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THE UC3886 PWM CONTROLLER USES AVERAGE CURRENT
MODE CONTROL TO MEET THE TRANSIENT REGULATION PERFORMANCE
OF HIGH END PROCESSORS
by Larry Spaziani
Applications Engineer
U-156
APPLICATION NOTE
ABSTRACT
The continuing development of high performance processors is imposing stringent requirements on their
respective power systems.Intel’s Pentium®Pro power system specification, for instance, demonstrates the
industry trend to operate at lower voltages and at higher currents, with tight regulation and fast transient
response.To meet these requirements, the UC3886 Average Current Mode PWM Controller is introduced.
This application note highlights the features of the UC3886 and details how this IC is ideal for creating the
optimal switching regulator for low voltage processor applications.
UC3886 FEATURES AND BENEFITS
• Average Current Mode Control direct from a sense resistor
No slope compensation networks required
• High Gain-Bandwidth Amplifiers for a fast transient response
• Programmable Current Sense Amplifier
Interfaces to a low power sense resistor
Accurate Over Current Protection
• On-board Reference for Standalone operation
• Direct Drive for Buck regulator NMOS high side switch
Eliminates need for separate Driver IC
decreases to limit the power dissipation on the
processor. Noise immunity decreases as well under
these conditions resulting in much tighter regulation
requirements. High efficiency must be maintained
even though currents are increasing and output volt-
ages are decreasing. Load current transients
become exceedingly fast as the processor goes into
and out of sleep modes of operation. Operating
voltage is no longer a standard but must be pro-
grammable for variations from supplier to supplier,
or even within a product line, such as with the
Intel Pentium®Pro. Of course, simplicity must be
maintained in order to keep parts count low and
cost down.
INTRODUCTION
The Intel Pentium®Pro microprocessor specifica-
tions underscore an ongoing trend in high end
digital systems. Clock frequencies are increasing
for improved throughput and the number of tran-
sistors is increasing as manufacturing technology
improves. Power management both within the
processor and power to the processor has been
elevated to a significant architectural design
consideration.
The consequence of these advances for the power
system is requirements that strike fear in the most
hardy power supply designers. Load current
increases as the number of transistors and the clock
frequency increase whereas operating voltage
