QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 662 36V 600MA 500KHZ SOT-23 STEP-DOWN REGULATOR LT1933 DESCRIPTION Demonstration circuit 662 is a monolithic step-down R1 installed). The internal soft start function reduces DC/DC switching converter featuring the LT 1933. input current surge during start-up. The board is optimized for 3.3V output at up to The LT1933 datasheet gives a complete description of 500mA load current for a steady state input voltage the part, operation and applications information. The range of 4.5V to 36V. With its wide input voltage datasheet must be read in conjunction with this Quick range, high current internal power switch, low drop- Start Guide for demonstration circuit 662. In particu- out operation, 500kHz switching frequency and tiny lar, the datasheet section on Thermal Calculations is SOT-23 package, the LT1933 is a very versatile and important for estimating whether a given applications powerful IC for DC/DC converters that require ex- combination of input voltage, load current, ambient tremely compact space, high efficiency and high input temperature and frequency will cause the LT1933 to voltage. exceed its absolute maximum rated junction tem- The LT1933 500kHz switching frequency allows all of perature. The LT1933 is assembled in a small low the components to be small, surface mount devices. profile SOT-23 ThinSOT package. Proper board layout The current-mode control topology creates fast tran- is essential for maximum thermal performance. See sient response and good loop stability with a mini- the datasheet section Layout Considerations. mum number of external components. The low resis- Design files for this circuit board are available. Call tance internal power switch (0.37V V sat) achieves CE the LTC factory. high efficiencies of up to 84%. The shutdown pin can LT is a trademark of Linear Technology Corporation be used to program undervoltage lockout or place the part in micropower shutdown, reducing supply cur- rent to less than 2A by driving the pin low (without QUICK START PROCEDURE Demonstration circuit 662 is easy to set up to evalu- higher ESR Aluminum Electrolytic capacitor, C4, is ate the performance of the LT1933. Refer to Figure 1 placed at the input terminals. This may protect for proper measurement equipment setup and follow against some, but not all, input transients due to hot- the procedure below: plugging with a power supply. See Application Note 88 for more details. NOTE: Make sure that the input voltage does not ex- ceed 36V. NOTE: Connect the power supply (with power off), load, and meters as shown in Figure 1. NOTE: The shutdown function is optional and its ter- minal can be left floating (disconnected) if shutdown 1. After all connections are made, turn on input power is not being used. and verify that the output voltage is 3.3V. NOTE: Do not hot-plug the input voltage terminal VIN. NOTE: If the output voltage is too low, temporarily The absolute maximum voltage on VIN is 36V and disconnect the load to make sure that the load is hot-plugging a power supply through wire leads to not set too high. the demonstration circuit can cause the voltage on 2. Once the proper output voltages are established, the extremely low-ESR ceramic input capacitor to ring adjust the load within the operating range and ob- to twice its DC value. In order to protect the IC, a 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 662 36V 600MA 500KHZ SOT-23 STEP-DOWN REGULATOR serve the output voltage regulation, ripple voltage, efficiency and other parameters. Figure 1. Proper Measurement Equipment Setup 2