QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C 1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER LT1766EFE DESCRIPTION Demonstration circuit 326C is a monolithic step-down program undervoltage lockout or place the part in mi- DC/DC switching converter featuring the LT1766EFE. The cropower shutdown, reducing supply current to 25A by board is optimized for 5V output at up to 1A load current driving the pin low. and a steady state input voltage range of 6V to 60V. With The LT1766EFE datasheet gives a complete description its wide input voltage range, 1.5A internal power switch, of the part, operation and applications information. The 200kHz switching frequency and thermally enhanced datasheet must be read in conjunction with this Quick package, the LT1766EFE is a very versatile and powerful Start Guide for demonstration circuit 326C. In particular, IC for DC/DC converters that require compact space, the datasheet section on Thermal Calculations is impor- high efficiency and tolerance to high input voltage tran- tant for estimating whether a given applications combi- sients. nation of input voltage, load current and frequency will The LT1766EFE 200kHz switching frequency allows all of cause the LT1766EFE to exceed its absolute maximum the components to be small, surface mount devices. rated junction temperature. The LT1766EFE is assembled Synchronization with an external clock of up to 700kHz in a small 16-pin thermally enhanced package with ex- is possible. The current-mode control topology creates posed pad where proper board layout is essential for fast transient response and good loop stability with a maximum thermal performance. See the datasheet sec- minimum number of external components. The low re- tion Layout Considerations. sistance internal power switch (0.2 ) achieves high effi- Design files for this circuit board are available. Call ciencies of up to 90%. The SHDN pin can be used to the LTC factory. Table 1. Typical Performance Summary (T = 25C) A PARAMETER CONDITION VALUE Steady State Input Voltage Range V = 5V, I 1A 660V OUT OUT Maximum Transient Input Voltage 60V V V = 6V to 60V, I 1A 5V 3% OUT IN OUT Maximum Output Current V = 5V 1A OUT V = 12V, I = 1A, V = 5V 20mV IN OUT OUT PKPK Output Voltage Ripple V = 48V, I = 1A, V = 5V 32mV IN OUT OUT PK-PK Switching Frequency V = 6V to 60V, I 1A 200kHz IN OUT V = 12V, I = 1A, V = 5V 87% IN OUT OUT Efficiency V = 48V, I = 1A, V = 5V 80% IN OUT OUT 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C 1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER QUICK START PROCEDURE Demonstration circuit 326C is easy to set up to evaluate the performance of the LT1766EFE. 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 and shutdown functions are optional and their pins can be left floating (discon- nected) if their functions are not being used. NOTE: Do not hot-plug the input voltage terminal VIN. Figure 1. Proper Measurement Equipment Setup The absolute maximum voltage on VIN is 60V and hot- plugging a power supply through wire leads to the dem- onstration circuit can cause the voltage on the extremely low-ESR ceramic input capacitor to ring to twice its DC CUSTOM OPTIONS value. This is due to high currents instantaneously gen- OUTPUT VOLTAGE erated in the inductive supply leads from an input volt- age step on the low-ESR ceramic input capacitor. A The components assembled on the board are optimized bulky higher-ESR capacitor, and an additional inductive for a wide input voltage range and a 5V output. The filter can be added to the circuit to dampen hot-plug feedback resistors (R2, R3) can be changed to adjust the transient ringing. See Application Note 88 for more de- output voltage according to the following equation: tails. V = 1.22 (1 + R2/R3) OUT 1. Connect the power supply (with power off), load and The Thevenin equivalent resistance seen at the FB pin meters as shown in Figure 1. should be less than 3.8k to maximize frequency foldback 2. After all connections are made, turn on input power during start-up and short-circuit. and verify that the output voltage is 5V. For output voltages below 3V, the boost diode should be NOTE: If the output voltage is too low, temporarily dis- moved from D2 to D3 to provide at least the minimum connect the load to make sure that the load is not set boost voltage required for the internal power switch. too high. Make sure that the boost capacitor (C4) has a voltage rating (1) greater than the output voltage (when the 3. Once the proper output voltages are established, ad- boost diode is placed in D2) or (2) greater than the input just the loads within the operating range and observe voltage (when the boost diode is placed in position D3). the output voltage regulation, ripple voltage, efficiency and other parameters. For output voltages greater than 5V, the optional block- ing zener diode D4 can be used to reduce the boost voltage across C4 to some lower voltage between 3V and 5V. The diode transfers power dissipation from in- side the LT1766EFE to the diode on the demonstration circuit, outside the LT1766EFE, allowing higher ambient temperature operation of the part. Maintaining boost voltage between 3V and 5V maximizes efficiency and 2