on the 14-lead PDIP. VSW: The Switch Output pin is the output of a 1A NPN totem-pole stage. The VSW pin turns the external FET on by pulling its gate high. Break-Before-Make action of 200ns on each switch edge is built in to eliminate cross conduc- tion currents. ILIM: The ILIM pin is the input to the current limit amplifier and requires the use of a noninductive, power sense resistor from ILIM to ground to set current limit. The typical current limit threshold voltage is 350mV. The typical input bias current is 100 µA out of the pin. |
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