Fairchild specifications RC5050 and RC5051 Description

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APPLICATION NOTE

AN50

 

 

I/O Controls

In addition to the Voltage Identification, there are several sig- nals that control the DC-DC converter or provide feedback from the DC-DC converter to the CPU. They are Power- Good (PWRGD), Output Enable (OUTEN), and Upgrade Present (UP#). These signals will be discussed later.

RC5050 and RC5051 Description

Simple Step-Down Converter

S1

L1

 

 

 

 

 

+

VIN

D1

C1

RL Vout

 

 

 

 

 

 

65-5050-06

Figure 1. Simple Buck DC-DC Converter

Figure 1 illustrates a step-down DC-DC converter with no feedback control. The derivation of the basic step-down con- verter is the basis for the design equations for the RC5050 and RC5051. Referring to Figure 1, the basic operation begins by closing the switch S1. When S1 is closed, the input voltage VIN is impressed across inductor L1. The current flowing in this inductor is given by the following equation:

IL =

(VIN – VOUT )TON

---------------------------------------------L1-

where TON is the duty cycle (the time when S1 is closed).

When S1 opens, the diode D1 conducts the inductor

current and the output current is delivered to the load accord- ing to the following equation:

IL =

VOUT(TS – TON )

-------------------------------------------L1

whereTS is the overall switching period and (TS - TON) is the time during which S1 is open.

By solving these two equations, we can arrive at the basic relationship for the output voltage of a step-down converter:

TON

V = V ----------

OUT IN TS

In order to obtain a more accurate approximation for VOUT, we must also include the forward voltage VD across diode D1 and the switching loss, VSW. After taking into account these factors, the new relationship becomes:

()TON VOUT = VIN + VD – VSW ----------– VD

TS

where VSW = MOSFET switching loss

=IL • RDS,ON

The RC5050 and RC5051 Controllers

The RC5050 is a programmable non-synchronous DC-DC controller IC. The RC5051 is a synchronous version of the RC5050. When designed around the appropriate external components, either of these devices can be configured to deliver more than 14.5A of output current. The RC5050 and RC5051 utilize both current-mode and voltage-mode PWM control to create an integrated step-down voltage regulator. The key differences between the RC5050 and RC5051 are listed in Table 4.

Table 4. RC5050 and RC5051 Differences

 

RC5051

RC5050

 

 

 

Operation

Synchronous

Non-Synchronous

 

 

 

Package

20-SOIC

20-SOIC

 

 

 

Output Enable/

Yes

Yes

Disable

 

 

 

 

 

Main Control Loop

Refer to the RC5051 Block Diagram illustrated in Figure 2. The control loop of the regulator contains two main sections; the analog control block and the digital control block. The analog section consists of signal conditioning amplifiers feeding into a set of comparators which provide the inputs to the digital control block. The signal conditioning section accepts inputs from the IFB (current feedback) and VFB (voltage feedback) pins and sets up two controlling signal paths. The voltage control path amplifies the VFB signal and presents the output to one of the summing amplifier inputs. The current control path takes the difference between the IFB and VFB pins and presents the resulting signal to another input of the summing amplifier. These two signals are then summed together with the slope compensation input from the oscillator. This output is then presented to a comparator, which provides the main PWM control signal to the digital control block.

The additional comparators in the analog control section set the point at which the current limit comparator disables the output drive signals to the external power MOSFETs.

The digital control block takes the comparator inputs and the main clock signal from the oscillator to provide the appropri- ate pulses to the HIDRV and LODRV output pins. These pins control the external power MOSFETs. The digital sec- tion utilizes high speed Schottky transistor logic, allowing the RC5050 and the RC5051 to operate at clock speeds as high as 1MHz.

High Current Output Drivers

The RC5051 contains two identical high current output drivers that utilize high speed bipolar transistors in a push-pull configuration. Each driver is capable of delivering 1A of current in less than 100ns. Each driver’s power and ground are separated from the chip’s power and ground for additional switching noise immunity.

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Contents Introduction Intel Pentium Pro Processor Power RequirementsOutput Voltage Programming Codes Output Ripple and NoiseEfficiency Processor Voltage IdentificationRC5050 and RC5051 Description Internal Voltage Reference Power Good PwrgdOutput Enable Outen Upgrade Present UP#Design Considerations and Component Selection Short Circuit ProtectionOscillator RC5051 Mosfet Selection Table Mosfet Selection CosiderationsLess heat sink required Calculation of Converter Efficiency Under Heavy Loads Converter EfficiencyShort Circuit Comparator Selecting the InductorImplementing Short Circuit Protection RC5050 Short Circuit Comparator Threshold VoltageRsense for Various Load Currents Comparison of Sense ResistorsSummary PC Trace Resistor Tolerance 16%Total Tolerance for PC Trace Resistor For each Mosfet RC5050 and RC5051 Short Circuit Current CharacteristicsManufacturer Schottky Diode SelectionSchottky Diode Selection Table Output Filter CapacitorsInput Filter Bill of MaterialBill of Materials for a 13A Pentium Pro Klamath Application ERJ-6ENF10.0KY PCB Layout Guidelines and ConsiderationsPCB Layout Guidelines ERJ-6GEY050YExample of a PC Motherboard Layout and Gerber File Performance Evaluation ProceduresGuidelines for Debugging and Performance Evaluations Debugging Your First Design Implementation486 11010 505 504 501 496 493 492 491 490989 488Device Description Case TemperatureEvaluation Summary Iload = 13.9AApplication Note RC5050 Evaluation Board SummaryDirectory of Component Suppliers Appendix aLife Support Policy