DC524A

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 524 POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER LTC3704 DESCRIPTION Demonstration circuit 524 is a positive input to negative 400kHz. Please refer to LTC3704 data sheet for design output converter featuring the LTC3704 switching con- details and applications information. troller. The DC524 converts a 5V15V input to a 5.2V The DC524 can produce up to 3A from a 5V input output and provides up to 2.5A of output current from source, 4A from 10V source and 5A from 15V source. 5V input. The converter operates at 300kHz with a high The actual current capability also depends on the operat- efficiency of 89%. The DC524 is set to operate in No ing temperature and available airflow. As MOSFET Q1 R mode with input voltages up to 15V. However, SENSE gets hot, its R increases and the available output DS(ON) DC524 can be modified for higher input voltages with an current decreases. optional current sense resistor. The circuit features op- Design files for this circuit board are available. Call tional soft-start circuitry that can be enabled or disabled the LTC factory. with a single jumper. Also, DC524 has a jumper to pro- gram either continuous or Burst Mode operation. The Burst Mode is a registered trademark and No R is a trademark of Linear Tech- SENSE nology Corporation DC524 can be synchronized to an external clock of up to Table 1. Performance Summary PARAMETER CONDITION VALUE Minimum Input Voltage 4.5V Maximum Input Voltage No R mode 18V (or higher with some modifications) SENSE V V = 5V to 15V, I = 0A to 2.5A 5.2V 3% OUT IN OUT1 Typical Output Ripple V V = 5V to 15V, I = 0A to 2.5 50mV OUT IN OUT1 PP Nominal Switching Frequency 300kHz QUICK START PROCEDURE Demonstration circuit 524 is easy to set up to evaluate sufficient current rating at minimum input voltage for the performance of the LTC3704. Refer to Figure 1 for the required output load. proper measurement equipment setup and follow the 3. Turn on the power at the input. procedure below: NOTE: Make sure that the input voltage does not ex- NOTE: When measuring the input or output voltage rip- ceed 15V. ple, care must be taken to avoid a long ground lead on 4. Check for the proper output voltage: Vout = 5.2V the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly across NOTE: If there is no output, temporarily disconnect the the Vin or Vout and GND terminals. See Figure 2 for load to make sure that the load is not set too high. proper scope probe technique. 5. Once the proper output voltage is established, adjust 1. Place jumper JP1 in the ENABLE position. the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency 2. With power off, connect the input power supply to Vin and other parameters. and GND. Make sure that the input power supply has 1 GND VIN QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 524 POSITIVE INPUT TO NEGATIVE OUTPUT DC/DC CONVERTER + + OPTIONAL EXTERNAL CLOCK SEEEXTERNAL CLOCK CONNECTIO SECTION + LOAD + Figure 1. Proper Measurement Equipment Setup Figure 2. Scope Probe Placement for Measuring Input or Output Ripple CONNECTING AN EXTERNAL CLOCK 3. Remove resistor R6. 1. Turn the input power source Off. 4. Install 0 resistor R7. 2. Remove the Burst Mode jumper. MONITORING THE MOSFET CURRENT 3. Set the clock source pulse amplitude to 5V and set Monitor the MOSFET current through the Rs1 resistor. the frequency between 330 kHz and 400kHz. 1. Connect a 50 coaxial cable across Rs1 with the 4. Turn the clock source off. shield connected to the ground connection of Rs1. 5. Connect the clock source (pulse generator with TTL 2. Use a 50 surface mount resistor to connect the cen- output) to the middle pin of Burst Mode jumper. ter conductor of the coaxial cable to the Rs1 terminal 6. Apply input power and observe the switching fre- which is connected to the source of MOSFET Q1. quency of 300kHz. 3. Set the scope input to 50 and connect the coaxial 7. Turn the clock source on and observe the switching cable to the scope. frequency go up to the switching frequency of the ex- 4. Set the scope to 5mV/division or less. The current ternal clock. scale with 1m Rs1 and 5mV/div is 10A/div (50 re- sistor at the input of coaxial cable forms 2:1 divider USING AN OPTIONAL CURRENT SENSE RESISTOR with 50 input impedance of scope). 1. Refer to LTC3704 data sheet to determine the proper resistor value. 2. Replace current sense resistor Rs1 with the correct value. 2 + + +