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型號(hào)：	RC5036
文件頁(yè)數(shù)：	474/1328頁(yè)
文件大?。?/td>	9519K
代理商：	RC5036

PRODUCT SPECIFICATION

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

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).

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,

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.

相關(guān)PDF資料	PDF描述
RC5036AP
RC5037A
RC5039
RC5040
RC5043

相關(guān)代理商/技術(shù)參數(shù)	參數(shù)描述
RC5036AP	制造商:未知廠家制造商全稱:未知廠家功能描述:
RC5036M	制造商:未知廠家制造商全稱:未知廠家功能描述:Current-Mode SMPS Controller
RC5037	制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:Adjustable Switching Regulator Controller
RC5037A	制造商:未知廠家制造商全稱:未知廠家功能描述:
RC5037A WAF	制造商:Fairchild Semiconductor Corporation 功能描述:

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