APPLICATION NOTE

AN42

 

 

where:

VIN = Input Voltage to the Converter

VSW = Voltage Across the MOSFET = ILOAD x RDS,ON

VD = Forward Voltage of the Schottky Diode

T = The Switching Period of the Converter = 1/fS, Where fS = Switching Frequency.

For an input voltage of 5V, an output voltage of 3.3V, an inductor value of 1.3H, and a switching frequency of 650KHz (using CEXT = 39pF), the inductor current can be calculated as follows:

(Ipk

– Imin )

(5.0 – 14.5 0.037 – 3.3)

----------------------------

=

------------------------------------------------------------

–6

 

-

 

2

1.3 10

 

 

 

 

 

 

 

 

 

 

(3.3 + 0.5)

 

1

 

 

 

(-------------------------------------------------------------5.0 – 14.5 0.037 + 0.5) -----------------------

650

3

= 1.048A

 

 

 

10

 

 

Therefore, for a continued load current of 14.5A, the peak current through the inductor, Ipk, is found to be:

Table 7. Comparison of Sense Resistors

ISC Iinductor = ILoad, max +

(IPK – Imin )

= 14.5 + 1 = 15.5A

-----------------------------2

For continuous operation at 14.5A, the short circuit detection threshold must be at least 15.5A.

The next step is to determine the value of the sense resistor. Including tolerance, the sense resistor value can be approxi- mated as follows:

RSENSE =

Vth,min

⋅ (1 – TF) =

Vth,min

⋅ (1 – TF)

----------------ISC

1.0---------------------------------+ ILoad,max-

 

 

 

where TF = Tolerance Factor for the sense resistor.

Several different types of sense resistors exist. Table 7 describes tolerance, size, power capability, temperature coefficient and cost of various sense resistors.

 

 

 

Discrete Metal

 

Discrete

 

 

Discrete Iron

Strip Surface

Discrete MnCu

CuNi Alloy

 

Motherboard

Alloy

Mount Resistor

Alloy Wire

Wire Resistor

Description

Trace Resistor

Resistor (IRC)

(Dale)

Resistor

(Copel)

 

 

 

 

 

 

Tolerance

±29%

±5%

±1%

±10%

±10%

Factor (TF)

 

(±1% available)

 

 

 

 

 

 

 

 

 

Size

2" x 0.2" x 0.001"

0.45" x 0.065" x

0.25" x 0.125" x

0.200" x 0.04" x

0.200" x 0.04" x

(L x W x H)

(1 oz Cu trace)

0.200"

0.025"

0.160"

0.100"

 

 

 

 

 

 

Power capability

> 50A/in

1 watt

1 watt

1 watt

1 watt

 

 

(3W and 5W

 

 

 

 

 

available)

 

 

 

 

 

 

 

 

 

Temperature

+4,000 ppm

+30 ppm

±75 ppm

±30 ppm

±20 ppm

Coefficient

 

 

 

 

 

 

 

 

 

 

 

Cost

Low

$0.31

$0.47

$0.09

$0.09

@10,000 piece

included in

 

 

 

 

 

motherboard

 

 

 

 

 

 

 

 

 

 

Refer to Appendix A for Directory of component suppliers

Based on the Tolerance in the above table, for embedded PC trace resistor and for Iload,max = 14.5A:

RSENSE = ---------------------------------------

Vth,min

⋅ (1 – TF) =

-

1.0A + ILoad, max

 

100mV

⋅ (1 – 29%) = 4.6mΩ

1.0A---------------------------------+ 14.5A

For a discrete resistor and Iload, max = 14.5A:

RSENSE = ---------------------------------------

Vth,min

⋅ (1 – TF) =

-

1.0A + ILoad, max

 

100mV

⋅ (1 – 5%) = 6.1mΩ

1.0A---------------------------------+ 14.5A

For user convenience, Table 8 lists the recommended values for sense resistor at various load currents using an embedded PC trace resistor or discrete resistor.

11

Page 11
Image 11
Fairchild RC5040, RC5042 specifications Discrete Metal, Description, Resistor IRC

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.