QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476 1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER LT1765EFE DESCRIPTION Demonstration circuit 476 is a 1.25MHz 3A monolithic 4.7V25V input, respectively, covering the common step-down DC/DC switching converter using the values used in cable modems, handhelds, automotive, LT1765EFE. The LT1765 features fast switching speed, and desktop computer applications. The 5V or 3.3V a 3A internal power switch, and a wide input voltage output voltage is jumper selectable. range, making it a versatile and powerful IC that fits This board is designed for applications that require 2A easily into space-constrained applications. The con- of load current from a wide input voltage range plus stant 1.25MHz switching frequency allows for the use simplicity, small circuit size, and low component of tiny, surface mount external components. The cur- count. The use of ceramic capacitors in this circuit not rent-mode control topology yields fast transient re- only demonstrates small size and low cost, but the sponse and good loop stability, requiring a minimum advantage of current-mode control in step-down ap- number of external compensation components and plications with a simple compensation network and a allowing the use of ceramic input and output capaci- feedforward capacitor for more rugged stability and tors. The low R internal power switch (0.09 ) DS(ON) excellent transient response. maintains high efficiencies (as high as 90%) over a Design files for this circuit board are available. Call wide range of input voltages and loads. Its 15A shut- the LTC factory. down current (activated via the SHDN pin) extends battery life. The wide V range of the LT1765 allows IN step-down configurations from 3V to 25V input. Syn- Higher input voltages may pulse-skip due to minimum on-time restrictions. chronization of switching frequencies up to 2MHz is Compensation component changes may be necessary to optimize pulse- skipping during high-temperature, high-voltage conditions and maintain possible. control of switch current. The demonstration circuit is designed to provide either 3.3V at 2A or 5V at 2A output, from a 7V25V input or Figure 1. Demonstration Circuit 476 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476 1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER TYPICAL PERFORMANCE SUMMARY (T = 25C) A Table 1. Step-Down Converter (V = 5V) Table 2. Step-Down Converter (with V = 3.3V) OUT OUT PARAMETER VALUE PARAMETER VALUE V 7V to 25V V 4.7V to 18V (up to 25V) IN IN V 5.0V V 3.3V OUT OUT I 2A(max) I 2A(max) OUT OUT Up to 90% at 1A out and up to 89% at Up to 88% at 1A out and up to 85% Efficiency Efficiency 2A output at 2A output 90 90 V = 8V IN V = 12V 85 85 V = 5V IN IN V = 8V V = 18V IN IN 80 80 V = 12V IN V = 24V IN 75 75 V = 18V IN 70 70 65 65 60 60 55 55 50 50 45 45 10 100 1k 10k 10 100 1k 10k LOAD CURRENT (mA) LOAD CURRENT (mA) Figure 2. Typical Efficiency of DC476A LT1765EFE, V = 5V Figure 3. Typical Efficiency of DC476A LT1765EFE, V = 3.3V OUT OUT QUICK START PROCEDURE Demonstration circuit 476 is easy to set up to evaluate 6. Connect a voltmeter across the VOUT and GND termi- the performance of the LT1765EFE. Refer to Figure 4 for nal to measure output voltage. proper measurement equipment setup and follow the 7. After all connections are made, turn on input power procedure below: and verify that the output voltage is either 5.0V or 1. Connect the 725V or 4.718V input power supply to 3.3V (the output voltage jumper setting). the VIN and GND terminals on the board. 8. The synchronization and shutdown functions are op- 2. Connect an ammeter in series with the input supply to tional and their terminals can be left floating (discon- measure input current. nected) if their functions are not being used. 3. Connect either power resistors or an electronic load MINIMUM INPUT VOLTAGE to the VOUT and GND terminals on the board. The minimum LT1765EFE operating input voltage is 4. Connect an ammeter in series with output load to 3.0V. Nevertheless, a DC/DC buck converter must have measure output current. an input voltage that is greater than the output voltage 5. Connect a voltmeter across the VIN and GND termi- by a certain margin in order to provide the desired regu- nals to measure input voltage. lated output voltage. Maximum duty cycle, switch on- resistance, and inductor and diode DC losses all play a 2 EFFICIENCY (%) EFFICIENCY (%)