Step-Down DC/DC Controller in 2mm × 3mm DFN Includes FET Drivers, DCR Sensing and Accepts Inputs to 38V Mike Shriver Wide input voltage range and reliability are desirable features in any buck converter. The LTC3854 delivers these features and more in a 2mm × 3mm DFN or MSE package. This current mode controller with integrated N-channel FET gate drivers can produce output voltages ranging from 0.8V to 5.5V over an input voltage range of 4.5V to 38V. SENSING THE CURRENT The LTC3854 employs a fixed frequency peak current mode topology, which provides a cycle-by-cycle current limit. Current can be sensed with discrete sense resistors in series with the inductor. Alternately, the DC resistance of the inductor (DCR) can be used to sense current instead of a sense resistor by placing an RC filter across the inductor to recreate the triangular current sense waveform. The advantage of DCR sensing is improved efficiency (thanks to the elimination of the power losses of the sense resistor), lower parts count and lower cost. The disadvantage is a less accurate current limit due to DCR variations part-to-part and over temperature. but with DCR sensing. The full load efficiency of the circuit with a sense resistor is 86.9%, while the full load efficiency increases to 88.7% (see Figure 3) with DCR sensing. The 400kHz switching frequency provides a good balance between high efficiency on one hand and a fast load step response on the other. The load step response shown in Figure 4 shows that the output voltage remains within ±50mV for a 50%-to-100% load step. Figure 1 shows a 1.5V, 15A converter that uses a sense resistor to sense the output current; Figure 2 shows the same circuit, Figure 1. A 1.5V, 15A converter with a sense resistor VIN M1 LTC3854 TG 0.1µF RUN/SS ITH 2200pF 5.62k 0.1µF VIN 4.5V TO 14V 180µF 16V BOOST L 0.56µH SW 47pF 10µF 16V ×2 RSENSE 2mΩ D1 4.7µF FB 17.4k SENSE– BG SENSE+ GND 1nF D1 = CMDSH-3 M1 = RENESAS RJK0305DPB M2 = RENESAS RJK0330DPB L = TOKO FDA1055-R56M COUT1 = SANYO 2R5TPE330M9 COUT2 = MURATA GRM31CR60J107ME39L 20 | July 2011 : LT Journal of Analog Innovation + INTVCC 20k 47Ω 47Ω M2 COUT2 100µF ×2 VOUT 1.5V 15A COUT1 330µF ×2 design features The wide input voltage range of the LTC3854 allows it to be used in a wide variety of applications, including automotive, industrial and communications. WIDE INPUT VOLTAGE RANGE APPLICATIONS The input voltage range of the LTC3854 allows it to be used in a wide variety of applications, including automotive, industrial and communications. One example of a wide input converter is the 5V, 10A converter in Figure 5, which can be used for an input voltage range of 6V to 38V. The strong gate drivers and low QG FETs allows for 94% efficiency at a 32V input voltage and 100% load as shown in Figure 6. The small size and low pin count of the LTC3854 allows the control section of the 2.5V/5A converter with a 4.5V to 26V input voltage range shown in Figure 7 to fit within a 180mm2 area. Dual channel FETs are used to minimize the size of the power stage. The 400kHz switching frequency allows the use of a small 2.2µ H inductor with a 7mm × 7mm footprint. The entire converter (minus the bulk input capacitor) can fit in a 430mm2 area. VIN LTC3854 TG 0.1µF Figure 2. Same converter shown in Figure 1, except with DCR sensing. The filter at R1 and C1 infers the current sense information from the inductor’s DCR. RUN/SS ITH 2200pF 5.62k 0.1µF 180µF 16V BOOST L 0.56µH SW 47pF VIN 4.5V TO 14V 10µF 16V ×2 M1 VOUT 1.5V 15A D1 4.7µF FB 17.4k SENSE– BG SENSE+ GND + 3.09k INTVCC 20k M2 0.1µF COUT2 100µF ×2 0.1µF D1 = CMDSH-3 M1 = RENESAS RJK0305DPB M2 = RENESAS RJK0330DPB L = TOKO FDA1055-R56M COUT1 = SANYO 2R5TPE330M9 COUT2 = MURATA GRM31CR60J107ME39L 92 Figure 3. Efficiency of the converter with a sense resistor versus efficiency with DCR sensing. By eliminating the sense resistor, the full load efficiency goes up by almost 2%. DCR SENSE EFFICIENCY (%) 90 RSENSE = 2mΩ 88 COUT1 330µF ×2 VOUT 50mV/DIV (AC-COUPLED) 1.8% 86 84 82 80 VIN = 12V 0 2 4 6 8 ILOAD (A) 10 12 14 16 Figure 4. 50% to 100% load step response of converter shown in Figure 2. ILOAD 10A/DIV 50µs/DIV VOUT = 1.5V ILOAD = 0A TO 15A July 2011 : LT Journal of Analog Innovation | 21 VIN RUN/SS 7.5k 0.1µF BOOST L 3.6µH SW 100pF RSENSE 3mΩ D1 42.2k SENSE– BG SENSE+ GND 98 COUT1 220µF 4.7µF FB 100 VOUT 5V 10A + INTVCC 8.06k 47µF 50V ×2 96 EFFICIENCY (%) ITH 2200pF 150µF 50V M1 LTC3854 TG 0.1µF VIN 6V TO 38V M2 4.7nF 94 92 90 16Ω D1 = ZETEX ZLLS1000 M1, M2 = INFINEON BSC093N04LS L = COILTRONICS HC1-3R6-R COUT1 = SANYO 6TPE220MI 16Ω 86 Figure 5. 5V, 10A converter with a 6V to 38V input voltage range SOFT-START, LOW DROPOUT AND HIGH STEP-DOWN RATIOS The LTC3854 features a programmable soft-start set with a capacitor on the RUN/SS pin. As the output voltage ramps up, the converter operates in discontinuous mode to prevent reverse current from flowing into the inductor. This allows the part to safely turn on into a prebiased output. After ramp-up, the part operates VIN = 6V VIN = 12V VIN = 24V VIN = 32V 88 0 1 2 3 4 5 6 ILOAD (A) 7 9 10 Figure 6. Efficiency of the 5V, 10A converter in continuous conduction mode. Other features are a maximum duty cycle of 97% for low dropout voltage applications, a low minimum on-time of 75ns for applications with high step-down ratios, foldback current limit in addition to the cycle-by-cycle current limit and a built-in 5V linear regulator for bias. CONCLUSION The LTC3854 delivers reliability and efficiency for cost-sensitive and space-constrained applications. The device can produce output voltages ranging from 0.8V to 5.5V over an input voltage range of 4.5V to 38V. The LTC3854’s 2mm × 3mm DFN and MSE packages, along with a 400kHz switching frequency, minimize solution size. n Figure 7. 2.5V, 5A converter with a 4.5V to 26V input voltage range VIN LTC3854 TG 0.1µF RUN/SS ITH 2200pF 15k M1T 0.1µF L 2.2µH SENSE– BG SENSE+ 10Ω 10Ω M1B COUT2 47µF 1nF D1 = CMDSH-3 M1T, M1B = VISHAY Si4816BDY L = TDK RLF7030T 2.2µH COUT1 = SANYO 4TPE220MF COUT2 = AVX 12106D476KAT2A RSENSE 5mΩ 4.7µF FB 43.2k 18µF 35V + INTVCC 20k + VIN 4.5V TO 26V BOOST SW 150pF 4.7µF 25V D1 22 | July 2011 : LT Journal of Analog Innovation 8 GND VOUT 2.5V 5A COUT1 220µF