QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549 2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER LT3431 DESCRIPTION Demonstration circuit 549 is a mono- SHDN pin can be used to program un- lithic step-down DC/DC switching con- dervoltage lockout or place the part verter using the LT3431. The board is in micropower shutdown, reducing sup- optimized for 5V output at up to 2A ply current to 30 A by driving the load current for a steady state input pin low. voltage range of 7.5V to 20V. The The LT3431 datasheet gives a complete LT3431 is capable of withstanding in- description of the part, operation put transients to 60V and runs at and applications information. The da- 500kHz with steady state input volt- tasheet must be read in conjunction ages that require duty cycles between with this Quick Start Guide for dem- 25% and 75%. For duty cycles below onstration circuit 549. In particu- 25% the part may pulse skip to effec- lar, the datasheet section on Ther- tively average the lower duty cycle mal Calculations is important for and still provide minimum on-time estimating whether a given applica- switch cycles. With its wide input tions combination of input voltage, voltage range, 3A internal power load current and frequency will cause switch, high 500kHz switching fre- the LT3431 to exceed its absolute quency and thermally enhanced pack- maximum rated junction temperature. age, the LT3431 is a very versatile The part is assembled in a small 16- and powerful IC for DC/DC converters pin thermally enhanced package with that require compact size, high effi- exposed pad where proper board layout ciency and tolerance to high input is essential for maximum thermal per- voltage transients. formance. See the datasheet section The LT3431 500kHz switching frequency Layout Considerations. allows all of the components to be NOTE: Do not hot-plug the input volt- small, surface mount devices. Syn- age terminal V . The absolute maxi- IN chronization with an external clock mum voltage on V is 60V and hot- IN of up to 700kHz is possible. The cur- plugging a power supply through wire rent-mode control topology creates leads to the demonstration card can fast transient response and good loop cause the voltage on the extremely stability with a minimum number of low-ESR ceramic input capacitor to external compensation components. In ring to twice its DC value. This is particular, the current-mode archi- due to high currents instantaneously tecture allows the use of ceramic in- generated in the inductive supply put and output capacitors for in- leads from an input voltage step on creased reliability, extremely low the low-ESR ceramic input capacitor. output ripple voltage and small com- An over-voltage can result if care is ponent size. The low resistance in- not taken to ensure that this does ternal power switch (0.1 ) maintains not happen. A bulky higher-ESR ca- high efficiencies of up to 87%. The pacitor, and an additional inductive 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549 2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER filter between the bulk capacitor and Design files for this circuit board the input capacitor can be added to are available. Call the LTC factory. the circuit to dampen hot-plug tran- sient ringing. See Application Note 88 for more details. Table 1. Typical Performance Summary PARAMETER CONDITION VALUE Steady State Input Voltage Range V = 5V, I 2A, T = 25C 7.520V OUT OUT A Maximum Transient Input Voltage 60V V I 2A 5V 3% OUT OUT Maximum Output Current 2A Typical Switching Frequency 500kHz Typical Efficiency V = 12V, V = 5V, I = 2A 83% IN OUT OUT 2