QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1126 3.3V/5A AND 2.5V/5A REGULATOR LTC3850EUF DESCRIPTION Demonstration circuit 1126 is a high density 3.3V/5A and 2.5V/5A step down converter featuring the LTC3850EUF. The power components, excluding the bulk output capacitors, fit within a 0.90” X 0.75” area on the top layer and the control circuit resides in a 0.70” X 0.65” area on the bottom layer. The package style for the LTC3850EUF is a 4mm X 4mm 28-lead QFN with an exposed ground pad. tors to save cost and board space and to improve efficiency. Other stuffing options allow the converter to be configured for either CCM (original setting), pulse skip or Burst Mode operation, or to be synchronized to an external clock. The main features of the board include rail tracking, an internal 5V linear regulator for bias, RUN pins for each output and a PGOOD signal. An optional DCR sense circuit allows the converter to use the inductor’s DCR as the sense element instead of the on-board sense resis- Design files for this circuit board are available. Call the LTC factory. The input voltage range is 6.5V to 14V. For applications that have a 5V +/- 0.5V input, the board has an optional resistor to tie the INTVCC pin to the VIN pin. Table 1. Performance Summary (TA = 25°C) PARAMETER CONDITION VALUE Minimum Input Voltage 6.5V Maximum Input Voltage 14V Output Voltage VOUT1 IOUT = 0A to 5A 3.3V ±2% Output Voltage VOUT2 IOUT = 0A to 5A 2.5V ±2% Nominal Switching Frequency 500kHz Full Load Efficiency VOUT1 = 3.3V, IOUT1 = 5A, VIN = 12V 91% Typical (see Figure 3 for efficiency curves) VOUT2 = 2.5V, IOUT2 = 5A, VIN = 12V 89% Typical QUICK START PROCEDURE Demonstration circuit 1126 is easy to set up to evaluate the performance of the LTC3850EUF. 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. Place jumpers in the following positions: JP1 ON JP2 ON 2. With power off, connect the input power supply to VIN and GND. 3. Turn on the power at the input. NOTE: Make sure that the input voltage does not exceed 15V. 4. Check for the proper output voltages. VOUT1 = 3.234V to 3.366V, VOUT2 = 2.450V to 2.550V 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1126 3.3V/5A AND 2.5V/5A REGULATOR NOTE: If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 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. RAIL TRACKING Demonstration circuit 1126 is setup for coincident rail tracking where VOUT2 tracks VOUT1 and the ramp-rate for VOUT1 is determined by the value of the TRK/SS1 capacitor at C11. See Figure 4. to Tables 2 and 3 for tracking options and to the data sheet for more details. 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 Table 2. VOUT1 Tracking Options TRACK1 DIVIDER TRK/SS1 CAP CONFIGURATION R13 R15 C11 Soft Start Without Tracking (original board) 0Ω Not Stuffed 10nF External Coincident Tracking 63.4kΩ 20.0kΩ Not Stuffed Table 3. VOUT2 Tracking Options TRACK2 DIVIDER TRK/SS2 CAP CONFIGURATION R3 R5 C3 Soft Start Without Tracking Not stuffed Not stuffed 10nF Coincident Tracking to VOUT1 (original board) 43.2kΩ 20.0kΩ Not Stuffed 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1126 3.3V/5A AND 2.5V/5A REGULATOR INDUCTOR DCR SENSING Demonstration circuit 1126 provides an optional circuit for DCR sensing. DCR sensing uses the DCR of the inductor to sense the inductor current instead of discrete sense resistors. The advantages of DCR sensing are lower cost, reduced board space and higher efficiency, but the disadvantage is a less accurate current limit. If DCR sensing is used, be sure to select an inductor current with a sufficiently high saturation current or use an iron powder type. Tables 4 and 5 show an example of how to modify the DC1126 for DCR sensing using the parameters below: VOUT1 = 3.3V / 5A VOUT2 = 2.5V / 5A VIN = 6.5V to 14V Fsw = 500kHz, typical L1,2 = Vishay IHLP2525CZ-11 2.2uH (DCR = 15.7mΩ typ, 16.5mΩ max) ILIM = INTVCC (R25 = 0Ω, R26 = OPEN) Table 4. VOUT1 Configured for 3.3V/5A Using DCR Sensing and Discrete Sense Resistors RSENSE FILTER RESISTORS SENSE FIILTER CAP DCR FILTER/DIVIDER RESISTORS TOP BOTTOM SENSE1- TO L1JUMPER CONFIGURATION RSNS1 L1 R7,R9 C4 R27 R29 R39 DCR Sensing Short with Cu strip or very short & thick piece of wire Vishay IHLP2525CZ-11 2.2uH Open 0.1uF 2.7kΩ 2.7kΩ 0Ω TDK RLF7030T – 2.2uH 10Ω 1nF Open Open Open Discrete RSENSE 8mΩ (original board) Table 5. VOUT2 Configured for 2.5V/5A Using DCR Sensing and Discrete Sense Resistors RSENSE FILTER RESISTORS SENSE FIILTER CAP DCR FILTER/DIVIDER RESISTORS TOP BOTTOM SENSE2- TO L2JUMPER CONFIGURATION RSNS2 L2 R20,R21 C16 R34 R32 R40 DCR Sensing Short with Cu strip or very short & thick piece of wire Vishay IHLP2525CZ-11 2.2uH Open 0.1uF 2.7kΩ 2.7kΩ 0Ω TDK RLF7030T – 2.2uH 10Ω 1nF Open Open Open Discrete RSENSE 8mΩ (original board) 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1126 3.3V/5A AND 2.5V/5A REGULATOR Iin Iout1 A + Vin V - Vin supply A + V Vout1 + - - V + Vout1 load - Vout2 + A Vout2 load + Iout2 Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Output or Input Voltage Ripple 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1126 3.3V/5A AND 2.5V/5A REGULATOR Vin = 12V, Fsw = 530kHz, CCM 100% 3.3Vout Efficiency 90% 2.5Vout 80% Each phase: Q = Si4816BDY L = TDK RLF7030 2.2uH Rsense = 8mOhms 70% 60% 0 1 2 3 4 5 6 Load current (Amps) Figure 3. Typical Efficiency vs Load Current 2msec / div 3.3V 2.5V ` VOUT1,2 1V / DIV Figure 4. Coincident Rail Tracking During Startup (original board). 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1126 3.3V/5A AND 2.5V/5A REGULATOR 6