QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING LTC3853EUJ DESCRIPTION Demonstration circuit 1254A is a polyphase 3-output step-down supply with tracking featuring the LTC3853EUJ. The entire circuit, excluding the bulk output capacitors, fits within a 1.5” X 1” area on all layers. The package style for the LTC3853EUJ is a 6mm x 6mm plastic QFN. The main features of the board include rail tracking, an internal 5V linear regulator for bias, RUN pins for each output, two PGOOD signals and a Mode selector that allow the converter to run in CCM, pulse skip or Burst Mode operation. Synchronization to an external clock is also possible through some minor component changes. The board is configured for resistor current sensing, but optional DCR sensing is possible through some component changes. The input voltage range is 6.5V to 24V. For applications with narrow, 5V ± 0.5V input range, the board has an optional resistor to tie the INTVCC pin to the VIN pin. The LTC3853 datasheet gives a complete description of the part, operation and application information and must be read in conjunction with this quick start guide for demo circuit 1254A. Design files for this circuit board are available. Call the LTC factory. Burst Mode is a trademark of Linear Technology Corporation Table 1. Performance Summary (TA = 25°C) PARAMETER CONDITION VALUE Input Voltage Range 6.5V - 24V Output Voltage VOUT1 VIN = 6.5V to 24V, IOUT1 = 0A to 5A 2.5V ±2% Output Voltage VOUT2 VIN = 6.5V to 24V, IOUT2 = 0A to 5A 1.8V ±2% Output Voltage VOUT3 VIN = 6.5V to 24V, IOUT3 = 0A to 5A 3.3V ±2% Nominal Switching Frequency Efficiency See Figures 4, 5 and 6 for efficiency curves 500kHz VOUT1 = 2.5V, IOUT1 = 5A; VIN = 12V 89.2% Typical VOUT2 = 1.8V, IOUT2 = 5A; VIN = 12V 86.6% Typical VOUT3 = 3.3V, IOUT3 = 5A; VIN = 12V 91.7% Typical QUICK START PROCEDURE Demonstration circuit 1254A is easy to set up to evaluate the performance of the LTC3853EUJ. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: 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. NOTE: When Place jumpers in the following positions: JP1 On JP2 On JP3 On JP4 Pulse Skip 2. With power off, connect the input power supply to Vin and GND. 3. Turn on the power at the input. 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING NOTE: Make sure that the input voltage does not ex- 5. 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. 6. Different operating modes can be evaluated by changing the position of jumper JP4. ceed 24V. 4. Check for the proper output voltages. Vout1 = 2.450V to 2.550V, Vout2 = 1.765V to 1.836V, Vout3 = 3.234V to 3.366V NOTE: If there is no output, temporarily disconnect the load to make sure that the load is not set too high. Iout1 A + + V Vout1 - Vout1 load Iout2 A + + V Vout2 - Iin Vout2 load - A Iout3 + Vin supply - Vin + A V - + + V - Vout3 Vout3 load - Figure 1. Proper Measurement Equipment Setup 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING GND VIN Figure 2. Measuring Input or Output Ripple Across Terminals or Directly Across Bulk Capacitor RAIL TRACKING Demonstration circuit 1231 is setup for coincident rail tracking where VOUT1 and VOUT2 track VOUT3 and the ramp-rate for VOUT3 is determined by the value of the TK/SS3 capacitor at C46 - See Figure 3. Please note that turning channel 3 off, will also turn off the other two channels, as they track channels 3. This board can be modified on the bench for external rail tracking or for independent turn-on of the rails. For the latter case, the ramp-rate for VOUT1 and VOUT2 will be determined by their respective TRK/SS capacitors. Refer to Table 2 for tracking options and to the data sheet for more details. Table 2. Output Tracking Options TK/SS1 RESISTOR AND CAPACITORS CONFIGURATION R31 R32 TK/SS2 RESISTOR AND CAPACITORS C44 R33 R34 TK/SS3 RESISTOR AND CAPACITORS C45 R35 R36 C46 Soft Start Without Tracking Vout1 (Default) Open Open 10nF Vout2 open Open 10nF Vout3 Vout3 equals External Ramp X Open 10nF 0Ω Open Open Ratiometric Tracking: Vout1 tracking Vout3 63.4kΩ 20.0kΩ Open Vout2 tracking Vout3 63.4kΩ 20.0kΩ Open 24.9kΩ 20.0kΩ Open Coincident Tracking (Default): Vout1 tracking Vout3 Vout2 tracking Vout3 Vout3 tracking ext. ramp 43.2kΩ 20.0kΩ Open Resistor divider Open 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING Vout3 (3.3V) Vout1 (2.5V) Vout2 (1.8V) Figure 3. Default coincidental startup tracking FREQUENCY SYNCHRONIZATION Demonstration circuit 1231’s Mode selector allows the converter to run in CCM, pulse skip or Burst Mode operation by changing position of jumper JP4. For synchronizing to an external clock source, however, some bench modification is needed. Refer to Table 3 and to the data sheet for more details. Table 3. Free Running and Synchronized Operation Options FREQ PIN COMPONENTS MODE SELECTOR CONFIGURATION R48 R63 R47 JP4 Free Running 10kΩ 0Ω 3.16kΩ FCC, Pulse Skip or Burst Mode Synchronized to External Clock open 10kΩ 0.01µF Burst Mode or Open INDUCTOR DCR SENSING AND RESISTOR SENSING The DCR sense circuit uses the resistive voltage drop across the inductor to estimate the current. In contrast to the traditional sense resistor current feedback, the DCR sensing circuit offers lower cost and higher efficiency, but results in less accurate current limit due to the large variation of the inductor DC resistance. For modifying the demo board for DCR sensing, please refer to Table 4. An efficiency improvement of 1% or more is still possible for optional DCR sensing. The typical efficiency versus load current for each of the outputs is given in Figures 4 to 6 respectively for a range of input voltages. 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING Table 4. DCR sensing component selection REMOVE RSENSE NETWORK ADD DCR SENSE NETWORK Vout1 R21, R22 = Open, RSNS1 = Short R51 = 1.69kΩ R52 = 22.6kΩ R53 = 0Ω C21 = 0.1µF Vout2 R23, R24 = Open, RSNS2 = Short R52 = 1.69kΩ R55 = 22.6k Ω R56 = 0Ω C22 = 0.1µF Vout3 R25, R26 = Open, RSNS3 = Short R53 = 2.74kΩ R58 = 7.5kΩ R59 = 0Ω C23 = 0.1µF 95 Efficiency (%) 90 85 80 6.5Vin, 2.5Vout 75 12Vin, 2.5Vout 16Vin, 2.5Vout 24Vin, 2.5Vout 70 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Load Current (A) Figure 4. Typical Efficiency vs. Load Current for Vout1 (2.5V) vs. Vin 95 Efficiency (%) 90 85 80 6.5Vin, 1.8Vout 75 12Vin, 1.8Vout 16Vin, 1.8Vout 24Vin, 1.8Vout 70 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Load Current (A) Figure 5. Typical Efficiency vs. Load Current for Vout2 (1.8V) vs. Vin 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING 95 Efficiency (%) 90 85 80 6.5Vin, 3.3Vout 75 12Vin, 3.3Vout 16Vin, 3.3Vout 24Vin, 3.3Vout 70 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Load Current (A) Figure 6. Typical Efficiency vs. Load Current for Vout3 (3.3V) vs. Vin 6 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING 7 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING 8