QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1092 36V 500KHZ H-GRADE DFN STEP-DOWN REGULATOR LT1933H DESCRIPTION Demonstration circuit 1092 is a monolithic step-down shutdown, reducing supply current to less than 2A DC/DC switching converter featuring the LT 1933H. by driving the pin low (without R1 installed). The in- The board is optimized for 3.3V output at up to ternal soft start function reduces input current surge 500mA load current for a steady state input voltage during start-up. range of 4.5V to 36V. With its H-grade temperature The LT1933H datasheet gives a complete description rating, wide input voltage range, high current internal of the part, operation and applications information. power switch, low dropout operation, 500kHz switch- The datasheet must be read in conjunction with this ing frequency and tiny DFN package, the LT1933H is Quick Start Guide for demonstration circuit 1092. In a very versatile and powerful IC for DC/DC converters particular, the datasheet section on Thermal Calcula- that require extremely compact space, high efficiency tions is important for estimating whether a given ap- and high input voltage with a high temperature range. plications combination of input voltage, load current, The LT1933H 500kHz switching frequency allows all ambient temperature and frequency will cause the of the components to be small, surface mount de- LT1933H to exceed its absolute maximum rated junc- vices. The board is designed to run at maximum tion temperature. The LT1933H is assembled in a power up to 85C ambient. The IC is rated for up to small DFN package. Proper board layout is essential 150C junction temperature. The current-mode con- for maximum thermal performance. See the data- trol topology creates fast transient response and sheet section Layout Considerations. good loop stability with a minimum number of exter- Design files for this circuit board are available. Call nal components. The low resistance internal power the LTC factory. switch (0.37V V sat) achieves high efficiencies of up CE LT is a trademark of Linear Technology Corporation to 84%. The shutdown pin can be used to program undervoltage lockout or place the part in micropower QUICK START PROCEDURE Demonstration circuit 1092 is easy to set up to evalu- to twice its DC value. In order to protect the IC, a ate the performance of the LT1933H. Refer to Figure higher ESR Aluminum Electrolytic capacitor, C4, is 1 for proper measurement equipment setup and fol- placed at the input terminals. This may protect low the procedure below: against some, but not all, input transients due to hot- plugging with a power supply. See Application Note NOTE: Make sure that the input voltage does not ex- 88 for more details. ceed 36V. NOTE: Connect the power supply (with power off), NOTE: The shutdown function is optional and its ter- load, and meters as shown in Figure 1. minal can be left floating (disconnected) if shutdown is not being used. 1. After all connections are made, turn on input power and verify that the output voltage is 3.3V. NOTE: Do not hot-plug the input voltage terminal VIN. The absolute maximum voltage on VIN is 36V and NOTE: If the output voltage is too low, temporarily hot-plugging a power supply through wire leads to disconnect the load to make sure that the load is the demonstration circuit can cause the voltage on not set too high. the extremely low-ESR ceramic input capacitor to ring 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1092 36V 500KHZ H-GRADE DFN STEP-DOWN REGULATOR 2. Once the proper output voltages are established, adjust the load within the operating range and ob- serve the output voltage regulation, ripple voltage, efficiency and other parameters. Figure 1. Proper Measurement Equipment Setup 2