High Current Path, Top Mosfet Turned On; High Current Paths During Abrupt Load Current Changes; Figure 52. Buck Voltage Regulator Example; Figure 53. High Current Path With Top Mosfet Turned On - Intel 855PM Design Manual

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Figure 52. Buck Voltage Regulator Example

5.8.1.

High Current Path, Top MOSFET Turned ON

The dashed/arrow line in Figure 53 indicates the high current path when the top MOSFET is ON.
Current flows from the V_DC power source, through the top MOSFET (There may be more than one of
these.), through the inductor and sense resistor and finally through the processor, R
components and current paths shown must be able to not only carry the high current through the
processor, but the power source and ground must also be adequate.

Figure 53. High Current Path With Top MOSFET Turned ON

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5.8.2.

High Current Paths During Abrupt Load Current Changes

During abrupt changes in the load current, the bulk and decoupling capacitors must supply current for
the brief period before the regulator circuit can respond. The dashed/arrow line in Figure 54 illustrates
this current path. Stray inductance and resistance become a major concern and if they are not
minimized, they can compromise the effectiveness of the capacitors. Bulk capacitors for Vcc should be
located at the highest current density points. These high-density points are located along the shortest
route between the processor core and the sense resistor. Using short, fat traces or planes can minimize
both stray inductance and resistance.
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Intel 855PM Chipset Platform Design Guide
NMOS
DRIVER
TG
CO
NMOS
BG
Voltage
Regulator
Control Circuitry
V_DC
NMOS
DRIVER
TG
NMOS
BG
Voltage
Feed back
Regulator
Control Circuitry
V_DC
SCHOTTKY
Feed back
SCHOTTKY
Platform Power Requirements
Output Vcc
RLoad
, to ground. The
Load
Output Vcc
RLoad
101