design features 2-Phase Synchronous Step-Down DC/DC Controller with Programmable Stage Shedding Mode and Active Voltage Positioning for High Efficiency and Fast Transient Response Jian Li and Charlie Zhao The LTC3856 is a versatile and feature-rich single-output 2-phase synchronous buck controller with on-chip drivers, remote output voltage sensing, inductor DCR temperature compensation, Stage Shedding™ mode and active voltage positioning (AVP). It is suitable for converting inputs of 4.5V–38V to outputs from 0.6V up to 5V. The LTC3856 facilitates the design of high efficiency, high power density solutions for telecom and datacom systems, industrial and medical instruments, DC power distribution systems and computer systems. The controller is available in 32-pin 5mm × 5mm QFN and 38-pin TSSOP packages. MAJOR FEATURES making it possible to produce a large stepdown ratio applications in very little space. The LTC3856’s constant-frequency peak current-mode control architecture allows a phase-lockable frequency of up to 770kHz. For high frequency applications, the LTC3856 can operate at low duty cycles due to its small minimum on-time (90ns), drops during heavy load conditions. Figure 1 shows a typical 4.5V~14V input, 1.5V/50A output application schematic. The LTC3856 includes a high speed differential amplifier for remote output voltage sensing, which can eliminate the regulation error due to PCB voltage The LTC3856’s two channels operate anti-phase, which reduces the input RMS current ripple and thus the input Figure 1. A 1.5V/50A, 2-phase converter featuring the LTC3856 VIN + VIN 1nF 0.1µF 2.2Ω S 5.6k 100pF 100Ω VIN 100Ω Q1 RJK0305DPB S S CLKOUT 100k, 1% 0.1µF VIN PLLIN TG1 RUN SW1 VFB S AVP 30.1k 0.1µF LTC3856 BOOST1 BG1 INTVCC PHASMD TG2 DIFFOUT BOOST2 D1, CMDSH-3 INTVCC 4.7µF D2, CMDSH-3 100k PGOOD EXTVCC 0.1µF SW2 BG2 4.7µF 6.3V 22µF Q7 RJK0305DPB Q4 RJK0330DPB L2 0.22µH 0.001Ω Q8 RJK0330DPB SENSE2+ MODE S 330µF 2.5V ×4 22µF INTVCC DIFFP ILIM 100µF 6.3V + ×4 VIN Q3 RJK0305DPB DIFFN INTVCC GND VOUT 1.5V/ 50A 0.001Ω INTVCC ITEMP ISET L1 0.22µH Q6 RJK0330DPB Q2 RJK0330DPB TK/SS VIN 4.5V TO 14V 22µF 22µF Q5 RJK0305DPB 1nF SENSE1+ FREQ ITH 20k SENSE1– 180µF 16V ×2 1nF PGOOD PGND SGND SENSE2– 100Ω 100Ω S 10Ω 10Ω July 2010 : LT Journal of Analog Innovation | 19 At light load, switching-related power losses dominate the total loss. With Stage Shedding mode, the LTC3856 can shut down one channel at light loads to reduce switching related losses. VOUT 50mV/DIV VOUT 100mV/DIV OVERSHOOT 36mV VOUT 100mV/DIV UNDERSHOOT 35mV IL1 10A/DIV IL2 10A/DIV 50A ILOAD 20A/DIV 25A 100µs/DIV VIN = 12V VOUT = 1.5V ILOAD = 25A TO 50A capacitance. Up to six LTC3856s can be combined for 12-phase operation by using the CLKOUT, PLLIN and PHASMD pins. Due to its peak current mode control architecture, the LTC3856 provides fast cycle-bycycle dynamic current sharing plus tight DC current sharing, as shown in Figure 2. The LTC3856’s maximum current sense voltage is selectable for either 30mV, 50mV or 75mV, allowing the use of either the inductor DCR or a discrete sense 1-PHASE 2-PHASE 90 EFFICIENCY (%) +7% AT 10% LOAD +1.7% AT 20% LOAD 85 80 +13% AT 5% LOAD 75 70 VIN = 12V VOUT = 1.5V 1 VSW1 10V/DIV VSW2 10V/DIV VSW2 10V/DIV VIN = 12V VOUT = 1.5V Figure 2. Load transient performance 95 VSW1 10V/DIV 10 LOAD CURRENT (A) 100 LTC3856 Stage Shedding MODE LTC3856 FORCED CONTINOUS MODE LTC3729 FORCED CONTINOUS MODE Figure 5. Efficiency comparison 20 | July 2010 : LT Journal of Analog Innovation 10µs/DIV Figure 3. Stage Shedding mode: 2-phase to 1-phase transition resistor as the sensing element. Inductor winding resistance (DCR) changes over temperature, so the LTC3856 senses the inductor temperature via the ITEMP pin and maintains a constant current limit over a broad temperature range. It makes high efficient inductor DCR sensing more reliable for high current applications. At heavy load, the LTC3856 operates in constant frequency PWM mode. At light loads, it can operate in any of three modes: Burst Mode® operation, forced continuous mode and Stage Shedding™ mode. Burst Mode operation switches in pulse trains of one to several cycles, with the output capacitors supplying energy during internal sleep periods. This provides the highest possible efficiency at very light load. Forced continuous conduction mode (CCM) offers continuous PWM operation from no load to full load, providing the lowest possible output voltage ripple. Programmable Stage Shedding mode is unique to the LTC3856. In Stage Shedding mode, one channel can be shut down at VIN = 12V VOUT = 1.5V 10µs/DIV Figure 4. Stage Shedding mode: 1-phase to 2-phase transition light load to reduce switching related losses, thus improving efficiency in the load range up to 20% of full load. The programmable active voltage positioning (AVP) is another unique design feature of the LTC3856. AVP modifies the regulated output voltage depending on its current loading. AVP can improve overall transient response and save output capacitors. STAGE SHEDDING MODE At light load, switching-related power losses dominate the total loss. With Stage Shedding mode, the LTC3856 can shut down one channel at light loads to reduce switching related losses. When the MODE pin is tied to INTVCC, the AVP RPRE-AVP LTC3856 DIFFP DIFFN RAVP Figure 6. Programmable AVP VOUT design features The LTC3856’s two channels operate anti-phase, which reduces the input RMS current ripple and thus the input capacitance. Up to six LTC3856s can be combined for 12-phase operation. Due to its peak current mode architecture, the LTC3856 provides fast cycle-by-cycle dynamic current sharing, plus tight DC current sharing. 108mV VOUT 50mV/DIV RDROOP 2.1mΩ VOUT 50mV/DIV 50A IL 20A/DIV 25A VIN = 12V VOUT = 1.5V 50A IL 20A/DIV 25A 100µs/DIV Figure 7. Transient performance without AVP LTC3856 enters Stage Shedding mode. This means that the second channel stops switching when ITH is below a certain programmed threshold. The threshold voltage VSHED on ITH is programmed according to the following formula: VSHED = 0.5 + 54mV 5 (0.5 − VISET ) 3 There is a precision 7.5µA flowing out of the ISET pin. Connecting a resistor to SGND sets the VISET voltage. Current mode control allows the LTC3856 to transition smoothly from 2-phase to 1-phase operation and vice versa, as shown in Figures 3 and 4. A voltage mode, multi phase supply cannot transition between 1- and 2-phase operation as smoothly. The efficiency improvements brought on by Stage Shedding mode are shown in Figure 5. Due to stronger gate driver and shorter dead-time, the LTC3856 can achieve around 4%~5% higher efficiency than the LTC3729, a comparable singleoutput, 2-phase controller, over the whole load range. With Stage Shedding VIN = 12V VOUT = 1.5V 100µs/DIV Figure 8. Transient performance with AVP mode, significant efficiency improvement is further achieved at light load. At 5% load, the efficiency is improved by 13%. ACTIVE VOLTAGE POSITIONING Transient performance is an important parameter in the requirements for the latest power supplies. To minimize the voltage excursions during a load step, the LTC3856 uses AVP to lower peak-topeak output voltage deviations caused by load steps without having to increase the output filter capacitance. Likewise, the output filter capacitance can be reduced in applications while maintaining peak-to-peak transient response. The LTC3856 senses inductor current information by monitoring the voltage across the sense resisters RSENSE or the DCR sensing network of the two channels. The voltage drops are added together and applied as VPRE-AVP between the AVP and DIFFP pins, which are connected through resistor RPRE-AVP. Then, VPRE-AVP is scaled through RAVP and added to the output voltage as the compensation for the load voltage drop. As shown in Figure 6, the load slope (RDROOP) is set to: RDROOP = RSENSE • R AVP V • RPRE - AVP A With proper design, AVP can reduce load transient-induced peak-to-peak voltage spikes by 50%, as shown in Figures 7 and 8. CONCLUSION The LTC3856 delivers an outsized set of features for its small 5mm × 5mm 32-pin QFN package. It can run at high efficiency using temperature compensated DCR sensing with Stage Shedding mode/ Burst Mode operation. AVP can improve the transient response even as output capacitance is reduced. Tracking, strong on-chip drivers, multichip operation and external sync capability fill out its menu of features. The LTC3856 is ideal for high current applications, such as telecom and datacom systems, industrial and computer systems applications. n July 2010 : LT Journal of Analog Innovation | 21