APPLICATION NOTE

AN42

 

 

Table 10. Schottky Diode Selection Table

Manufacturer

 

Forward Voltage

Model #

Conditions

VF

Philips

IF = 20A; Tj=25°C

< 0.84v

PBYR1035

IF = 20A; Tj=125°C

< 0.72v

Motorola

IF = 20A; Tj=25°C

< 0.84v

MBR2035CT

IF = 20A; Tj=125°C

< 0.72v

Motorola

IF = 15A; Tj=25°C

< 0.84v

MBR1545CT

IF = 15A; Tj=125°C

< 0.72v

Motorola

IF = 20A; Tj=25°C

< 0.58v

MBR2015CTL

IF = 20A; Tj=150°C

< 0.48v

Output Filter Capacitors

Output ripple performance and transient response are functions of the filter capacitors. Since the 5V supply of a PC motherboard may be located several inches away from the DC-DC converter, the input capacitance can play an impor- tant role in the load transient response of the RC5040.

The higher the input capacitance, the more charge storage is available for improving the current transfer through the FET(s). Capacitors with low Equivalent Series Resistance (ESR) are best for this type of application and can influence the converter's efficiency if not chosen carefully. The input capacitor should be placed as close to the drain of the FET as possible to reduce the effect of ringing caused by long trace lengths.

With this in mind, correct calculation of the output capaci- tance is crucial to the performance of the DC-DC converter. The output capacitor determines the overall loop stability, output voltage ripple, and load transient response. The calcu- lation is as follows:

C(∝F)

IO ⋅ ΔT

= ------------------------------------ΔV – IO ESR-

where ΔV is the maximum voltage deviation due to load transients, ΔT is the reaction time of the power source, and IO is the output load current. ΔV is the loop response time of the RC5040 and RC5042, approximately 8s.

For IO = 10A and ΔV = 165mV, the bulk capacitance required can be approximated as follows:

IO ⋅ ΔT

10A 8s

= 1454F

C(∝F) = ------------------------------------ΔV – IO ESR- =

165mV--------------------------------------------------------- 10A 11mΩ

Input filter

The DC-DC converter design should include an input induc- tor between the system +5V supply and the converter input as described below. This inductor will serve to isolate the +5V supply from noise occurring in the switching portion of the DC-DC converter and also to limit the inrush current into the input capacitors during power up. An inductor value of around 2.5H is recommended, as illustrated in Figure 15.

ESR is the resonant impedance of the capacitor, and it is dif- ficult to quantify. Since the capacitor is actually a complex impedance device having resistance, inductance, and capaci- tance, it is natural for it to have a resonant frequency. As a rule, the lower the ESR, the better suited the capacitor is for use in switching power supply applications. Many manufac- turers do not supply ESR data, but a useful estimate can be

5V2.5H

0.1F

Vin

1000F, 10V Electrolytic

65-AP42-17

obtained using the following equation:

DF

ESR = ------------

2πfC

where DF is the dissipation factor of the capacitor, f is the operating frequency, and C is the capacitance in farads.

Figure 15. Input Filter

Bill of Materials

The Bill of Materials for the application circuits of Figures 2 through 4 is presented in Table 11.

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

C4, C5, C7, C8, C9,

Panasonic

0.1F 50V capacitor

 

C10

ECU-V1H104ZFX

 

 

 

 

 

 

C6

Panasonic

4.7F 16V capacitor

 

 

ECSH1CY475R

 

 

 

 

 

 

Cext

Panasonic

39pF capacitor

 

 

ECU-V1H121JCG

 

 

 

 

 

 

C12

 

 

 

 

 

 

 

C1, C2, C3

United Chemicon

1000F 6.3V electrolytic

ESR<0.047Ω

 

LXF16VB102M

capacitor 10mm x 20mm

 

 

 

 

 

C11

Panasonic

0.22F 50V capacitor

 

 

ECU-V1H224ZFX

 

 

 

 

 

 

15

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Fairchild RC5040, RC5042 Schottky Diode Selection Table, Output Filter Capacitors, Input filter, Bill of Materials

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.