QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING LTC3853EUJ DESCRIPTION Demonstration circuit 1349A is a high current 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 1.5” area on all layers. The package style for the LTC3850EUJ 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 14V, but 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 LT3853 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 1349A. 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 - 14V Output Voltage VOUT1 VIN = 6.5V to 14V, IOUT1 = 0A to 15A 1.8V ±2% Output Voltage VOUT2 VIN = 6.5V to 14V, IOUT2 = 0A to 15A 1.2V ±2% Output Voltage VOUT3 VIN = 6.5V to 14V, IOUT3 = 0A to 15A 2.5V ±2% Nominal Switching Frequency Efficiency See Figures 4, 5 and 6 for efficiency curves 400kHz VOUT1 = 1.8V, IOUT1 = 15A; VIN = 12V 89.2% Typical VOUT2 = 1.2V, IOUT2 = 15A; VIN = 12V 86.6% Typical VOUT3 = 2.5V, IOUT3 = 15A; VIN = 12V 91.7% Typical QUICK START PROCEDURE Demonstration circuit 1349A 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 Forced Continuous 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 1349A HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING NOTE: Make sure that the input voltage does not exceed 14V. Check for the proper output voltages. Vout1 = 1.765V to 1.836V, Vout2 = 1.176V to 1.224V, Vout3 = 2.450V to 2.550V NOTE: If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 4. 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. Iout1 A + V Vout1 - + Vout1 load - Iout2 A + V A + - Vout2 load - - Iin Vin V Vout2 + + Iout3 Vin supply A - + V - Vout3 + Vout3 load - Figure 1. Proper Measurement Equipment Setup 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING Figure 2. Measuring Input or Output Ripple Directly Across Bulk Capacitor RAIL TRACKING Demonstration circuit 1349 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 C44 - See Figure 3. Please note that turning channel 3 off, will also turn off the other two channels, as they are tracking channel 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 C46 R33 R34 TK/SS3 RESISTOR AND CAPACITORS C45 R35 R36 C44 Soft Start Without Tracking Vout1 Open Open 10nF Vout2 open Open 10nF Vout3 (Default) Vout3 equals External Ramp X Open 10nF 0Ω Open Open Ratiometric Tracking: Vout1 tracking Vout3 43.2kΩ 20.0kΩ Open Vout2 tracking Vout3 43.2kΩ 20.0kΩ Open 10.0kΩ 20.0kΩ Open Coincident Tracking (Default): Vout1 tracking Vout3 Vout2 tracking Vout3 Vout3 tracking ext. ramp 24.9kΩ 20.0kΩ Open Resistor divider Open 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING Vout3 (2.5V) Vout1 (1.8V) Vout2 (1.2V) Figure 3. Default coincidental startup tracking FREQUENCY SYNCHRONIZATION Demonstration circuit 1349’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 R65 R47 JP4 Free Running 10.0kΩ 0Ω 2.55kΩ 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. A full load efficiency improvement of 1% - 2% 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. Table 4. DCR sensing component selection REMOVE RSENSE NETWORK DCR NETWORK Vout1 R21, R22 = Open, RSNS1 = Short R51 = 511Ω R52 = OPEN R53 = 0Ω C21 = 0.47µF Vout2 R23, R24 = Open, RSNS2 = Short R52 = 402Ω R55 = OPEN R56 = 0Ω C21 = 0.47µF Vout3 R25, R26 = Open, RSNS3 = Short R53 = 511Ω R58 = OPEN R59 = 0Ω C21 = 0.47µF 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING SINGLE OUTPUT / DUAL PHASE OPERATION The demonstration circuit 1349A has been designed to support a 2+1 configuration (2 phase output plus a nd single phase 2 output) as described in the LT3853 datasheet. Although the design of such a two phase output circuit is beyond the scope of the guide, some comments on how to implement this on this demo board should be mentioned. Make sure the power components for ch1 and ch2 are the same (All components on right side of IC on 2. Remove all components connected to VFB2 (pin12) and ITH2 (pin 13). Remove feedback resistor R37. 3. Install optional components R63, R64, R66 and R68 with 0Ω jumpers and R67 with a 100kΩ Ω pullup. 4. Install the correct feedback (R1, R2 and C1) and compensation (C4, C5 and R7). 94 92 90 88 86 Efficiency (%) 1. schematic for these two channels). Short Vout1 to Vout2 using the pads provided. 84 82 80 78 1.8Vout, 6.5Vin 76 1.8Vout, 14Vin 74 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Load Current (A) Figure 4. Typical Efficiency vs. Load Current for Vout1 (1.8V) vs. Vin 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING 94 92 90 88 Efficiency (%) 86 84 82 80 78 1.2Vout, 6.5Vin 76 1.2Vout, 14Vin 74 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Load Current (A) Figure 5. Typical Efficiency vs. Load Current for Vout2 (1.2V) vs. Vin 94 92 90 88 Efficiency (%) 86 84 82 80 78 2.5Vout, 6.5Vin 76 2.5Vout, 14Vin 74 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Load Current (A) Figure 6. Typical Efficiency vs. Load Current for Vout3 (2.5V) vs. Vin 6 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING 7