2-Phase Synchronous Buck Controller Features Light Load Stage Shedding Mode, Active Voltage Positioning, Low RSENSE and Remote VOUT Sensing Design Note 486 Charlie Zhao and Jian Li Introduction Today’s computer, datacom, and telecom systems demand power supplies that are efficient, respond quickly to load transients and accurately regulate the voltage at the load. For example, load current can be measured by using the inductor DCR, thus eliminating the need for a dedicated sense resistor. Inductor DCR sensing increases efficiency—especially at heavy load—while reducing component cost and required board space. The LTC ®3856 single-output 2-phase synchronous buck controller improves the accuracy of inductor DCR sensing by compensating for changes in DCR due to temperature. age sensing, Stage Shedding™ mode for improved light load efficiency and adaptive voltage positioning for fast transient response. The LTC3856 can convert a wide input voltage range, 4.5V to 38V, to outputs from 0.6V to 5V. Despite the many features, the chip is small, available in 32-pin 5mm × 5mm QFN and 38-pin TSSOP packages. High Efficiency, 2-Phase, 4.5V to 14V Input, 1.5V/50A Output Converter Figure 1 shows a typical LTC3856 application in a 4.5V to 14V input, 1.5V/50A output converter. The LTC3856’s two channels operate out-of-phase, which reduces the input RMS current ripple and thus the required input capacitance. Up to six LTC3856s can be paralleled for DCR temperature compensation is just one of many performance enhancing features offered in the LTC3856. It also includes on-chip gate drivers, remote output volt- L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks and Stage Shedding is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN 1nF S 0.1μF 2.2Ω 5.6k 100pF 100Ω Q1 RJK0305DPB S S CLKOUT 100k, 1% 0.1μF VIN PLLIN SENSE1+ FREQ TG1 RUN SW1 VFB S AVP 30.1k 0.1μF LTC3856 BOOST1 BG1 INTVCC D1, CMDSH-3 INTVCC 4.7μF ITEMP PHASMD TG2 DIFFOUT BOOST2 D2, CMDSH-3 PGOOD 0.1μF SW2 EXTVCC BG2 Q7 RJK0305DPB L2 0.22μH Q4 RJK0330DPB 0.001Ω Q8 RJK0330DPB SENSE2+ MODE S 22μF 22μF INTVCC ISET ILIM 100μF 6.3V + s4 VIN Q3 RJK0305DPB 1nF PGOOD PGND SGND SENSE2– 100Ω 100Ω S 10Ω 10Ω Figure 1. 1.5V/50A, 2-Phase Synchronous Buck Converter Featuring the LTC3856 01/11/486 180μF 16V s2 VIN 4.5V TO 14V GND VOUT 1.5V/ 50A 0.001Ω INTVCC DIFFP INTVCC L1 0.22μH Q6 RJK0330DPB Q2 RJK0330DPB DIFFN 100k 22μF 22μF Q5 RJK0305DPB 1nF TK/SS ITH 20k SENSE1– + VIN 100Ω 330μF 2.5V s4 4.7μF 6.3V up to 12-phase operation. The LTC3856 has a phaselocked loop (PLL) and can be synchronized to an input frequency between 250kHz and 770kHz. Due to its peak current-mode control architecture, the LTC3856 provides fast cycle-by-cycle dynamic current sharing plus tight DC current sharing, as shown in Figure 2. Stage Shedding Mode At light loads, the LTC3856 can be programmed to operate in one of three modes: Burst Mode® operation, forced continuous mode or Stage Shedding mode. With Stage Shedding mode, the LTC3856 can shut down one channel to reduce switching related loss which is the dominant loss at light loads. Stage Shedding mode is selected by simply tying the MODE pin to INTVCC. The efficiency improvements achieved by Stage Shedding mode are shown in Figure 3. Due to strong gate drivers and shorter dead-time, the LTC3856 can achieve 4% ~ 5% higher efficiency than the LTC3729, a comparable single-output, 2-phase controller, over the whole load range. With Stage Shedding mode, significant efficiency improvement is further achieved at light load. At 5% load, the efficiency is improved by 13%. VOUT 50mV/DIV IL1 10A/DIV IL2 10A/DIV ILOAD 20A/DIV dn486 F02 100μs/DIV Figure 2. Load Transient and Current Sharing: VIN = 12V, 25A to 50A Load Step 95 1-PHASE 2-PHASE 90 EFFICIENCY (%) 1.7% AT 20% LOAD 85 7% AT 10% LOAD 80 LTC3856, CCM LTC3856, PHASE SHEDDING LTC3729, CCM 75 13% AT 5% LOAD 70 10 0 20 30 LOAD CURRENT (A) 40 50 dn486 F03 Figure 3. Efficiency Comparison: VIN = 12V, VO = 1.5V, FSW = 400kHz, L = 220nH, RSENSE = 1mΩ, QT = RJK0305DPB, QB = 2xRJK0330DPB Data Sheet Download www.linear.com Current mode control allows the LTC3856 to transition smoothly from 2-phase to 1-phase operation and vice versa. Active Voltage Positioning User-selectable active voltage positioning (AVP) is another unique design feature of the LTC3856. AVP improves overall transient response and reduces required output capacitance by modifying the regulated output voltage depending on its current loading. With proper design, AVP can reduce load transient-induced peak-to-peak voltage spikes by 50%. Inductor DCR Sensing Temperature Compensation Although not used here, inductor DCR sensing offers a lossless method of sensing the load current. The problem is that the DCR of the inductor typically has a positive temperature coefficient, causing the effective current limit of the converter to change with inductor temperature. The LTC3856 can sense the inductor temperature with an NTC thermistor, thereby adjusting the current limit based on the temperature. The result is a constant current limit over a broad temperature range. This improves inductor DCR sensing reliability in high current applications. Output Voltage Remote Sensing For high output current, low voltage applications, board or wire interconnect resistance can cause a severe load regulation problem. To solve this problem, the LTC3856 includes a low offset, unity-gain, high bandwidth differential amplifier for true remote sensing. Common mode noise and ground loop disturbances can be rejected, and load regulation is greatly improved, especially when there are long trace runs between the load and the converter output. Conclusion The LTC3856 is a feature-rich single output, 2-phase synchronous step-down DC/DC controller. It achieves high efficiency in both heavy load and light load conditions, with temperature compensated DCR sensing and Stage Shedding mode or Burst Mode operation. AVP improves transient response even when the output capacitance is reduced. Remote sensing, a tight ±0.75% reference voltage accuracy over temperature, voltage 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 and can meet the high standards of today’s power supplies for telecom and datacom, industrial and computer applications. 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