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PRODUCT SPECIFICATION
RC5035
7
A
Dual Power Supply Application
Some CPU power applications such as the Intel Pentium
P55C will require separate voltages for the CPU core and I/O
circuitry. Currently, the voltage requirements for this proces-
sor are 2.8V for the CPU core and 3.3V for the I/O circuitry.
The circuit in figure 1 addresses this requirement using a
minimum of external components. The RC5035 includes an
internal operational amplifier that can be combined with an
external N-channel power MOSFET and 1% resistors (see
Design Equations) to form a second power supply. In this
configuration, both the switched-mode and linear power
supplies can be programmed between 2.0V and 3.6V to meet
a variety of dual voltage requirements. In the circuit configu-
ration of figure 1, the switched-mode portion is used for the
5V to 2.8V conversion for the CPU core to realize the
optimal package efficiency. The op-amp, power FET and
resistors are combined to generate the 3.3V required by the
I/O power plane.
Analysis of the linear regulator portion from an efficiency
standpoint reveals that for a 5V to 3.3V conversion, the
efficiency is roughly 3.3/5, or 66%. For loads of 4A or lower,
the power dissipation of the external MOSFET should not
pose any thermal design problems if it is chosen wisely.
The N-channel MOSFET must exhibit a low Rdson and
should still be inexpensive and readily available. The overall
efficiency of the dual power supply will vary depending
upon the burden on each output. Therefore, the switched-
mode supply should typically be utilized as the primary
supply with the linear regulator portion being used to
provide power for the output that is more lightly loaded.
Design Equations
Linear Regulator
Switched-mode Regulator
Autodetecting Single/Dual Power
Supply for a Flexible Motherboard
Design
Further analysis of the Intel Pentium 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 processors
may occupy the same CPU socket, distinct differences exist
in their power supply requirements. The present generation
P54C uses a single supply for both the processor core and
the I/O. For the higher performance devices, the supply volt-
age required is 3.5V
±
100mV (VRE s-specification).
V
OUT
V
REF
R2
R3
+
R3
è
=
V
OUT
V
REF
R4
R5
+
R4
è
=
For the lower performance models, a 3.3V
±
5% supply is
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 2 directly addresses this
requirement. The basic purpose of this design is to provide
an automatic switch between a single switched-mode power
supply and a dual switching/linear power supply depending
upon which CPU occupies the socket.
To ease the task of identifying the CPU, the P55C processor
includes a single-bit identification pin,
VCC2DET,
at
location 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. Therefore, the addition of an external pull-up
resistor allows the user to easily identify which processor
occupies the CPU socket. The circuit in figure 2 also uses the
CPU identification pin to select the appropriate output
voltage for the CPU core power island and switch off the
linear regulator output when only a single output is neces-
sary. To optimize the overall efficiency of the power supply,
the switching converter should be used to power the load that
is most heavily burdened. Therefore, the obvious choice is to
assign the switching converter as the CPU core supply.
Using this configuration, the switched-mode supply will
always be loaded whereas the linear regulator portion will
only be burdened when the dual voltages are required.
Because the I/O circuitry will always operate from a nominal
3.3V supply, the linear regulator output is set at a fixed 3.3V
output using resistors R2 and R3. The CPU core supply is
thus switched between 2.8V and 3.5V using an external FET
and the appropriate combination of resistors. Using this cir-
cuit configuration, the switched-mode supply can source up
to 10A, while the linear regulator portion can be loaded to
5A. These current ranges will easily accomodate the stan-
dard Pentium P54C/P54C and the P55C as well as other
Pentium compatible processors such as the Cyrix 6x86.
Detailed Product Description
The RC5035 combines a programmable voltage step-down
DC-DC controller with a similarily programmable low drop-
out linear regulator. When designed around the appropriate
external components, it can be configured to deliver more
than 5A of load current with both outputs loaded and up to
10A from the switched-mode regulator. During all loading
conditions the switched-mode portion of the RC5035 func-
tions as a constant-on-time PWM step-down regulator.