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Maxim > Design Support > Technical Documents > Reference Designs > Automotive > APP 3218
Maxim > Design Support > Technical Documents > Reference Designs > Power-Supply Circuits > APP 3218
Keywords: AMD K8, MAX1937, active offset, K8 mobile cpu power, low power K8 mobile power supply
REFERENCE DESIGN 3218 INCLUDES: Tested Circuit Schematic BOM Description
Power Supply Meets AMD K8 Low-Power Mobile
Specification
Apr 30, 2004
Abstract: This application note describes a two-phase, synchronous, step-down converter that is fully
compliant with the AMD® K8 Low-Power Mobile Specification. It includes details of the circuit operation,
schematic, bill of materials, and a 1.2 volt, 27.3 Amp reference design with test data.
This application note describes a two-phase, synchronous, step-down converter that uses the MAX1937
to achieve a power supply that is fully compliant with the AMD K8 Low-Power Mobile Specification.
The design uses the standard K8 VID table with a -100mV offset from the selected VID voltage. An
active offset method implements the required -100mV offset. The specified "droop" or voltage positioning
value for this application is 50mV.
Details of the active offset circuit are presented, along with the schematic and bill of materials for the
complete power supply. A 1.2 volt, 27.3 Amp reference design along with test data is also presented.
For more details on the 2-phase synchronous step down converter (voltage positioning etc.), please refer
to the MAX1937-MAX1939 Data Sheet.
Active Offset
The active offset circuit is shown in Figure 1. It is essentially a precision 1-mA current source. The
circuit consists of resistors R51, R52, R53, R54, R55 and the LMX321 op amp U2. The circuit accepts a
2V input from the reference voltage (pin 12) of the MAX1937 and maintains 2V, with the polarity shown,
across the 2kΩ resistor R55. The resulting 1mA current in the resistor flows in the 100Ω feedback
resistor R8 connected to the FB pin (pin 14) of the MAX1937 and produces a negative offset of -100mV.
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