High Current Paths During Switching Dead Time; High Current Path With Bottom Mosfet(S) Turned On; Figure 54. High Current Path During Abrupt Load Current Changes; Figure 55. High Current Path With Top And Bottom Mosfets Turned Off (Dead Time) - Intel 855PM Design Manual

Platform Power Requirements

Figure 54. High Current Path During Abrupt Load Current Changes

CO
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)

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.
102
NMOS
DRIVER
TG
NMOS
BG
Voltage
Regulator
Control
Circuitry
NMOS
DRIVER
TG
CO
NMOS
B G
Voltage
Regulator
Control
Circuitry
V_DC
SCHOTTKY
Feed back
V_DC
SCHOTTKY
Feed back
®
Intel 855PM Chipset Platform Design Guide
R
Output Vcc
RLoad
Output Vcc
RLoad