APPLICATION NOTE

AN42

 

 

Over-Voltage Protection

The RC5040 and RC5042 constantly monitor the output voltage for protection against over voltage. If the voltage at the VFB pin exceeds 20% of the selected program voltage, an over-voltage condition is assumed, and the controller dis- ables the output drive signal to the external MOSFET(s).

Short Circuit Protection

A current sense methodology is implemented to disable the output drive signal to the MOSFET(s) when an over-current condition is detected. The voltage drop created by the output current flowing across a sense resistor is presented to an internal comparator. When the voltage developed across the sense resistor exceeds the comparator threshold voltage, the controller disables the output drive signal to the MOSFET(s).

The DC-DC converter returns to normal operation after the fault has been removed, for either an over voltage or a short circuit condition.

Oscillator

The RC5040 oscillator section is implemented using a fixed current capacitor charging configuration. An external capacitor (CEXT) is used to preset the oscillator frequency between 200KHz and 1MHz. This allows maximum flexibil- ity in setting the switching frequency and in choosing exter- nal components.

In general, a lower operating frequency increases the peak ripple current flowing through the output inductor, allowing the use of a larger inductor value. Operation at lower fre- quencies increases the amount of energy storage that the bulk output capacitors must provide during load transients that occur due to the slower loop response of the controller.

In addition, note that the efficiency losses due to switching are relatively fixed per switching cycle. Therefore, as the switching frequency increases, the contribution toward effi- ciency due to switching losses also increases.

RC5040 has an optimal operating frequency of 650KHz. This frequency allows the use of smaller inductive and capacitive components while optimizing peak efficiency under all operating conditions.

Design Considerations and Component Selection

Application Circuits

Figure 3 illustrates a typical non-synchronous application using the RC5040. Figure 4 shows a typical synchronous application using the RC5040, and Figure 5 shows a typical non-synchronous application using the RC5042.

 

L2

 

 

 

 

 

 

 

 

 

 

 

VCC

2.6H

 

 

 

 

 

 

 

 

 

 

 

C4

 

 

C3

C5

 

 

 

 

 

 

 

C1

C2

 

 

 

 

 

 

 

 

0.1F

1000F

1000F

1000F

0.1F

 

 

 

 

 

 

 

 

 

 

 

 

 

DS2

C8

C9

 

 

 

 

 

 

 

 

 

 

1N5817

 

 

 

 

 

 

 

 

 

 

0.1F

0.1F

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

M1

M2

 

 

 

 

 

 

11

 

 

10

C12

 

2SK1388

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

12

 

 

9

1F

 

 

 

 

 

 

R7

 

13

 

 

8

2SK1388

L1

RSENSE

 

 

 

 

 

 

 

 

 

 

 

 

14

 

 

7

 

 

 

 

 

 

VO

10K

 

 

 

 

 

1.3H

8mΩ

 

 

 

15

RC5040

6

C6

 

1500F

1500F

1500F

 

 

 

 

 

 

 

4.7F

 

 

 

VREF

 

16

 

 

5

 

 

 

 

17

 

 

4

 

 

 

 

 

 

 

C7

 

 

 

 

DS1

 

 

 

 

 

 

18

 

 

3

 

 

 

C13

C14

C15

 

 

 

 

 

MBR1545CT

 

0.1F

 

19

 

 

2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

GND

 

20

 

 

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CEXT

 

 

 

 

 

 

 

 

 

 

 

39pF

 

 

 

 

 

 

 

 

VID3

R1

 

10K

 

 

VCC

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VID2

R2

 

10K

 

R6

 

 

 

 

 

 

 

 

 

 

 

10K

 

 

 

 

 

 

 

 

R3

 

10K

 

 

 

 

 

 

 

 

VID1

 

 

 

PWRGD

 

 

 

65-AP42-03

 

 

 

 

 

 

 

 

 

 

VID0

R4

 

10K

 

VCC

C11

 

 

 

 

 

 

 

 

 

 

0.22F

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

OUTEN

 

 

 

 

R5

 

 

 

 

 

 

 

 

 

 

 

10K

 

 

 

 

 

 

 

 

 

 

 

C10

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.1F

 

 

 

 

 

 

 

Figure 3. Non-Synchronous DC-DC Converter Application Schematic Using RC5040

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Fairchild RC5040, RC5042 Design Considerations and Component Selection, Over-Voltage Protection, Short Circuit Protection

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.