Fairchild RC5042 PCB Layout Guidelines and Considerations, Motorola Shottky Diode, 320-6110

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AN42APPLICATION NOTE

Table 11. Bill of Materials for a 14.5A Pentium Pro Motherboard Application

C13, C14, C15

Sanyo

1500∝F 6.3V electrolytic

ESR < 0.047 Ω

 

6MV1500GX

capacitor 10mm x 20mm

 

 

 

 

 

DS1

Motorola

Shottky Diode

Vf<0.72V @ If = 15A

(note 1)

MBR1545CT

 

 

 

 

 

 

DS2

General Instruments 1N5817

Schottky Diode

1A, 20V

 

 

 

 

L1

Skynet 320-8107

1.3∝H inductor

 

 

 

 

 

L2*

Skynet

2.5∝H inductor

*Optional – will help re-

 

320-6110

 

duce ripple on 5v line

 

 

 

 

M1, M2, M3

Fuji

N-Channel Logic Level

RDS(ON) < 37m ohm

(note 2)

2SK1388

Enhancement Mode MOSFET

VGS < 4V, ID > 20A

 

 

 

 

Rsense

COPEL

6 milliohm CuNi Alloy Wire

 

 

A.W.G. #18

resistor

 

 

 

 

 

R1, R2, R3, R4, R6,

Panasonic ERJ-6ENF10.0KV

10K 5% Resistors

 

R7

 

 

 

 

 

 

 

U1

Raytheon

DC-DC Converter for Pentium

 

 

RC5042M or RC5040M

Pro

 

 

 

 

 

Refer to Appendix A for Directory of component suppliers.

Notes:

1.In synchronous mode using the RC5040, a 1A schottky diode (1N5817) may be substituted for the MBR1545CT.

2.MOSFET M3 is only required for the RC5040 synchronous application.

PCB Layout Guidelines and Considerations

PCB Layout Guidelines

•Placement of the MOSFETs relative to the RC5040 is critical. The MOSFETs (M1 & M2), should be placed such that the trace length of the HIDRV pin to the FET gate is minimized. A long lead length causes high amounts of ringing due to the inductance of the trace and the large gate capacitance of the FET. This noise radiates all over the board, and because it is switching at a high voltage and frequency, it is very difficult to suppress.

Figure 16 shows an example of proper MOSFET placement in relation to the RC5040. It also shows an example of problematic placement for the MOSFETs.

In general, noisy switching lines should be kept away from the quiet analog section of the RC5040. That is, traces that connect to pins 12 and 13 (HIDRV and VCCQP) should be kept far away from the traces that connect to pins 1 through 5, and pin 16.

•Place the 0.1∝F decoupling capacitors as close to the RC5040 and RC5042 pins as possible. Extra lead length negates their ability to suppress noise.

•Each VCC and GND pin should have its own via to the appropriate plane on the board to add isolation between pins

•The CEXT timing capacitor should be surrounded with a ground trace. The placement of a ground or power plane underneath the capacitor provides further noise isolation, and helps to shield the oscillator from the noise on the PCB. This capacitor should be placed as close to pin 1 as possible.

•Group the MOSFETs, inductor, and Schottky diode as close together as possible. This minimizes ringing derived from the inductance of the trace and the large gate capacitance of the FET. Place the input bulk capacitors as

close to the drains of MOSFETs as possible. In addition, place the 0.1∝F decoupling capacitors right on the drain

of each MOSFET. This helps to suppress some of the high frequency switching noise on the DC-DC converter input.

•The traces that run from the RC5040 IFB (pin 4) and VFB (pin 5) pins should be run next to each other and be Kelvin connected to the sense resistor. Running these lines together helps to reject some of the common mode noise to the RC5040 feedback input. Run the noisy switching signals (HIDRV & VCCQP) on one layer, and use the inner layers for power and ground only. If the top layer is being used to route all of the noisy switching signals, use the bottom layer to route the analog sensing signals VFB and IFB.

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Contents Pentium Pro DC Power Requirements IntroductionInput Voltages DC Voltage RegulationOutput Ripple and Noise EfficiencyProcessor Voltage Identification ControlsSimple Step-Down Converter RC5040 and RC5042 DescriptionRC5040 and RC5042 Controllers Power Good Pwrgd Output Enable OutenUpgrade Present UP# Main Control LoopDesign Considerations and Component Selection Over-Voltage ProtectionShort Circuit Protection OscillatorRC5042 Mosfet Selection Two MOSFETs in ParallelThermal Conditions1 Manufacturer & Model # Typ MaxCharge Pump or Bootstrap Mosfet Gate BiasConverter Efficiency Implementing Short Circuit Protection Selecting the InductorShort Circuit Comparator Discrete Metal DescriptionResistor IRC ResistorResistor mΩ = 2000miFor each Mosfet RC5040 and RC5042 Short Circuit Current Characteristics⋅ .2 = 0.74W Schottky Diode Selection Schottky Diode Selection Table Output Filter CapacitorsInput filter Bill of MaterialsPCB Layout Guidelines and Considerations PCB Layout GuidelinesMotorola Shottky Diode 320-6110Example of Proper MOSFETs Placements PC Motherboard Layout and Gerber FileApplication Note Guidelines for Debugging and Performance Evaluations TroubleshootingDebugging Your First Design Implementation Performance Evaluation Vout+ 80.0mV Device Description Iload =13.9ASummary RC5040/RC5042 Evaluation BoardAppendix a Directory of Component Suppliers Life Support Policy

RC5040, RC5042 specifications

The Fairchild RC5042 and RC5040 are versatile integrated circuits that stand out in the realm of high-performance analog applications. Designed to meet the demands of modern electronic systems, these devices integrate various features and technologies that contribute to their effectiveness in a multitude of applications.

The RC5040 is a precision voltage reference that offers a stable, low-noise output, making it ideal for applications such as instrumentation, data acquisition systems, and RF circuits. It boasts an operating temperature range of -40°C to +85°C, ensuring reliability in diverse environments. One of its most significant characteristics is its low-temperature drift, which minimizes variations in output voltage over temperature fluctuations, thereby enhancing the accuracy of devices that utilize it.

On the other hand, the RC5042 is designed as a high-speed comparator with an integrated voltage reference. This dual functionality allows for a more compact design in applications where space is a premium. The RC5042 features an ultra-fast response time and high input impedance, which contribute to its capability to handle rapidly changing signals without distortion. This makes it particularly useful in applications like analog signal processing and threshold detection.

Both devices utilize Fairchild's advanced BiCMOS technology, which combines the benefits of bipolar and CMOS processes. This technology allows the devices to operate with low power consumption while maintaining high speed and operational efficiency. The RC5042 and RC5040 also incorporate noise-reduction techniques, which help in minimizing unwanted disturbances that could impact circuit performance.

Another noteworthy characteristic of both the RC5040 and RC5042 is their ease of integration. They come in compact package sizes, making them easier to incorporate into various designs without compromising on performance. Furthermore, the availability of multiple output options allows engineers the flexibility to choose configurations that best suit their specific applications.

In conclusion, the Fairchild RC5042 and RC5040 are robust devices that offer essential functionality for various high-performance analog applications. With their precision, fast response time, and exceptional reliability, these integrated circuits are a valuable asset in the design of modern electronic systems, catering to the growing demands of the technology landscape.
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