Current Mode Switching Supply with Ultralow Inductor DCR Sensing for High Efficiency and High Reliability Jian Li, Haoran Wu and Gina Le Current mode switching supplies have several advantages over voltage-mode switching supplies: (1) high reliability with fast, cycle-by-cycle current sensing and protection; (2) simple and reliable loop compensation—stable with all ceramic output capacitors; (3) easy and accurate current sharing in high current PolyPhase supplies. In high current applications, power losses in the current sensing component are a concern, so the resistance of the sense component must be as low as possible. The problem is that low resistance sensing elements produce reduced signal-to-noise ratios, such that switching jitter becomes an issue in high current, high density applications. The LTC3866 solves this problem by making it possible to build reliable current mode switching supplies with <0.5mΩ current sensing resistance. This single-phase synchronous buck controller drives all N-channel power MOSFET switches with onboard gate drivers. It employs a unique architecture that enhances the signal-tonoise ratio of the current sense signal, allowing the use of a very low DC resistance (DCR) power inductor or low value current sensing resistor to maximize efficiency in high current applications. This feature reduces the switching jitter commonly found in low DCR applications. The controller has a wide 4.5V–38V input range, remote output voltage sensing with accurate 0.5% reference, programmable and temperature-compensated current limit when using inductor DCR sensing, short-circuit soft recovery without overshoot, and chip thermal shutdown. The LTC3866 facilitates the design of high efficiency, high power density and high reliability solutions for telecom systems, industrial and medical instruments, and DC power distribution systems. The controller is available in a low thermal 24 | April 2012 : LT Journal of Analog Innovation Figure 1. LTC3866 current sensing scheme with ultralow inductor DCR. High current paths are shown with thicker lines. VIN INTVCC VIN BOOST INDUCTOR LTC3866 RITEMP ITEMP TG DCR VOUT BG PGND RS 22.6k SNSD+ SNS– RNTC 100k L SW RP 90.9k SNSA+ R1 R2 C1 C2 SGND PLACE C1, C2 NEXT TO IC PLACE R1, R2 NEXT TO INDUCTOR R1C1 = 5 • R2C2 Figure 2. High efficiency, 1.5V/30A step-down converter with very low DCR sensing 100k 0.1µF FREQ MODE/PLLIN RUN PGOOD TK/SS ITEMP 30.1k 220pF 20k 10k 1.5nF C1 220nF C2 220nF 4.7µF EXTVCC ITH VFB 220µF VIN 4.5V TO 20V LTC3866 VIN DIFFOUT INTVCC DIFFP BOOST DIFFN TG SNSD+ SW SNS– SNSA+ ILIM 0.1µF 0.33µH DCR = 0.32mΩ BG PGND CLKOUT SGND R2 931Ω R1 4.64k COUT 470µF ×2 VOUT 1.5V 30A design features The LTC3866 employs a unique architecture that enhances the signal-to-noise ratio of the current sense signal, allowing the use of a very low DC resistance (DCR) power inductor or low value current sensing resistor to maximize efficiency in high current applications. 3 Burst Mode® OPERATION PULSE-SKIPPING MODE CCM 90 EFFICIENCY (%) 80 70 TOP OF BOARD BOTTOM OF BOARD 60 50 40 INDUCTOR TOP FET BOTTOM FET 30 20 VIN = 12V VOUT = 1.5V FSW = 400kHz 10 0 0.01 0.1 1 ILOAD (A) 10 100 LTC3866 12V INPUT 1.5V/30A OUTPUT NO AIRFLOW Figure 3. Efficiency of the circuit in Figure 2 Figure 4. Thermal test of the circuit in Figure 2 impedance 24-lead 4mm × 4mm QFN and 24-lead exposed pad FE packages. It is especially well suited to low voltage, high current supplies because of a unique architecture that enhances the signal-to-noise ratio of the current sense circuit. This allows it to operate with the small sense signals produced by very low DCR, 1mΩ or less, inductors, which improve power efficiency in high current supplies. The improved SNR minimizes FEATURES The LTC3866 uses a constant frequency peak current mode control architecture, guaranteeing cycle-by-cycle peak current limit and current sharing between different power supplies. Figure 5. Switching node jitter comparison at 12V input, 1.5V/25A output STANDARD DCR SENSING 160ns jitter due to switching noise, which could corrupt the signal. The LTC3866 can sense a DCR value as low as 0.2mΩ with careful PCB layout, though in this extreme situation, the additional PCB and solder resistance should be considered. As shown in Figure 1, the LTC3866 comprises two positive sense pins, SNSD+ and SNSA+, to acquire signals and processes Figure 6. Short circuit test LTC3866 ENHANCED DCR SENSING 60ns VOUT 1V/DIV VSW 10V/DIV VSW 10V/DIV IL 10A/DIV VIN = 12V VOUT = 1.5V ILOAD = 25A 100ns/DIV VIN = 12V VOUT = 1.5V ILOAD = 25A 100ns/DIV 500µs/DIV April 2012 : LT Journal of Analog Innovation | 25 APPLICATIONS R2 • C2 = R1 • C1/5. An additional, optional temperature compensation circuit guarantees the accurate current limit over a wide temperature range, especially important in DCR sensing. them internally to provide a 14dB (5×) signal-to-noise ratio improvement in response to low voltage sense signals. The current limit threshold is still a function of the inductor peak current and its DCR value, and can be accurately set from 10mV to 30mV in a 5mV steps with the ILIM pin. The part-to-part current limit error is only about 1mV over the full temperature range. Figure 2 shows a high efficiency, 1.5V/30A step-down converter with very low DCR sensing. An inductor with DCR = 0.32mΩ is used in this design to maximize efficiency. The LTC3866 also features a precise 0.6V reference with a guaranteed limit of ±0.5% that provides an accurate output voltage from 0.6V to 3.5V. Its differential remote VOUT sensing amplifier makes the LTC3866 ideal for low voltage, high current applications. The filter time constant, R1 • C1, of the SNSD+ path should equal the L/DCR of the output inductor, while the filter at SNSA+ path should have a bandwidth five times larger than SNSD+, namely The efficiency in different operation modes is shown in Figure 3. The full load efficiency is as high as 90.3% at 12V input voltage. It is about 1.4% improvement over the supply using a 1mΩ sense resistor with the same power stage design. The hot spot (bottom MOSFET) temperature rise is only 39.6°C without any Figure 7. A high efficiency, 1.5V/80A power supply based on parallel LTC3866s and power blocks 100k 100k INTVCC1 ITH ITEMP PGOOD VIN 100k ITH VFB PGOOD ITEMP INTVCC PGND CLKOUT ILIM SNSA+ SNS– SGND VGATE TEMP– GND CS– GND CS+ BG INTVCC2 + 330µF + 330µF GND 10Ω 10Ω 4.75k 10µF VOUT VIN 4.7µF VIN1 VOUT1 VIN2 VOUT2 100µF PWMH CMDSH-3 BOOST TG 100µF VIN VIN SW 26 | April 2012 : LT Journal of Analog Innovation TEMP+ 470µF INTVCC2 SNSD+ 47nF PWML ACBEL POWER BLOCK VRA001-4C1G 2.2Ω DIFFP 47nF VOUT2 INTVCC2 EXTVCC LTC3866EUF VIN2 VOUT 1.5V 80A GND 1µF MODE/PLLIN FREQ RUN TK/SS 100k DIFFN 0.1µF INTVCC1 47nF VOUT1 GND BG PGND CLKOUT SW DIFFOUT CMDSH-3 BOOST SNSD+ VIN1 PWMH INTVCC TG 47nF VOUT 4.7µF EXTVCC 10µF VIN INTVCC1 DIFFP ILIM 20k DIFFN SNSA+ 30.1k 470µF 2.2Ω LTC3866EUF SGND 220pF DIFFOUT SNS– 5.36k VFB MODE/PLLIN RUN FREQ TK/SS 0.1µF 1500pF VIN 1µF 0.1µF PWML TEMP+ VGATE TEMP– GND CS– GND CS+ ACBEL POWER BLOCK VRA001-4C1G 330µF + 330µF GND GND GND + 4.75k design features 2.2Ω LTspice IV circuits.linear.com/548 10µF ×2 1µF VIN VIN 180µF 12V ×2 MODE/PLLIN FREQ 20k RUN TK/SS 0.1µF VOUT 500mV/DIV ILOAD 50A/DIV DIFFP BOOST DIFFN TG 28.7k SNSD+ SW 100pF SNS– SNSA+ PGND ILIM D1: CMDSH-3 M1: BSC024NE2LS M2: BSC010NE2LS D1 INTVCC C1 220nF 4.7µF VOUT LTC3866 DIFFOUT 1nF 120k ITEMP EXTVCC ITH VFB IL1 & IL2 10A/DIV PGOOD L1 1µH DCR = 1.3mΩ M1 M2 BG R1 3.48k CLKOUT SGND 100µF ×2 330µF ×2 VOUT 5V 25A 10µs/DIV Figure 8. Current sharing performance of the 1.5V/80A supply in Figure 7 Figure 9. High efficiency power supply, 12V input to 5V/25A output airflow, as shown in Figure 4, where the ambient temperature is about 23.8°C. output inductor signal and connect to the SNSA+ pin. If the RC filter is used, its time constant, R • C, is set equal to L/DCR of the output inductor. In these applications, the current limit, VSENSE(MAX), is five times larger for the specified ILIM, and the operating voltage range of SNSA+ and SNS– is 0V to 5.25V. Without using the internal differential amplifier, the output voltage of 5V can be generated as shown in Figure 9. The thermal test shows that the hot spot (the inductor) temperature is about 57.3°C at full load without any airflow, as shown in Figure 10, where the ambient temperature is 25°C. The unique design improves the efficiency, as well as the noise sensitivity. The worst case switching node jitter is reduced by 60%, as shown in Figure 5, with a very low 0.32mΩ inductor DCR. Another unique feature of LTC3866 is short-circuit soft recovery. The internal soft recovery circuit guarantees that there is no overshoot when the power supply recovers from a short-circuit condition as shown in Figure 6. The LTC3866 can be used with a power block for a more compact design and very high current. Figure 7 shows a dual-phase, high efficiency, 1.5V/80A power supply based on a 2× parallel LTC3866 + power block scheme. Although the DCR of the inductor in the power block is only 0.53mΩ, the current sharing performance is excellent in both DC and transient conditions, as shown in Figure 8. In applications where higher value DCR inductor or RSENSE is used, the LTC3866 can be used like any typical current mode controller by disabling the SNSD+ pin, shorting it to ground. An RSENSE resistor or a RC filter can be used to sense the R3 R2 20k 147k CONCLUSION The LTC3866 delivers an outsized set of features for its small 4mm × 4mm 24-pin QFN package. The unique, ultralow DCR current sensing with current mode control makes the LTC3866 a good fit for low voltage, high current applications with high efficiency and high reliability. Tracking, strong on-chip drivers, multichip operation and external sync capability fill out its menu of features. The LTC3866 is ideal for computer and telecom systems, industrial and medical instruments, and DC power distribution systems. n Figure 10. Thermal test of the circuit in Figure 9 TOP OF BOARD INDUCTOR BOTTOM FET TOP FET BOTTOM OF BOARD LTC3866 13V INPUT 5V/25A OUTPUT NO AIRFLOW April 2012 : LT Journal of Analog Innovation | 27