QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL LT3080EDD DESCRIPTION Demonstration circuit 1294A shows adjustable linear regulators in parallel using LT3080. 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 LT3080 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 DC1294A has a reduced input voltage 25V due to input capacitor voltage rating, and is capable of delivering up to 4.4A output current by paralleling 4 LT3080s with only 20m PCB ballast resistors. The DC1294A can be used as a high current linear regulator, post regulator for switching supply, vari- able voltage supply or low output voltage power supply. Internal protection circuitry includes current limiting and thermal limiting. LT3080 regulator is offered in 8-lead MSOP (with an Exposed Pad for better thermal characteristics), a 3mm × 3mm DFN, 5-lead TO-220 and a simple-touse 3-lead SOT-223 version. The LT3080 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 1294A. 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=3.3V 3.8V Maximum Vin Voltage Minimum Vcontrol Voltage 25V Vout=3.3V 4.9V Maximum Vcontrol Voltage 25V Output Voltage 3.3V ±3% Minimum Output Current 4mA Maximum Output Current Vin-Vout<2.5V 4.4A 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL QUICK START PROCEDURE The DC1294A is easy to set up to evaluate the performance of the LT3080. Refer to Figure 1. for proper measurement equipment setup and following the procedures below: 1. Apply 3.8V across Vin (to Gnd), and 4.9V across Vcontrol. Draw 4.4A of load current. The measured Vout should be 3.3V ± 3% (3.2V to 3.4V). 2. Vary Vin from 3.8V to 25V, Vcontrol from 4.9V to 25V and the load current from 4mA to 4.4A. Vout should measure 3.3V ± 3% (3.2V to 3.4V). Note: Make sure the power dissipation is limited below the thermal limit. Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Input or Output Ripple 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL INPUT VOLTAGE RANGE The guaranteed Vin dropout voltage is 0.5V at 4.4A, the guaranteed Vcontrol dropout voltage is 1.6V at 4.4A. 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 zero ohm jump resistor R1 on board, a single supply is sufficient to drive the demo circuit 1294A. 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. THERMAL IMAGE An example thermal image shows the temperature distribution on board. The test is done in still air at room temperature with average 1.7W power dissipation in each LT3080. Thermoteknix 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL 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. PARALLEL OPERATION A small ballast resistor is required for parallel operation, such as 20m . A short PCB trace can easily serve that purpose. Refer to the datasheet for trace length calculation. THE BOTTOM PAD OF IC IS THE OUTPUT. The bottom pad can be tied to the output pin respectively, however, each bottom pad should isolated from the others, thus force the output current go through the ballast resistors. 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL 5