QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481 1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE LT1976 DESCRIPTION Demonstration circuit 481 is a monolithic step- zero load current to under 100A at most input down DC/DC switching converter featuring the voltages (see Figure 3) while maintaining a regu- LT1976. The board is optimized for 3.3V output at lated output. A power good comparator and a tim- up to 1A load current for an input voltage range of ing delay can be used for additional system diag- 4V to 60V. Minimum on-time restrictions and 3.3V nostics and sequencing. The soft start function output may limit the steady state maximum input reduces inrush current at soft start and output voltage to 42V (up to 47V for some boards) before voltage overshoot. pulse skipping occurs. With its wide input voltage The LT1976 datasheet gives a complete descrip- range, 1.5A internal power switch, 100mA supply tion of the part, operation and applications infor- current (at input voltages above 14V), 200kHz mation. The datasheet must be read in conjunc- switching frequency and thermally enhanced tion with this Quick Start Guide for demonstration package, the LT1976 is a very versatile and pow- circuit 481. In particular, the datasheet section on erful IC for DC/DC converters that require com- Thermal Calculations is important for estimating pact space, high efficiency at both heavy and light whether a given applications combination of input loads and high input voltage. voltage, load current and frequency will cause the The LT1976 200kHz switching frequency allows all LT1976 to exceed its absolute maximum rated of the components to be small, surface mount de- junction temperature. The LT1976 is assembled in vices. Synchronization with an external clock of up a small 16-pin thermally enhanced package with to 700kHz is possible. The current-mode control exposed pad where proper board layout is essen- topology creates fast transient response and good tial for maximum thermal performance. See the loop stability with a minimum number of external datasheet section Layout Considerations. components. The low resistance internal power Design files for this circuit board are available. switch (0.2W) achieves high efficiencies of up to Call the LTC factory. 90%. The SHDN pin can be used to program un- dervoltage lockout or place the part in micropower Burst Mode is a trademark of Linear Technology Corporation. shutdown, reducing supply current to less than 1A by driving the pin low. Burst Mode reduces Table 1. Typical Performance Summary (T = 25C) A PARAMETER CONDITION VALUE Steady State Input Voltage Range V = 3.3V, I 1A 442V OUT OUT Maximum Transient Input Voltage 60V V V = 4V to 60V, I 1A 3.3V 3% OUT IN OUT Maximum Output Current V = 3.3V 1A OUT V = 12V, I = 1A, V = 3.3V 34mV IN OUT OUT PKPK Output Voltage Ripple V = 42V, I = 1A, V = 3.3V 42mV IN OUT OUT PK-PK Switching Frequency V = 4V to 42V, I 1A 200kHz IN OUT V = 12V, I = 1A, V = 3.3V 80% IN OUT OUT Efficiency V = 42V, I = 1A, V = 3.3V 72% IN OUT OUT 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481 1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE QUICK START PROCEDURE Demonstration circuit 481 is easy to set up to evaluate the performance of the LT1976. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: NOTE: Make sure that the input voltage does not exceed 60V. NOTE: The synchronization, shutdown, and power good functions are optional and their terminals can be left floating (disconnected) if their functions are not being used. NOTE: Do not hot-plug the input voltage terminal VIN. The absolute maximum voltage on VIN is Figure 1. Proper Measurement Equipment Setup 60V and hot-plugging a power supply through wire leads to the demonstration circuit can cause the voltage on the extremely low-ESR ceramic input capacitor to ring to twice its DC value. This is due FUNCTIONS & OPTIONS to high currents instantaneously generated in the inductive supply leads from an input voltage step BURST MODE on the low-ESR ceramic input capacitor. A bulky Burst Mode operation reduces light load quiescent higher-ESR capacitor, and an additional inductive current by disabling switching for a number of filter can be added to the circuit to dampen hot- switch cycles and reducing supply current briefly plug transient ringing. See Application Note 88 for until switching begins again. Bursts of switch more details. In order to protect the IC, a transient pulses are enough to maintain output voltage voltage suppressor diode can be added between regulation at light load. Figure 3 shows the supply VIN and GND terminals to absorb any high volt- current versus input voltage. At input voltages age transient ringing that may occur due to hot- above 12V, zero load supply current is less than plugging. 110A. NOTE: Connect the power supply (with power off), Burst Mode extends battery life in applications load, and meters as shown in Figure 1. with long periods of inactivity. The tradeoff of hav- ing Burst Mode is creating increased output volt- 1. After all connections are made, turn on input age ripple at light load. For loads that require ex- power and verify that the output voltage is 3.3V. tremely well-regulated output voltages, additional NOTE: If the output voltage is too low, temporar- filtering may be used on the output to achieve ex- ily disconnect the load to make sure that the tremely low output voltage ripple. At higher input load is not set too high. voltages (above 35V ), Burst Mode may occur IN below load currents as high as 300mA to 500mA. 2. Once the proper output voltages are estab- lished, adjust the load within the operating OUTPUT VOLTAGE range and observe the output voltage regula- The components assembled on the board are op- tion, ripple voltage, efficiency and other parame- timized for a wide input voltage range and a 3.3V ters. 2