QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 423 1A, 500KHZ HIGH VOLTAGE BUCK CONVERTER LT1956 DESCRIPTION Demonstration circuit 423 is a 60V, 500kHz, 1A load cur- transient operation up to 60V. Synchronization of switch- rent, monolithic step-down DC/DC switching converter ing frequencies up to 700kHz is possible. using the LT1956. With its wide input voltage range, Demonstration Circuit 423 is designed for 7.5V to 25V 1.5A internal power switch, and thermally enhanced input (with up to 60V transient) to 5.0V output at 1A package, the LT1956 is a very versatile and powerful IC load current for automotive, industrial, portable com- for DC/DC converters that require compact space and a puters, battery chargers, and distributed power systems. wide variety of input voltage ranges. The constant This board was designed for wide input voltage range 500kHz switching frequency allows all of the compo- applications where simplicity, small circuit size, and low nents to be small, surface mount devices. The current- component count are important as well as up to 1A of mode control topology creates fast transient response load current. The extremely low output voltage ripple and good loop stability with a minimum number of ex- ceramic capacitor not only demonstrates small size and ternal compensation components and the use of ceramic low cost, but the advantage of current-mode control in input and output capacitors. The low resistance internal step-down applications with a simple compensation power switch (0.2 ) maintains high efficiencies >75% network and a feedforward capacitor for more rugged and up to 90%. The SHDN pin and 25A shutdown cur- stability and excellent transient response. Demonstration rent extends battery life. The wide VIN range of the Circuit 423 is best used in 500kHz applications where LT1956 allows steady state input voltages from 7.5V the maximum duty cycle does not exceed 75% and the input up to 25V input. The LT1956 is capable of steady minimum steady state duty cycle remains above 25%. state operation of up to 60V if the output voltage is set Duty cycles below 25% are capable, with the restriction higher than 5V and the duty cycle remains high enough of minimum on-time, the part may begin to skip pulses (typically above 25%). Minimum on-time and maximum to effectively lower the overall minimum on-time. output power at high input voltages may have thermal Design files for this circuit board are available. Call restrictions (please see datasheet for more details). the LTC factory. However, with 5V output, the LT1956 is designed for Table 1. Typical Performance Summary PARAMETER CONDITION VALUE Steady State Input Voltage Range V = 5V, I 1A 7.525V OUT OUT Maximum Transient Input Voltage 60V V I 1A 5V 3% OUT OUT Maximum Output Current 1A Typical Switching Frequency 500kHz Typical Efficiency V = 12V, V = 5V, I = 1A 83% IN OUT OUT 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 423 1A, 500KHZ HIGH VOLTAGE BUCK CONVERTER QUICK START PROCEDURE Demonstration circuit 423 is easy to set up to evaluate CUSTOMIZING THE BOARD the performance of the LT1956. Refer to Figure 1 for The components assembled on the board are optimized proper measurement equipment setup and follow the for a wide input voltage range and a 5V output. The procedure below: bandwidth can be increased for more specific input volt- Make sure that the input voltage does not exceed 60V. ages such as 12V 10% or 18V24V with 36V transient if desired (just a few examples). The adjustable feedback The synchronization and shutdown functions are op- resistors allow the output voltage to be customized. For tional and their pins can be left floating (disconnected) if output voltages below 3.0V, the boost diode should be their functions are not being used. moved from D2 to D3 to provide the minimum boost 1. Connect the 7.5V25V input power supply to the Vin voltage required for the internal power switch. Make and Gnd terminals on the board. sure that the boost capacitor (C8) has a voltage rating greater than or equal to the output voltage for applica- 2. After all connections are made, turn on input power tions where the boost diode is placed in D2. However, and verify that the output voltage is 5.0V. the boost capacitor must have a voltage rating greater ++ than the input voltage whenever the boost diode is OPTIONAL placed in position D3. + + D4 is an optional boost voltage blocking diode. For custom applications in which the output voltage is sig- nificantly greater than 5V, this blocking diode reduces the boost driver voltage, reducing the power (and thus heat) dissipation inside the IC. It is recommended that a CMHZ5236B zener diode is used in D4 when V = 12V OUT and the junction temperature is hot without the use of the Zener. Note that in order to properly install D4, the small trace connecting the anode and cathode of D4 on the board must be separated (an Exacto knife works well) before D4 is soldered to the board. 100 90 + 80 70 OPTIONAL + 60 + LOAD 8VIN 50 12VIN 40 Figure 1. Proper Measurement Equipment Setup 18VIN 30 24VIN 20 10 0 0 200 400 600 800 1k LOAD CURRENT (mA) Figure 2. Efficiency 2 EFFICIENCY (%)