DEMO CIRCUIT 1359 QUICK START LT3640 GUIDE LT3640 Dual Monolithic Buck Regulator with PowerOn Reset and Watchdog Timer DESCRIPTION Demonstration circuit 1359 is a Dual Monolithic Buck Regulator with Power-On Reset and Watchdog Timer featuring the LT3640. The board operates from inputs up to 35V and withstands transients up to 55V. The outputs are 3.3V, 0.8A and 1.2V, 1A. At light loads, both regulators operate in low ripple Burst Mode® to maintain high efficiency and low output ripple. Users can populate R13 on the EN/UVLO pin to provide a programmable under voltage lockout. Both channels have cycle-by-cycle current limit, providing protection against shorted outputs. The power-on reset and watchdog timer periods are independently adjustable using external capacitors. Tight accuracy specifications and glitch immunity ensure reliable operation of the circuit. Watchdog can be enabled or disabled by JP1. The circuit can be synchronized to an external clock connected to the SYNC terminal. If the SYNC function is used, the Rt resistor (R9) should be chosen to set the LT3640 internal switching frequency at least 20% below the lowest synchronization input frequency. The LT3640 datasheet gives complete descriptions of the part, operation and application information. The datasheet must be read in conjunction with this quick start guide for working on or modifying the demo circuit 1359. Design files for this circuit board are available. Call the LTC factory. L, LTC, LTM, LT are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. PERFORMANCE SUMMARY Specifications are at TA = 25°C SYMBOL PARAMETER VIN Input Supply Range VOUT1 VOUT2 IOUT1 IOUT2 IQ Output Voltage 1 Output Voltage 2 Output Current 1 Output Current 2 No Load Quiescent Current FSW TWDU TWDL TRST Switching Frequency Watchdog Upper Boundary Period Watchdog Lower Boundary Period Programmed Reset Period CONDITIONS VOUT1 = 3.3V, IOUT1 = 0.8A VOUT2 = 1.2V, IOUT2 = 1A VIN = 12V, IOUT1 = 0.8A VIN = 12V, IOUT2 = 1A MIN 5 3.20 1.16 0 0 VIN = 12V, VOUT1 = 3.3V, VOUT2 = 1.2V, No Load MAX 35 3.30 1.20 2 55.5 3.5 55.5 UNITS V 3.40 1.24 0.8 1 V V A A mA 2.35 MHz ms ms ms 0.33 1.75 C7 = 1500pF C7 = 1500pF C8 = 1500pF TYP 1 LT3640 QUICK START PROCEDURE Demonstration circuit 1359 is easy to set up to evaluate the performance of the LT3640. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: 5. Check for the proper output voltages: 1. Place Jumper JP1 in the following position: 6. Once the proper output voltages are established, adjust the loads within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. OFF: Watchdog Disabled ON: Watchdog Enabled 2. With power off, connect the input power supply to VIN and GND. 3. With power off, connect loads from VOUT1 to GND and VOUT2 to GND. 4. Turn on the power at the input. NOTE. Make sure that the input voltage does not exceed 35V. 2 VOUT1 = 3.3V, VOUT2 = 1.2V NOTE. If there is no output, temporarily disconnect the load to make sure that the load is not set too high or is shorted. 7. To test the watchdog timer, connect a clock input to the WDI terminal. Observe the output at the WDO terminal while the clock parameters are adjusted. 8. To test Power-On Reset, observe output at the RESET terminals: RST1 and RST2. LT3640 Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple 3 LT3640 4