QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185 DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER LTC3850EUF DESCRIPTION Demonstration circuit 1185 is a dual phase/dual output synchronous buck converter featuring the LTC3850EUF. The demo board comes in two versions. The output voltages for version -A are 2.0V/10A and 1.8V/10A and the output voltages for version -B are 1.5V/15A and 1.2V/15A. The input voltage range is 6.5V to 14V for both versions. For applications that have a 5V +/- 0.5V input, the board has an optional resistor to tie the INTVCC pin to the VIN pin. The demo board uses a high density, two sided drop-in layout. The power components, excluding the bulk output and input capacitors, fit within a 1.35” X 0.75” area on the top layer. The control circuit resides in a 0.60” X 0.75” area on the bottom layer. The package style for the LTC3850EUF is a 4mm X 4mm 28-lead QFN with an exposed ground pad. The main features of the board include an internal 5V linear regulator for bias, RUN pins for each output, an EXTVCC pin and a PGOOD signal. The board can be configured for either CCM (original setting), Burst Mode, or pulse skip operation with the MODE jumper. The board also has optional resistors for single output / dual phase operation, rail tracking, DCR sensing and synchronization to an external clock. Design files for this circuit board are available. Call the LTC factory. Table 1. Performance Summary (TA = 25°C) PARAMETER CONDITION VALUE Minimum Input Voltage 6.5V Maximum Input Voltage 14V Version -A Output Voltage VOUT1 IOUT1 = 0A to 10A 2.0V ±2% Output Voltage VOUT2 IOUT2 = 0A to 10A 1.8V ±2% Nominal Switching Frequency 500kHz Full Load Efficiency VOUT1 = 2.0V, IOUT1 = 10A , VIN = 12V 88.0% (see Figure 3 for efficiency curves) VOUT2 = 1.8V, IOUT2 = 10A , VIN = 12V 87.0% Output Voltage VOUT1 IOUT1 = 0A to 15A 1.5V ±2% Output Voltage VOUT2 IOUT2 = 0A to 15A 1.2V ±2% Version -B Nominal Switching Frequency 400kHz Full Load Efficiency VOUT1 = 1.5V, IOUT1 = 15A , VIN = 12V 87.4% (see Figure 4 for efficiency curves) VOUT2 = 1.2V, IOUT2 = 15A , VIN = 12V 85.3% 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185 DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER QUICK START PROCEDURE Demonstration circuit 1185 is easy to set up to evaluate the performance of the LTC3850EUF. Refer to Figure 1 for the proper measurement equipment setup and follow the procedure below: NOTE: When measuring the output or input voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. See Figure 2 for the proper scope probe technique. Short, stiff leads need to be soldered to the (+) and (-) terminals of an output capacitor. The probe’s ground ring needs to touch the (-) lead and the probe tip needs to touch the (+) lead. Place jumpers in the following positions: JP1 RUN1 ON JP2 RUN2 ON JP3 MODE CCM Check for the proper output voltages. Version –A: Vout1 = 1.960V to 2.040V Vout2 = 1.764V to 1.836V Version –B: Vout1 = 1.470V to 1.530V Vout2 = 1.176V to 1.224V 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. NOTE: Do With power off, connect the input power supply to VIN and GND. not apply load across the VOSn+ and VOSnturrets. These turrets are only intended to Kelvin sense the output voltage across COUT1 and COUT4. Heavy load currents may damage the output voltage sense traces. Turn on the power at the input. NOTE: Make sure that the input voltage does not exceed 15V. SINGLE OUTPUT / DUAL PHASE OPERATION A single output / dual phase converter may be preferred for high output current applications. The benefits of single output / dual phase operation is lower ripple current through the input and output capacitors, improved load step response and simplified thermal design. To implement single output / dual phase operation, make the following modifications: 3. Tie VFB1 to VFB2 by stuffing 0Ω at R50. 4. Tie TRK/SS1 to TRK/SS2 by stuffing 0Ω at R52. 5. Tie RUN1 to RUN2 by stuffing 0Ω at R55. 6. Remove the redundant ITH compensation network and VFB divider. 1. Tie VOUT1 to VOUT2 by tying together the exposed copper pads near J3 and J5 at the edge of the board. Use a piece of heavy copper foil. 2. Tie ITH1 to ITH2 by stuffing 0Ω at R49. 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185 DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER RAIL TRACKING Demonstration circuit 1185 is setup for independent turn-on of VOUT1 and VOUT2. The ramp-rate for VOUT1 is determined by the TRK/SS1 cap at C2 and the ramprate for VOUT2 is determined by the TRK/SS2 cap at C47. The turn-on of one rail will not affect the other for the original demo board. This board can be modified on the bench to allow VOUT1 to track an external signal. It can also be modified to allow VOUT2 to track VOUT1 or to allow VOUT2 to track an external signal. Tables 2 and 3 cover the rail tracking options for each rail, with the –B version used as an example. Table 1. VOUT1 Tracking Options for a 1.5V Output. TRACK1 DIVIDER TRK/SS1 CAP CONFIGURATION R3 R2 C2 Soft Start Without Tracking (original board) 0Ω Not stuffed 0.1uF External Coincident Tracking 17.8kΩ 20.0kΩ Not Stuffed Table 2. VOUT2 Tracking Options for a 1.2V Output. TRACK2 DIVIDER TRK/SS2 CAP CONFIGURATION R36 R34 R37 C47 Soft Start Without Tracking (original board) 0Ω Not stuffed Not stuffed 0.1uF Coincident Tracking to VOUT1 (1.5V) 0Ω 10.0kΩ 20.0kΩ Not Stuffed External Coincident Tracking 10.0kΩ Not stuffed 20.0kΩ Not Stuffed INDUCTOR DCR SENSING Demonstration circuit 1185 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 3 and 4 show an example of how to modify the DC1185 for DCR sensing using these parameters: VOUT1 = 2.0V / 10A VOUT2 = 1.8V / 10A VIN = 6.5V to 14V Fsw = 500kHz, typical L1,2 = Toko FDU0650-R56M=P3 (0.56uH, DCR = 2.45mΩ typ, 3.2mΩ max) ILIM = FLOATING (R42,R44 = OPEN) 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185 DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER Table 3. VOUT1 Configured as a 2.0V/10A Converter Using DCR Sensing and Discrete Sense Resistors CONFIGURATION RS1 DCR Sensing Short with Cu strip or very short & thick piece of wire L1 Toko SENSE FIILTER CAP DCR FILTER/DIVIDER RESISTORS SENSE1- TO L1JUMPER TOP BOTTOM R29,R30 C14 R45 R47 R61 Open 0.1uF 2.37kΩ 6.49kΩ 0Ω 100Ω 1nF Open Open Open FDU0650-R56M=P3 Discrete RSENSE 3mΩ Toko (original board) FDU0650-R56M=P3 2010 pkg RSENSE FILTER RESISTORS Table 4. VOUT2 Configured as a 1.8V/10A Converter Using DCR Sensing and Discrete Sense Resistors CONFIGURATION RS2 DCR Sensing Short with Cu strip or very short & thick piece of wire L2 Toko SENSE FIILTER CAP DCR FILTER/DIVIDER RESISTORS SENSE1- TO L1JUMPER TOP BOTTOM R39,R40 C15 R51 R53 R62 Open 0.1uF 2.37kΩ 6.49kΩ 0Ω 100Ω 1nF Open Open Open FDU0650-R56M=P3 Discrete RSENSE 3mΩ Toko (original board) FDU0650-R56M=P3 2010 pkg RSENSE FILTER RESISTORS 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185 DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER SYNCHRONIZATION TO AN EXTERNAL CLOCK The LTC3850 uses a phase lock loop which forces its internal clock to be synchronized to an external clock. Once synchronized, the rising edge of the top FET gate is aligned to the rising edge of the external clock. The external clock signal needs to be applied to the LTC3850’s MODE pin which is tied to the turret labeled SYNC. The internal phase lock loop is stabilized by a network on the FREQ pin of the LTC3850. To setup the DC1185 for synchronization to an external clock, follow the steps below. 2. Stuff 10kΩ at R8. 3. Stuff 10nF at R10. 4. Leave 1nF at C12. 5. Float the MODE pin by placing the MODE jumper in the BM position. 6. Apply the external clock from the turret labeled SYNC to GND. 1. Remove R7. 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185 DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER Vout1 V + - Iin A Iout1 + Vin supply A A - Iout2 Vout1 load + + Vout2 load - + - V Vin Vout2 V - + Figure 1. Proper Measurement Equipment Setup + COUT VOUT - GND Figure 2. Measuring Output Voltage Ripple 6 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185 DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER 2.0V / 10A and 1.8V / 10A efficiency at Vin = 12V and Fsw = 500kHz Efficiency 95% 90% 2.0V 1.8V 85% Qtop = Si4420BDY Qbottom = Si4630DY No schottky L = Toko FDU0650 0.56uH Rsense = 3m 80% 75% 70% 0 2 4 6 8 10 12 Load current (Amps) Figure 3. Efficiency Curves for the DC1185A-A. 1.5V / 15A and 1.2V / 15A efficiency at Vin = 12V and Fsw = 400kHz 95% Efficiency 90% 1.5V 1.2V 85% Qtop = RJK0305DPB Qbottom = RJK0301DPB No schottky L = Vitec 59PR9875 (0.4uH) Rsense = 2m 80% 75% 70% 0 3 6 9 12 15 18 Load current (Amps) Figure 4. Efficiency Curves for the DC1185A-B. 7 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185 DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER 8