Step-Down Converter Delivers 25A at 12VOUT from Inputs Up to 60V
Linear Technology » LTC3890, LTC3891
Victor Khasiev, Linear Technology
Design Note 1024
The LTC®3890 (dual outputs) and LTC3891 (single output) step-down DC/DC controllers directly accept inputs from 4 V to 60 V. 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.
High Efficiency 2-Phase Converter Produces 12 V at 25 A
Figure 1 shows the LTC3890 in a 2-phase single output step-down converter configuration that delivers 25 A at 12 V, which can be scaled up to 75 A 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. Although the ITH pins are connected together, each is terminated to a separate 47 pF capacitor to compensate for possible noise from interconnecting traces. A relatively low switching frequency, around 150 kHz, and a relatively high phase inductance of 10 μH 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.
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 25 A 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 0.5 mA. Figure 4 shows a thermal map of the board with no air flow present at VIN = 20 V and VOUT = 12 V at 25 A (300 W).
Two values define selection of the inductor: RMS current (IRMS) and saturation current (IPK):
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 5.1 V. This means that logic level MOSFETs must be used in the design.
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.
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