DEMO CIRCUIT 1441A QUICK START GUIDE LTC3855EUJ DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER DESCRIPTION Demonstration circuit DC1441A is a dual output synchronous buck converter featuring the LTC3855EUJ. The board provides two outputs of 1.8V/17A and 1.2V/17A from an input voltage of 4.5V to 14V at a switching frequency of 400kHz. 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 1.4” X 0.75” area on the top layer. The control circuit resides in a 0.8” X 0.9” area on the bottom layer. The package style for the LTC3855EUJ is a 40-lead 6mm X 6mm QFN. The main features of the board are listed below: • Differential amplifier for remote sensing VOUT2 which is configured for 1.2V. • Diff amp bypass for VOUT2 ≥ 3.3V. • Optional resistors for single output dual phase operation. • Optional resistors for DCR sensing and for NTC compensated DCR sensing. • PLLIN pin for synchronization to an external clock which can be used in conjunction with PHASMD pin and CLKOUT pin for up to 12-phase operation. • Selectable light load operating modes of pulse skip, Burst Mode® or FCM. • TRACK/SS pins for external rail tracking. • RUN pins and PGOOD pins for each phase. 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 4.5V Maximum Input Voltage 14V Output Voltage VOUT1 IOUT1 = 0A TO 17A, VIN = 4.5V to 14V 1.8V ± 1.75% Output Voltage VOUT2 IOUT2 = 0A TO 17A, VIN = 4.5V to 14V 1.2V ± 1.50% Nominal Switching Frequency 400kHz Efficiency VOUT1 = 1.8V, IOUT1 = 17A, VIN = 12V 88.3% typical See Figures 3-5 VOUT2 = 1.2V, IOUT2 = 17A, VIN = 12V 85.0% typical 1 LTC3855EUJ QUICK START PROCEDURE Demonstration circuit 1441 is easy to set up to evaluate the performance of the LTC3855EUJ. Refer to Figure 1 for the proper measurement equipment setup and follow the procedure below: 1. Place jumpers in the following positions: JP1 RUN1 ON JP2 RUN2 ON JP3 MODE FCM 2. With power off, connect the input power supply to VIN and GND. Turn on the power at the input and increase the input voltage to 4.5V or higher. 3. Check for the proper output voltages. Vout1 = 1.769V to 1.832V Vout2 = 1.182V to 1.218V 4. 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. Do not apply load between the VO1+ and VO1- pins or between the VO2_SNS+ and VO2_SNSpins. These pins are only intended to Kelvin sense the output voltage across COUT1 and COUT4. Heavy load currents applied across the VO1+/- sense pins will damage these sense traces. Heavy load currents across the VO2_SNS+/- pins will damage the converter. Note 1. When measuring the output or input voltage ripple, do not use the 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. Note 2. 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: • Tie VOUT1 to VOUT2 by tying together the exposed copper pads on the VOUT shapes with pieces of heavy copper foil. • Tie ITH1 to ITH2 by stuffing 0Ω at R49. • Tie VFB1 to VFB2 by stuffing 0Ω at R50. • Tie TRK/SS1 to TRK/SS2 by stuffing 0Ω at R52. • Tie RUN1 to RUN2 by stuffing 0Ω at R55. • Keep the ILIM pins at the same potential or tie them together by stuffing 0Ω •at R71. • • • Keep the ITEMP pins at the same potential or tie them together by stuffing 0Ω •at R67. Remove the redundant ITH compensation network, VFB divider and TRACK/SS cap. Re-compensate if necessary. 2 LTC3855EUJ INDUCTOR DCR SENSING Demonstration circuit 1441 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 since the controller can not detect saturation when DCR sensing is used. This means using a ferrite with a high saturation current rating or using an iron powder type whose inductance will drop off much more gradually. Refer to tables 2 and 3 to see an example of how to setup the two rails for DCR sensing. The original RSENSE setup is shown for comparison. These parameters are used: • • • • • • • • VOUT1 = 1.8V / 17A VOUT2 = 1.2V / 17A VIN = 12V Fsw = 400kHz, typical L1,2 = Vishay IHLP4040DZ-01 0.56uH (0.56uH, DCR = 1.7mΩ typ, 1.8mΩ max) ILIM = FLOATING No temperature compensation The DC1441A also has footprints for an NTC compensation network on the ITEMP pins for DCR sensing which increases the current sense threshold as the inductor temperature increases. As a result, the current limit falls less with temperature. RN1 detects the temperature for L1, RN2 detects the temperature for L2. RN3 is used for single output dual phase applications and is placed next to both inductors. See the data sheet for more details on NTC compensated DCR sensing. VOUT2 >= 3.3V The 1.2V output on phase 2 uses the internal differential amplifier to sense the output voltage. However, its common mode range only goes up to 3.5V. Therefore, for margin, the nominal output voltage should not exceed 3.3V. For outputs of 3.3V and higher, the diff amp should be bypassed. To bypass the diff amp, follow these steps: • Remove R11 to disconnect the output of the diff amp from the feedback divider. • Stuff R12 with 0 Ω to tie the feedback divider to VOUT2 (through R65). • Remove R70. • Stuff R69 with 0 Ω. 3 LTC3855EUJ Table 2. CONFIGURATION DCR Sensing Discrete RSENSE (original board) Table 3. CONFIGURATION DCR Sensing Discrete RSENSE (original board) VOUT1 Setup for 1.8V/17A with DCR Sensing and with Discrete Sense Resistors RSENSE SENSE DCR FILTER/DIVIDER FILTER FIILTER RESISTORS RESISTORS CAP TOP BOTTOM RS1 Short with Cu strip or very short & thick piece of wire 2mΩ 2010 pkg L1 Vishay SENSE1- TO L1JUMPER R29, R30 Open C14 0.1uF R56 3.09kΩ R57 100kΩ R58 0Ω 100Ω 1nF Open Open Open IHLP4040DZ-01 0.56uH Iron powder Vitec 59PR9875 0.4uH ferrite, Isat = 23A VOUT2 Setup for 1.2V/17A with DCR Sensing and with Discrete Sense Resistors RS2 Short with Cu strip or very short & thick piece of wire 2mΩ 2010 pkg L2 Vishay RSENSE FILTER RESISTORS SENSE FIILTER CAP DCR FILTER/DIVIDER RESISTORS TOP BOTTOM SENSE2- TO L2JUMPER R40,R39 Open C15 0.1uF R59 3.09kΩ R60 Not stuffed R62 0Ω 100Ω 1nF Open Open Open IHLP4040DZ-01 0.56uH Iron powder Vitec 59PR9875 0.4uH ferrite, Isat = 23A 4 LTC3855EUJ Vout1 V + - Iin A Iout1 + Vin supply Vout1 load + A A - Iout2 + Vout2 load - - V + + Vout2 V - Vin Figure 1. Proper Measurement Equipment Setup + COUT VOUT - GND Figure 2. Measuring Output Voltage Ripple 5 LTC3855EUJ 1.8V / 17A and 1.2V / 17A Efficiency at VIN = 12V, FSW = 400kHz, mode = FCM 95 Efficiency (%) 90 1.8V 1.2V 85 For each phase ● QT = RJK0305DPB ● QB = RJK0330DPB ● L = Vitec 59PR9875 (0.4uH) ● Rsense = 2mOhms 80 75 0 2 4 6 8 10 12 14 16 18 Load current (Amps) Figure 3. Typical Efficiency Curves in FCM VOUT = 1.8V, VIN = 12V, FSW = 400kHz 100 90 80 Efficiency (%) 70 60 FCM Burst Mode Pulse Skip 50 40 Parameters ● QT = RJK0305DPB ● QB = RJK0330DPB ● L = Vitec 59PR9875 (0.4uH) ● Rsense = 2mOhms 30 20 10 0 0.01 0.10 1.00 10.00 100.00 Load current (Amps) Figure 4. Typical Efficiency Curves for the 1.8V rail in FCM, Burst Mode and Pulse Skip Mode. 6 LTC3855EUJ VOUT = 1.2V, VIN = 12V, FSW = 400kHz 100 90 80 Efficiency (%) 70 60 FCM Burst Mode Pulse Skip 50 40 Parameters ● QT = RJK0305DPB ● QB = RJK0330DPB ● L = Vitec 59PR9875 (0.4uH) ● Rsense = 2mOhms 30 20 10 0 0.01 0.10 1.00 10.00 100.00 Load current (Amps) Figure 5. Typical Efficiency Curves for the 1.2V rail in FCM, Burst Mode and Pulse Skip Mode. 7 R37 PGOOD1 X2 1 PGOOD2 X1 on PCB Tooling Holes 1 JP3 0.1uF C47 OPT 1uF C50 4 1 2 3 R46 100K 1% 3 R40 100 20.0K R43 S2- OPT TRK/SS1 R52 TRK/SS2 ITEMP1 SGND DIFFP SENSE2- SENSE2+ ILIM2 RUN2 OPT R42 OPT R69 R67 OPT ITEMP2 OPT R71 OPT OPT VFB1 R50 ILIM1 RUN1 R55 OPT VFB2 ITH2 0 R70 1nF TK/SS2 ITH2 VFB2 SGND VFB1 ITH1 39 ILIM1 OPT RN1 ITEMP1 R44 OPT R30 100 C14 TK/SS1 R11 0 41 10 9 8 7 6 5 RUN2 INTVCC R39 100 1nF C15 TRK/SS2 ITH2 VFB2 3 VFB1 4 2 1 ITH1 TRK/SS1 S1+ S1- R29 100 ITEMP1 U1 LTC3855EUJ O PT IONAL JUM PERS FOR SINGLE OUT PUT /DUAL PHASE OPERAT ION OPT R12 OPT C49 20.0K R33 20.0K S2+ 40.2K OPT R32 R27 OPT OPT R10 0 R28 C38 C37 JP1 ITH1 R49 R66 100K 1% INTVCC OPT OPT OPT R38 R41 VIN 1.5nF C48 INTVCC JP2 R35 5.49K 1% 150pF C42 C44 C41 1nF C43 150pF R31 18.2K 1% 0.1uF C2 R34 EXTVCC OPT 1 E16 E4 OPT R2 EXTVCC Repres ents PGOOD2 PGOOD1 E2 E3 0 R3 20.0K R36 0 VOUT1 E9 E10 GND EXTVCC TRK/SS2 TRK/SS1 PLLIN E1 R68 2 INTVCC 3 RUN1 R7 100K 1% INTVCC 1 R63 OPT 21 22 23 24 25 26 27 28 CMDSH-3 D2 1uF C17 2.2 R18 OPT R45 OPT R47 OPT C51 OPT RN3 INTVCC 4.7uF 10V C11 4 4 Q3 RJK0305DPB Q2 RJK0330DPB 4 4 Q4 RJK0330DPB INTVCC VIN INTVCC 4 Q1 RJK0305DPB CMDSH-3 D1 0.1uF C20 E15 4 VIN 4 4 Q6 OPT VIN D4 OPT D3 Q8 OPT OPT Q7 OPT OPT RN2 ITEMP2 OPT R51 OPT R53 OPT C52 OPT R62 OPT L2- OPT R60 L1- OPT R57 COUT1 100uF 6.3V + COUT2 330uF 2.5V + COUT3 330uF 2.5V VOUT1 CIN1 270uF 16V C32 10uF 10V + J1 E5 VIN VIN+ VIN- GND J3 VOUT1 VO1+ E11 E6 J2 J4 GND VO1- E12 2010 0.002 RS2 R64 10 COUT4 100uF 6.3V R65 10 COUT5 COUT6 + 330uF + 330uF 2.5V 2.5V VOUT2 E13 E14 C36 10uF 10V J5 VOUT2 NOTE 4, ONLY APPLY LOAD FROM J3 TO J4 FOR VOUT1 ONLY APPLY LOAD FROM J5 TO J6 FOR VOUT2 DO NOT CONNECT SCOPE GROUND TO E14 2010 0.002 RS1 VIN 1 NOTE 3, FOR VOUT2 >= 3.3V, REMOVE R11 AND R70. STUFF R12 AND R69 WITH 0 OHM RESISTORS. 0.4uH L2 R59 OPT CIN5 10uF R58 OPT CIN3 0.4uH L1 R56 OPT OPT 10uF Q5 OPT CIN4 CIN2 NOTE 2, TO IMPLEMENT DCR SENSING ON PHASE 2, SHORT RS2. REMOVE R39 AND R40. STUFF R62 WITH A 0 OHM RESISTOR. THE DCR SENSE FILTER CONSISTS OF C15, R59 AND R60. CALCULATE THESE VALUES PER THE DATA SHEET. OPT INTVCC 0.1uF C21 R25 0 0 BG2 EXTVCC BG1 TG1 R9 29 SW1 CLKOUT 30 R61 SW2 TG2 BOOST2 PGND2 BG2 EXTVCC INTVCC VIN BG1 PGND1 BOOST1 TG1 JP4 4 CLKOUT TEMPERATURE COMPENSATION NETWORK FOR DCR SENSING ILIM2 2 OPT 3 1 2 35 NOTE 1, TO IMPLEMENT DCR SENSING ON PHASE 1, SHORT RS1. REMOVE R29 AND R30. STUFF R58 WITH A 0 OHM RESISTOR. THE DCR SENSE FILTER CONSISTS OF C14, R56 AND R57. CALCULATE THESE VALUES PER THE DATA SHEET. 1 2 3 ITEMP2 40 S E N S E 1D IF F N 11 S E N S E 1+ 38 13 D IF F O U T 12 R UN 1 37 2 1 S 1+ R26 2 1 N FF O O 2 GND VO2_SNSVO2_SNS+ J6 A 71 / V 2. 1 1 R UN2 34 FR E Q 36 IT E M P 1 ILIM 1 14 IT E M P 2 ILIM 2 15 M O D E /P LLIN P G O O D1 16 P G O O D2 17 33 P HS A S MD NC 18 31 SW1 32 C LK O U T SW2 19 TG 2 20 5 6 7 8 5 6 7 8 1 2 3 5 6 7 8 1 2 3 5 6 7 8 1 2 3 5 6 7 8 1 2 3 5 6 7 8 1 2 3 1 2 1 1 2 3 5 6 7 8 5 6 7 8 1 2 3 2 N U R 2 M E C D F O M M B S P S 1- F F O A 71 / V 8. 1 S 2+ N O G E D G E D G E D V 4 1V 5. 4 S 2- 1 N U R 06 D M S 09 A H P 02 1 2 VIN LTC3855EUJ 8