QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1021A HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH DIFFERENTIAL OUTPUT SENSING AND TRACKING LTC3823EUH DESCRIPTION Demonstration circuit 1021A is a high efficiency synchronous step-down DC/DC converter with 4.5V to 16V input range. It can provide 2.5V output with 10A maximum current. The demo board features the LTC®3823EUH controller. The LTC3823 has a high accuracy +/-0.67% 0.6V reference voltage and provides many advanced functions including output tracking, differential output sensing, phase-lock loop frequency synchronization, programmable current limit, output over voltage protection and input under voltage lockout, power good monitor, programmable soft start, selectable discontinuous operation mode or forced continuous mode at light load, and adjustable dead time between the top gate and bottom gate signals to optimize the efficiency. Design files for this circuit board are available. Call the LTC factory. LTC is a registered trademark of Linear Technology Corporation Table 1. Performance Summary (TA = 25°C) PARAMETER CONDITION VALUE Input Voltage Range 4.5V to 16V Output Voltage VIN = 4.5-16V, IOUT = 0A to 10A, 2.5V ±2% Maximum load current VIN = 4.5-16V 10A Typical Output Ripple VIN = 12V, IOUT = 10A (20MHz BW) 25mVP–P Typical Switching Frequency VIN = 12V, IOUT = 10A 500kHz OPERATING PRINCIPLES The LTC3823 uses a constant on-time, valley current mode control architecture able to deliver very low duty cycles with fast transient response without requiring a sense resistor. The switching frequency of the converter can be selected by an external resistor Ron and is compensated for variations on input supply voltage. An internal phase-lock loop allows the controller to be synchronized to an external clock. Figure 3 gives the efficiency curves. to the external voltage to be tracked. The tracking can be coincident or ratiometric, as shown in Figure 4. The LTC3823 has true remote differential output sensing. When using remote sensing, connect Voutsense+ pin to the positive terminal of the remote output load, and connect Voutsense- pin to the negative terminal of the remote output load. The LTC3823 has output voltage up and down tracking capability. Using the track/SS pin, the converter output can track another supply’s output voltage. To select tracking function, place JP2 at TRACK, remove the soft start cap Css and connect a resistor divider 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1021A HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH DIFFERENTIAL OUTPUT SENSING AND TRACKING QUICK START PROCEDURE Demonstration circuit 1021A is easy to set up to evaluate the performance of LTC3823. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: NOTE: When measuring the input or output voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly across the Vin or Vout and GND terminals. See Figure 2 for proper scope probe technique. 1. Initial jumper positions: JP1: DCM (discontinuous mode); JP2: SS (soft start) 2. With power off, connect the input power supply to +Vin (4.5V-16V) and GND (input return). 3. Connect the 2.5V load between Vout and GND. (Initial load: 0 A) 4. Connect the DVMs to the input and output. 5. Turn on the input power supply and check for the proper output voltages. Vout should be 2.5V+/-2%. 6. Once the proper output voltage is established, adjust the load within the operating range and observe the output voltage regulation, ripple voltage and other parameters. Figure 1. Proper Measurement Equipment Setup 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1021A HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH DIFFERENTIAL OUTPUT SENSING AND TRACKING Figure 2. Measuring Input or Output Ripple 12V input to 2.5V output 95 90 efficiency 85 80 FC 75 DCM 70 65 60 0 1 2 3 4 5 6 7 8 9 10 load current (A) Figure 3. Efficiency vs load current 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1021A HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH DIFFERENTIAL OUTPUT SENSING AND TRACKING (a) Coincident tracking (b) Ratiometric tracking Figure 4. (a)(b) Up/down output tracking. Upper waveform: external voltage; lower waveform: output voltage of the converter 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1021A HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH DIFFERENTIAL OUTPUT SENSING AND TRACKING 5