QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 502 1.25MHZ WIDE INPUT RANGE STEP DOWN DC/DC CONVERTER LT1767 DESCRIPTION Demonstration circuit 502 is a input to 5.0V output 1A or for 4.5- 1.25MHz 1.5A monolithic step-down 25V input to 3.3V output 1A for ca- DC/DC switching converter using the ble modem, handheld, automotive, and LT1767 buck regulator. With its fast desktop computer applications. The switching speed, 1.5A internal power output voltage is jumper selectable switch, and wide input voltage range, (5V or 3.3V). This board was designed the LT1767 is a very versatile and for wide input voltage range applica- powerful IC for DC/DC converters that tions where simplicity, small circuit require compact space and a wide va- size, and low component count are im- riety of input voltage ranges. The portant as well as up to 1A of load constant 1.25MHz switching frequency current. The use of ceramic capaci- allows all of the components to be tors not only demonstrates small size very small, surface mount devices. and low cost, but the advantage of The current-mode control topology current-mode control in step-down ap- creates fast transient response and plications with a simple compensation good loop stability with a minimum network and a place-holder for a number of external compensation com- feedforward capacitor for more rugged ponents and ceramic input and output stability and optimal transient re- capacitors. The low resistance inter- sponse if required. nal power switch (0.22 ) maintains Design files for this circuit board high efficiencies >80% and up to 90%. are available. Call the LTC factory. The SHDN pin and 6uA shutdown current extends battery life. The wide V IN range of the LT1767 allows step-down Higher input voltages may pulse-skip due to configurations from 3Vin up to 25Vin. minimum on-time restrictions. Compensation Synchronization of switching frequen- component changes may be necessary to opti- cies up to 2MHz is possible. mize pulse-skipping during high-temperature, high-voltage conditions and maintain control Demonstration Circuit 502 is a step- of switch current. down converter using the LT1767EMS8E. The circuit is designed for 6.6-25V Table 1. Typical Performance Summary PARAMETER CONDITION VALUE Input Voltage Range V = 5V (Jumper Selection) 6.6V25V out Input Voltage Range V = 3.3V (Jumper Selection) 4.5V18V out V V = 5V (Jumper Selection) 5V 3% out out V V = 3.3V (Jumper Selection) 3.3V 3% out out Maximum Output Current 1A Typical Switching Frequency 1.25MHz Typical Efficiency V = 12V, V = 5V, I = 1A 84% in out out 1 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 502 1.25MHZ WIDE INPUT RANGE STEP DOWN DC/DC CONVERTER QUICK START PROCEDURE Demonstration circuit 502 is easy to 4. Connect an ammeter in series with set up to evaluate the performance of output load to measure output cur- the LT1767. Refer to Figure 1 for rent. proper measurement equipment setup 5. Connect a voltmeter across the Vin and follow the procedure below: and Gnd terminals to measure input NOTE: Make sure that the input voltage voltage. does not exceed 25V. 6. Connect a voltmeter across the Vout 1. Connect the 7V25V or 4.5V18V in- and Gnd terminal to measure output put power supply to the Vin and Gnd voltage. terminals on the board. 7. After all connections are made, 2. Connect an ammeter in series with turn on input power and verify that the input supply to measure input the output voltage is either 5.0V current. or 3.3V (the output voltage jumper setting). 3. Connect either power resistors or an electronic load to the Vout and 8. The synchronization and shutdown Gnd terminals on the board. functions are optional and their pins can be left floating (discon- nected) if their functions are not being used. ++ + LTC1767EMS8E 1.25MHz Wide Input Range Linear Technology Step Down DC/DC Converter (408) 432-1900 Demo Circuit 502 www.linear.com Vin GND SYNC + VOUT Vout GND SHDN 3.3V 5V + + + LOAD Figure 1. Proper Measurement Equipment Setup 2 2