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PRODUCT SPECIFICATION
RC5102
7
P
Auto Switching Single/Dual Power
Supply for a Flexible Motherboard
Design
A detailed analysis of the new Pentium-class processors
reveals the requirement for an open-ended motherboard
power supply design that can accomodate different CPUs in
a single system. As an example, consider the Intel P54C
and P55C Pentium processors. Although these two proces-
sors may occupy the same CPU socket, distinct differences
exist in their power supply requirements. The present gener-
ation P54C uses a single supply for both the processor core
and the I/O. For the higher performance devices, the supply
voltage required is 3.5V
±
100mV (VRE s-specification).
For the lower performance models, a 3.3V
acceptable. For improved compatibility, Intel has now re-
specified its 3.3V standard CPUs for operation at the new
3.5V VRE level. The P55C multimedia upgrade processor,
due to be released in the latter part of 1996, requires separate
voltages for the core and I/O circuitry. The nominal core
voltage is currently 2.8V
±
100mV, while the I/O supply
remains at a nominal 3.3V. It is therefore desirable to imple-
ment a power supply design that will automatically detect
the CPU model present and program each output voltage
accordingly. The circuit in figure 4 directly addresses this
requirement. The basic theory of this design is to provide an
automatic switch between a single and a dual linear power
supply depending upon which CPU occupies the socket.
±
5% supply is
To ease the task of identifying the CPU, the P55C processor
includes a single-bit identification pin
AL1, to distinguish itself from the standard Pentium P54C
processor. This pin is always bonded to ground on the P55C
CPU, while it is an internal no connect on the P54C. There-
fore, the user can easily identify which processor occupies
the CPU socket by direct monitoring of this pin. The circuit
in figure 4 uses the CPU identification pin to select either
a single or a dual output as well as select the appropriate
output voltage for the CPU core power island.
VCC2DET
, at location
Because the I/O circuitry can always operate from a nominal
3.5V supply, output 1 is set at a fixed 3.5V output. The CPU
core supply is thus switched between 2.8V and 3.5V using
an external FET and the appropriate resistor values. Using
this circuit configuration, both output s can source up to 5A.
These current ranges will easily accomodate the standard
Pentium P54C and the P55C as well as other Pentium
compatible processors. For selected processors, the load
currents required by each output will force the use of a
MOSFET or Darlington pair as the pass elements. Using
these higher-powered devices, the RC5102 can source up to
10A given the appropriate thermal requirements are also
met. Please consult Fairchild Semiconductor Applications
for additional details regarding CPU applications.
Current Sharing Option
If the RC5102 is to be used in an application that must
address several CPUs, additional load current capability may
be required from one of the outputs. For example, consider
the Cyrix 6x86 microprocessor. Although its specifications
are very similar to those of the Intel Pentium P54, it
requires as much as 10A under worst-case conditions. To
remedy this situation without adding additional costly pass
elements, the RC5102 can be configured to allow both
outputs to share the load current. In order to achieve accept-
able performance, the layout of the output traces must be
carefully routed. If the traces from the emitter of the pass
transistors to the point where the two outputs are joined
together, both outputs will share the load current equally.
If the series impedances of each trace are different however,
one output will tend to provide more than 50% of the load
current while the other output will not be heavily burdened.
Using this option, low cost NPN transistors may be used
instead of MOSFETs to deliver up to 10A loads. Again, the
overall power limitation will depend heavily upon the level
of thermal management for the pass elements. Please consult
Fairchild Semiconductor Applications for additional details
on this and other possible configurations using the RC5102.