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Integrated Current Sensing Technology for Synchronous Buck Converters
Keywords: integrated current sensing, buck converter, 3D packaging
INTRODUCTION: Multiphase buck converters rely on per phase current sensing to, at a minimum, achieve phase balance and increasingly to enable current-mode based control to improve the transient response. In the past, DCR sensing of the output inductor has been relied upon to provide the current information to the controller. But this technique requires additional board components and external temperature compensation, which add to layout complexity and limit the accuracy of the current sense for practical implementations. In addition, DCR based sensing places a lower limit on the DC resistance of the inductor, limiting the achievable system level efficiency. A lossless current sensing technique is proposed, which takes advantage of the on-resistance of the sync FET used in the buck converter to sense the current flowing through the device and reconstruct an emulated version of the inductor current. The current sensing circuit is integrated into the MOSFET driver and co-packaged with a set of FETs in a stacked die arrangement with common lead-frame shared between driver and sync FET to enable accuracy equivalent to or better than DCR sensing without the limitations mentioned above. CURRENT SENSING / SIGNAL RECONSTRUCTION: The proposed current sensing technique is based on sensing of the sync FET drain to source voltage (VDS) during the on-state of the device. Ordinarily with shunt based current sensing the sense resistance must be highly accurate and temperature stable. The on-resistance of a typical MOSFET can vary by +/- 20% part to part. On top of that the on-resistance will increase approximately 25% over the typical temperature range in which the device operates. The stacked die common lead-frame technology enables a solution to overcome these shortcomings. The proposed method uses a variable gain current sensing amplifier (CSA) to sense the VDS voltage. Co-packaging enables trimming out of manufacturing variability, since each CSA is inextricably tied to a single sync FET. The stacked die common lead-frame technology enables accurate inexpensive temperature sensing of the die to compensate for the temperature dependence of the FET. In order to produce an equivalent to the DCR sensed signal, an emulation scheme is employed. The emulation scheme uses a triangle wave generator that is regulated by feedback from the current sense described above and the timing information provided by the driver. In addition to steady-state operation, modern multiphase controllers are required to drive the buck converter in other operating modes including: Diode Emulation Mode (DEM), body-braking, tri-state (phase off). These additional modes are also correctly emulated by the current sense logic. EXPERIMENTAL RESULTS The proposed technology has been implemented and verified in a multiphase environment with a VR12.5 compliant controller. Waveforms will be presented showing steady-state performance, transient response, phase balance and special modes of operation. CONCLUSION An on-resistance based integrated current sensing technique was described for use with buck converters for multiphase VR applications. The technique relies on real-time die temperature measurements enabled by the use of common lead-frame technology. In addition, co-packaging enables highly accurate sensing without additional board components or layout sensitivity. The technique was demonstrated in a multiphase VR12.5 type application.
Evan Reutzel, Systems and Applications Engineer
Texas Instruments Inc.
Bethlehem, PA

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