Fairchild RC5042, RC5040 specifications Schottky Diode Selection

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AN42

 

APPLICATION NOTE

 

 

 

 

PD, Diode = IF, ave ⋅ VF ⋅ (1 – DutyCycle) =

 

14 ⋅ 0.45 ⋅ 0.8 ≈ 5W

 

 

 

Thus for the Schottky diode, the thermal dissipation during

 

a short circuit is greatly magnified and requires that the

 

thermal dissipation of the diode be properly managed by the

 

appropriate choice of a heat sink. In order to protect the

 

Schottky from being destroyed in the event of a short, we

 

should limit the junction temperature to less than 130°C.

 

Using the equation for maximum junction temperature,

 

we can arrive at the thermal resistance required below:

 

TJ(max) – TA

 

 

 

PD = -------------------------------

 

 

Figure 14A. VCCQP Output Waveform for Normal

RΘJA

 

 

Assuming that the ambient temperature is 50°C, we get:

Operation Condition with Vout = 3.3V@10A

 

TJ(max) – TA

130 – 50

 

RΘJA = -------------------------------

= --------------------= 16°C ⁄ W

 

PD

5

 

Thus we need to provide for a heat sink that will give the Schottky diode a thermal resistance of at least 16°C/W or lower in order to protect the device during an indefinite short.

In summary, with proper heat sink, the Schottky diode is not being over stressed during a short circuit condition.

Figure 14B. VCCQP Output Waveform for

Output Shorted to Ground

The Schottky diode has a power dissipation consideration during the short circuit condition. During normal operation, the diode dissipates power when the power MOSFET is off. The power dissipation is given by:

PD, Diode = IF ⋅ VF ⋅ (1 – DutyCycle) =

14.5 ⋅ 0.5V ⋅ (1 – 0.62) = 2.75W

In short circuit mode, the duty cycle is dramatically reduced to approximately 20%. The forward current during a short circuit condition decays exponentially through the inductor. The power dissipated on the diode during the short circuit condition, is approximated by:

1

1.5us

IF, ending = Isc ⋅ e– L-----------⁄R = 20A ⋅ e–

1.3us-------------≈ 7.9A

IF, ave ≈ (20A + 7.9A) ⁄ 2 ≈ 14A

 

Schottky Diode Selection

The application circuits of Figures 3, 4, and 5 show two Schottky diodes, DS1 and DS2. In synchronous mode, DS1 is used in parallel with M3 to prevent the lossy diode in the FET from turning on. In non-synchronous mode, DS1 is used as a flyback diode to provide a constant current path for the inductor when M1 is turned off.

The Schottky diode DS2 serves a dual purpose. As config- ured in Figures 3, 4, and 5, DS2 allows the VCCQP pin on the RC5040 to be bootstrapped up to 9V using capacitor C12. When the lower MOSFET M3 is turned on, one side of capacitor C12 is connected to ground while the other side of the capacitor is being charged up to voltage VIN – VD through DS2. The voltage that is then applied to the gate of the MOSFET is VCCQP – VSAT, or typically around 9V. DS2 also provides correct sequencing of the various supply voltages by assuring that VCCQP is not enabled before the other supplies.

A vital selection criteria for DS1 and DS2 is that they exhibit a very low forward voltage drop, as this parameter can directly affect the regulator efficiency. Table 10 lists several suitable Schottky diodes. Note that the MBR2015CTL has a very low forward voltage drop. This diode is ideal for appli- cations where output voltages less than 2.8V are required.

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Contents Input Voltages Pentium Pro DC Power RequirementsIntroduction DC Voltage RegulationProcessor Voltage Identification Output Ripple and NoiseEfficiency ControlsRC5040 and RC5042 Controllers RC5040 and RC5042 DescriptionSimple Step-Down Converter Upgrade Present UP# Power Good PwrgdOutput Enable Outen Main Control LoopShort Circuit Protection Design Considerations and Component SelectionOver-Voltage Protection OscillatorRC5042 Thermal Mosfet SelectionTwo MOSFETs in Parallel Conditions1 Manufacturer & Model # Typ MaxCharge Pump or Bootstrap Mosfet Gate BiasConverter Efficiency Short Circuit Comparator Selecting the InductorImplementing Short Circuit Protection Resistor IRC Discrete MetalDescription ResistorResistor mΩ = 2000mi⋅ .2 = 0.74W RC5040 and RC5042 Short Circuit Current CharacteristicsFor each Mosfet Schottky Diode Selection Input filter Schottky Diode Selection TableOutput Filter Capacitors Bill of MaterialsMotorola Shottky Diode PCB Layout Guidelines and ConsiderationsPCB Layout Guidelines 320-6110Example of Proper MOSFETs Placements PC Motherboard Layout and Gerber FileApplication Note Debugging Your First Design Implementation TroubleshootingGuidelines for Debugging and Performance Evaluations 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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