Platform Power Requirements
R
102
Intel
®
855PM Chipset Platform Design Guide
Figure 54. High Current Path During Abrupt Load Current Changes
Output Vcc
Feed back
SCHOTTKY
RLoad
+
V_DC
NMOS
NMOS
DRIVER
CO
TG
BG
Voltage
Regulator
Control
Circuitry
5.8.3.
High Current Paths During Switching Dead Time
When the top MOSFET turns OFF and before the bottom MOSFET (again there may be more than one
of these.) is turned ON, The pattern of current flow changes. The inductor is no longer being supplied
current through the top MOSFET starts to collapse its magnetic field. The inductor literally becomes a
generator, at this point. The dashed/arrow line in Figure 55 shows the current path during the time that
both top and bottom MOSFETs are OFF. This is termed “Dead Time.” During Dead Time there is a
high current flow through the inductor, processor, ground, and the Schottky diode. The diode and its
traces must be laid out in such as to minimize both stray inductance and resistance with short, fat traces
or planes.
Figure 55. High Current Path with Top and Bottom MOSFETs Turned Off (Dead Time)
Output Vcc
Feed back
SCHOTTKY
RLoad
+
V_DC
NMOS
NMOS
DRIVER
CO
TG
BG
Voltage
Regulator
Control
Circuitry
5.8.4.
High Current Path with Bottom MOSFET(s) Turned ON
A few nanoseconds after the top MOSFET is turned OFF, the bottom MOSFET(s) is turned ON. The
high current path now switches from the Schottky diode to the bottom MOSFET(s), the current path
shown by the dashed/arrow line in Figure 56. Minimize stray inductance and resistance with short, fat
traces or planes.
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