advertisement Step-Down Converter Delivers 25A at 12VOUT from Inputs Up to 60V Design Note 1024 Victor Khasiev Introduction The LTC ® 3890 (dual outputs) and LTC3891 (single output) step-down DC/DC controllers directly accept inputs from 4V to 60V. This wide input range covers input voltages for single or double battery automotive environments, thus eliminating the need for snubbers and voltage suppression circuitry typically required to protect ICs during load dumps. This range also encompasses 48V telecom applications. If no galvanic isolation is required between the input and output voltages, the LTC3890 and LTC3891 can replace expensive and bulky transformer-based converters. When compared to a transformer-based solution, an LTC3890 or LTC3891 step-down converter increases efficiency, reduces power loss in the supply lines, simplifies layout and significantly reduces the bill of materials. 5 30.1k 2 7 0.1µF 8 29 47pF 4.7nF 9.76k 13 30 12 47pF 35.7k 499k 10pF 21 27 1µF PLLIN LTC3890 FREQ VIN 14 28 20 22 SENSE1– 1 SENSE1+ 32 RUN2 SS1 TG1 SS2 SW1 ITH1 ITH2 BOOST1 VFB1 VFB2 PGND PGOOD1 INTVCC TG2 SW2 PGOOD2 ILIM 2.2Ω 2.2pF RUN1 BG1 31 11 VOUT Although the ITH pins are connected together, each is terminated to a separate 47pF capacitor to compensate L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN, 16V TO 60V 1M 57.6k High Efficiency 2-Phase Converter Produces 12V at 25A Figure 1 shows the LTC3890 in a 2-phase single output step-down converter configuration that delivers 25A at 12V, which can be scaled up to 75A by adding more LTC3890 ICs to increase the number of power phases. For lower output current, the single-phase LTC3891 can be used. Implementing a 2-phase converter simply requires tying together the independent channel pins of the LTC3890, namely, FB1 and FB2, TRACK/SS1 and TRACK/SS2, RUN1 and RUN2, ITH1 and ITH2. BOOST2 EXTVCC BG2 SENSE2+ SENSE2– 2.2µF/100V ×4 1µF/100V VIN 100Ω 26 RJK0651DPB 25 1 0.1µF DFLS1100 INT 23 19 + 150µF VOUT 12V AT 25A VIN RJK0651DPB L2 10µH 0.1µF DFLS1100 3m 10µF ×2 INT 18 9 10µF ×2 4.7µF 16 10 3m RJK0653DPB INT 15 17 L1 10µH RJK0653DPB 100Ω L1, L2: WÜRTH 7443631000 2.2pF DN1024 F01 Figure 1. High Efficiency Converter Produces 25A at 12VOUT from Inputs Up to 60V 0513/1024 + 150µF for possible noise from interconnecting traces. A relatively low switching frequency, around 150kHz, and a relatively high phase inductance of 10µH are used to reduce switching losses at high input voltages. The output voltage is fed to the EXTVCC pin to reduce losses associated with biasing the chip and internal gate drivers at high input voltages. Circuit Performance Efficiency is shown in Figure 2, measured without cooling air flow. Efficiency peaks close to 98% in the middle of the load range and declines to 96% at the 25A maximum load. Figure 3 shows the average input current vs input voltage at no load in Burst Mode ® operation. The value of this current is below 0.5mA. Figure 4 shows a thermal map of the board with no air flow present at VIN = 20V and VOUT = 12V at 25A (300W). 99.0 20V 36V 50V 98.5 EFFICIENCY (%) 98.0 97.5 97.0 96.5 96.0 95.5 95.0 1 6 11 16 21 LOAD (A) 26 DN1024 F02 Figure 2. Efficiency at VIN = 20V, 36V and 50V 0.22 INPUT CURRENT (mA) 0.20 Figure 4. Temperature Hot Spots with No Air Flow Component Selection Two values define selection of the inductor: RMS current (IRMS) and saturation current (IPK ): ∆I2 12 (V – V )•D ∆I= IN OUT L•f V D= OUT VIN I IPH = k • OUT 2 ∆I IPK =IPH + 2 IRMS = (IPH)2 + where f is the switching frequency and k is a coefficient defined by the current imbalance between the phases. For converters based on the LTC3890, k = 1.08, assuming current sense resistors with a 1% tolerance. Selection of power MOSFETs and input/output capacitors is described in detail in the LTC3890 data sheet. It is important to note that the typical internal VCC voltage and, consequently, the MOSFET gate voltage is 5.1V. This means that logic level MOSFETs must be used in the design. 0.18 0.16 0.14 0.12 0.10 20 25 30 35 VIN 40 45 50 DN1024 F03 Figure 3. Average Input Current vs Input Voltage at No Load. VOUT is 12V. Data Sheet Download www.linear.com/LTC3890 Conclusion The LTC3890 dual output, synchronous step-down converter can be easily configured as a single output, dual phase converter for high input voltage, high output current automotive and telecom applications. For applications help, call (408) 432-1900, Ext. 3161 Linear Technology Corporation dn1024 LT 0513 REV A • PRINTED IN THE USA (408) 432-1900 LINEAR TECHNOLOGY CORPORATION 2012 1630 McCarthy Blvd., Milpitas, CA 95035-7417 ● FAX: (408) 434-0507 ● www.linear.com