QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1401A ADJUSTABLE SINGLE RESISTOR LDO REGULATOR LT3085EDCB DESCRIPTION Demonstration circuit 1401A is an adjustable 500mA linear regulator featuring LT®3085. Architected as a precision current source and voltage follower, it allows this new regulator to be used in many applications requiring high current, adjustability to zero output, and no heat sink. Also the device brings out the collector of the pass transistor to allow low dropout operation when used with multiple supplies. A key feature of the LT3085 is the capability to supply a wide output voltage range. By using a reference current through a single resistor, the output voltage can be programmed to any level between zero and 36V. The DC1401A has a reduced input voltage 25V due to input capacitor voltage rating. And DC1401A is capable of delivering up to 500mA output current. Therefore, the DC1401A can be used as a high current linear regulator, post regulator for switching supply, variable voltage supply or low output voltage power supply. Internal protection circuitry includes current limiting and thermal limiting. LT3085 regulator is offered in 8-lead MSOP (with an Exposed Pad for better thermal characteristics) and 6-lead 2mm × 3mm DFN packages. The LT3085 datasheet gives a complete description of the part, operation and application information. The datasheet should be read in conjunction with this quick start guide for working on or modifying the demo circuit 1401A. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. Table 1. Performance Summary ( TA = 25oC ) PARAMETER CONDITION VALUE Minimum Vin Voltage Vout=1.2V 1.7V Maximum Vin Voltage Minimum Vcontrol Voltage 25V Vout=1.2V Maximum Vcontrol Voltage Output Voltage 25V JP1, Vout=1.2V 1.2V ±3% JP1, Vout=1.8V 1.8V ±3% JP1, Vout=2.5V 2.5V ±3% JP1, Vout=3.3V 3.3V ±3% JP1, Vout=5.0V 5.0V±3% Minimum Output Current Maximum Output Current 2.8V 1mA Vin-Vout<6V 500mA 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1401A ADJUSTABLE SINGLE RESISTOR LDO REGULATOR QUICK START PROCEDURE The DC1401A is easy to set up to evaluate the performance of the LT3085. Refer to Figure 1. for proper measurement equipment setup and following the procedures below: 1. Before proceeding to test, use jumper JP1 for the desired output voltage (1.2V, 1.8V, 2.5V, 3.3V or 5.0V). If the output voltage is different from the above values, use the USER option and install R6. Select R6 according to the following equation: R6 = VOUT . Vcontrol. Draw 500mA of load current. The measured Vout should be 1.2V ± 3% (1.174V to 1.246V). 3. Vary Vin from 1.7V to 25V, Vcontrol from 2.8V to 25V and the load current from 1mA to 500mA. Vout should measure 1.2V ± 3% (1.174V to 1.246V). Note: Make sure the power dissipation is limited below the thermal limit. 10µA 2. Assume 1.2V is the desired output. Apply 1.7V across Vin (to Gnd), and 2.8V across Figure 1. Proper Measurement Equipment Setup 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1401A ADJUSTABLE SINGLE RESISTOR LDO REGULATOR Figure 2. Measuring Input or Output Ripple INPUT VOLTAGE RANGE The guaranteed Vin dropout voltage is 0.5V at 500mA, the guaranteed Vcontrol dropout voltage is 1.6V at 500mA. The maximum Vin and maximum Vcontrol is reduced to 25V due to the input capacitor voltage rating. DUAL SUPPLY OR SINGLE SUPPLY Use two separate supplies for Vin and Vcontrol, a low dropout voltage can be achieved on the Vin pin and the power dissipation is minimized. Alternatively, Tying the Vcontrol to Vin through a short wire, a single supply is sufficient to drive the demo circuit 1401A. AVOID USING A LONG WIRE TO TIE VCONTROL AND VIN. OUTPUT CURRENT The output current will decrease at high input-tooutput differential. The actual current output is further limited by the thermal shutdown function, which is related to the board thermal dissipation. DO NOT CONTINUOUSLY OPERATE THE LT3085 TO THE THERMAL LIMIT, WHICH MAY IMPAIR THE DEVICE RELIABILITY. MAXIMUM JUNCTION TEMPERATURE IS 125oC. 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1401A ADJUSTABLE SINGLE RESISTOR LDO REGULATOR THERMAL IMAGE An example thermal image shows the temperature distribution on board. The test is done in still air at room temperature with 2.0W power dissipation in LT3085. BYPASS CAPACITOR Since the SET pin is a high impedance node, unwanted signals may couple into the SET pin and cause erratic behavior. This will be most noticeable when operating with minimum output capacitors at full load current. The easiest way to remedy this is to bypass the SET pin with a small amount of capacitance from SET to ground, 10pF to 20pF is sufficient. A 0.01uF bypass capacitor is used on the demo board to provide a lownoise output. Please refer to datasheet for details. 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1401A ADJUSTABLE SINGLE RESISTOR LDO REGULATOR 5