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60V Low IQ, Dual Output Synchronous Step-Down Controller with Adjustable Gate Drive

Victor Khasiev - Applications Engineer Mar 31st 2017

Design Note 562: Introduction

The LTC®3892 is a dual output synchronous buck controller featuring a wide input voltage range of 4.5V to 60V and an output voltage of 0.8V to 99% of VIN. The combination of the LTC3892’s built-in FET drivers and external FETs yield high efficiency and high power capability, up to 97% efficiency and over 60A output current2. Its robust design—an IC operating junction temperature range of –55°C to 150°C—meets the needs of industrial, automotive, telecom, military, aerospace and distributed systems.

The LTC3892’s low IQ is indispensable in batterypowered applications where idle current must be kept low. It features 29μA no-load quiescent current and 3.9μA shutdown current, maintaining battery charge as long as possible. The LTC3892’s wide input voltage range satisfies the demanding requirements of industrial and automotive applications, which are distinguished by their lack of stable, high quality voltage sources. For example, in automotive environments, a 12V nominal voltage rail can range from a low 5V to above 50V during cold starts and load dumps. The LTC3892’s ability to sustain the output in the face of high voltage inputs eliminates costly voltage suppressors; its low minimum input maintains an uninterrupted logic supply when the input voltage dips far below nominal.

Another key feature of the LTC3892 is its programmable gate voltage that can be set from 5V to 10V. This is important for many reasons. A designer can choose a standard (10V) gate drive level and select switching devices from the wide variety of MOSFETs offered by multiple vendors, optimizing the supply efficiency and reducing overall cost. However, if gate driver loss is important, then logic level (5V) FETs can be used. As a compromise, the gate voltage can be programmed to any intermediate value, providing acceptable gate and minimum conduction losses1. The LTC3892 features programmable undervoltage lockout (UVLO) and programmable EXTVCC via the DRVUV pin.

Circuit Description

Figure 1 shows a single LTC3892, dual-output solution: 12V out at 5A and 5V out at 8A. The LTC3892 controls two power trains, each with a pair of switching MOSFETs, inductor and output filter. This circuit demonstrates the ability for VOUT1 to be set to a fixed level by programming the VPRG1 pin. In this schematic it’s set to 5V1; consequently VOUT1 is connected directly to the FB1 pin. The standard gate drive level is selected by tying the DRVSET pin to the INTVCC pin.

Figure 1. LTC3892, Dual Output DC/DC Converter SchematicFigure 1. LTC3892, Dual Output DC/DC Converter Schematic

In the solution of Figure 1, the VIN pin and the drains of the top MOSFETs of both outputs are connected to the same input voltage VIN. However, these three ports can be connected to different voltages or power supplies if needed. Designers can bias the LTC3892 through the VIN pin and apply a voltage from different sources to the drains of Q1 or Q3. Figures 2 and 3 illustrate the efficiency of the converter in Figure 1 at various input voltages.

Demonstration circuits 2 are available from the factory, designed to display a variety of load capabilities. Incorporating current mode control, a LTC3892 supply can be wired as a dual output supply as on the DC1998A demo board, as a 2-phase single output supply delivering 12V at 30A (DC2190A-B), or as a 4-phase single output supply generating 12V at 60A (DC2190A-A).

Figure 2. LTC3892, VOUT1, 5V Efficiency CurvesFigure 2. LTC3892, VOUT1, 5V Efficiency Curves

Figure 3. LTC3892, VOUT2, 12V Efficiency CurvesFigure 3. LTC3892, VOUT2, 12V Efficiency Curves

Conclusion

The LTC3892 is a versatile 60V input, dual output buck controller, optimized for high efficiency DC/ DC solutions in automotive, industrial and telecom fields. Other salient features include programmable gate voltage, extremely low no-load and shutdown quiescent current, programmable frequency, internal bootstrap diodes and easy current sharing for high current designs.