QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 774 HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH MARGINING AND TRACKING LTC3770EUH DESCRIPTION Demonstration circuit 774 is a high efficiency synchro- monitor, programmable soft start, selectable discontinu- nous step-down DC/DC converter with 5V to 28V input ous operation mode or forced continuous mode at light range. It can provide 2.5V output with 10A maximum load, and adjustable dead time between the top gate and current. The demo board features the LTC3770EUH con- bottom gate signals to optimize the efficiency. troller. The LTC3770 has a high accuracy +/-0.67% 0.6V reference voltage and provides many advanced functions Design files for this circuit board are available. Call including output tracking, programmable voltage mar- the LTC factory. gining, programmable current limit, output over voltage protection and input under voltage lockout, power good Table 1. Performance Summary (T = 25C) A PARAMETER CONDITION VALUE Input Voltage Range 5V to 28V Output Voltage V = 5-28V, I = 0A to 10A, 2.5V 2% IN OUT Maximum load current V = 5-28V 10A IN Typical Output Ripple V = 12V, I = 10A (20MHz BW) 60mV IN OUT PP Typical Switching Frequency V = 12V, I = 10A 250kHz IN OUT OPERATING PRINCIPLES The LTC3770 uses a constant on-time, valley current external voltage to be tracked. The tracking can be coin- mode control architecture able to deliver very low duty cident or ratiometric, as shown in figure 4. cycles with fast transient response without requiring a Voltage margining is the dynamic adjustment of the out- sense resistor. The switching frequency of the converter put voltage to its worst case operating range during test can be selected by an external resistor Ron and is com- to screen the components, verify some con- pensated for variations on input supply voltage. An in- trol/protection functionality of the board and improve the ternal phase-lock loop allows the controller to be syn- system reliability. The LTC3770 has two logic control chronized to an external clock. Figure 3 gives the effi- pins, MARGIN1 and MARGIN0, to enable and set up ciency curves (JP1: DCM). margin up (higher output voltage) or margin down The LTC3770 has the output voltage up and down track- (lower output voltage). Table 2 is a summary of the con- ing capability. Using the TRACK/SS pin, the converter figurations. output can track another supplys output voltage. To se- When the margining function is enabled, the error ampli- lect tracking function, place JP2 at TRACK, take off the fier reference voltage will be adjusted to: V =0.6V +/- REFIN soft start cap Css and connect a resistor divider to the 1.18VxR5/R6 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 774 HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH MARGINING AND TRACKING By selecting the ratio of R5 to R6, the margin voltage is programmable and the converter output can be set to a desirable testing range. QUICK START PROCEDURE Demonstration circuit 774 is easy to set up to evaluate 2. With power off, connect the input power supply to the performance of LTC3770. Refer to Figure 1 for VIN (5V-28V) and GND (input return). proper measurement equipment setup and follow the 3. Connect the 2.5V load between VOUT and GND. (Ini- procedure below: tial load: 0 A) NOTE: When measuring the input or output voltage rip- 4. Connect the DVMs to the input and output. ple, care must be taken to avoid a long ground lead on 5. Turn on the input power supply and check for the the oscilloscope probe. Measure the input or output proper output voltages. VOUT should be 2.5V+/-2%. voltage ripple by touching the probe tip directly across the VIN or VOUT and GND terminals. See Figure 2 for 6. Once the proper output voltage is established, adjust proper scope probe technique. the load within the operating range and observe the output voltage regulation, ripple voltage and other pa- 1. Initial jumper positions: rameters. JP2: SS JP1: FC Figure 1. Proper Measurement Equipment Setup 2