3-Phase Buck Controller for Intel VRM9/VRM10 with Active Voltage Positioning

Introduction

Each new generation of CPUs demands more from power supplies than the last: more power, tighter voltage regulation and faster transient response. Meeting all of the new requirements is a difficult proposition, but the LTC3738 helps power supply designers do just that. It is a 3-phase buck controller with active voltage positioning specifically designed for Intel VRM9 and VRM10 (Figure 1).

Figure 1. This 3-phase power supply manages the thermal problems inherent in high current Intel VRM10 applications.

High Power and Thermal Management

The LTC3738 can easily work with the 3-phase LTC3731 to form a 6-phase (up to 12-phase interleaved) power supply to deliver more than 100A current to its load. For such high currents, proper thermal management is crucial. The current mode architecture of the LTC3738 evenly distributes the load, and thus thermal stress, across the channels. This improves the thermal performance and reliability of the entire power solution. The LTC3738 also includes a thermal detector that generates a VR_HOTB warning signal when chip itself gets hot (around 120°C) plus a self-protect thermal detector that shuts down the device when chip becomes extremely hot and endangers the safety of the power supply. The LTC3738 also has a comparator for external thermal detection. Power designers can put thermal detection resistors at the hottest spot on the board and let the LTC3738 send a VR_HOTB signal to the CPU when its thermal comparator trips.

Accurate Load Line Control

The tight load line window of the VRM9/VRM10 specification asks for accurate static and dynamic voltage control. The ±1% DC regulation accuracy and precise programmable active voltage positioning of LTC3738 helps power designers meet the load line window easily. The unique active voltage positioning solution of LTC3738 makes the load line slope control easy and accurate. The slope is programmed by the ratio of two external resistors. LTC3738 senses true load current including ripple current of all three channels and generates an accurate AVP control voltage. The precise regulation of the LTC3738 gives more range for output voltage ripple. Hence power designers can use smaller output capacitor values and lower their total solution cost. Smaller output capacitor values also speed up the changing of the output voltage when the CPU generates a different VID code.

Other Features

The LTC3738 has a differential amplifier for remote sensing of both the high and low sides of the output voltage. There is no reverse current during start-up, which allows the LTC3738 to power up into a pre-biased output without sinking current from the output. The LTC3738 also has a defeatable short-circuit shutdown timer. Three operation modes—PWM, pulse skip and Stage Shedding—allow power supply designers to optimize for efficiency and noise.

Conclusion

LTC3738 is specifically designed to simplify power supply designs for Intel VRM9/VRM10 applications. It is a complete power supply solution with essential thermal management features, accurate load line control, precise output voltage sensing, and comprehensive fault protection.

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Xiaoyong Zhang