Power supplies that meet AMD K8 low-power mobile processor specifications

This application note describes the implementation of a two-phase, synchronous-rectifying buck regulator using the MAX1937 that fully meets the AMD K8 low-power mobile processor specifications.

This design uses the standard K8 VID platform and adds a -100mV offset to the selected VID voltage. An active offset method achieves the required -100mV offset. The fixed offset or voltage position specified in this application is at 50mV.

Details of the active offset circuit, schematic, and complete component list of the power supply are listed below. A 1.2V, 27.3A reference design and test data are also given below.

For details on the two-phase, synchronous-rectification buck regulators (voltage positioning, etc.), please refer to the MAX1937-MAX1939 data sheets.

active offset

Figure 1 shows the active offset circuit, which is essentially a 1mA precision current source. The circuit includes R51, R52, R53, R54, R55 and operational amplifier LMX321 (U2). This circuit receives a 2V input from the MAX1937's reference voltage (pin 12) and maintains a 2V voltage across 2kΩ resistor R55 with the polarity shown in the figure. This creates a 1mA current in the resistor, which creates a negative offset of -100mV through the 100Ω resistor R8 connected to the MAX1937 FB pin (pin 14).

Figure 1. Active offset circuit