Fairchild RC5040 Schottky Diode Selection Table, Output Filter Capacitors, Input filter, Model #

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 8∝s.

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

IO ⋅ ΔT	10A ⋅ 8∝s	= 1454∝F
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 2.5∝H 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

5V2.5∝H

0.1∝F

Vin

1000∝F, 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	0.1∝F 50V capacitor
C10	ECU-V1H104ZFX
C6	Panasonic	4.7∝F 16V capacitor
	ECSH1CY475R
Cext	Panasonic	39pF capacitor
	ECU-V1H121JCG
C12
C1, C2, C3	United Chemicon	1000∝F 6.3V electrolytic	ESR<0.047Ω
	LXF16VB102M	capacitor 10mm x 20mm
C11	Panasonic	0.22∝F 50V capacitor
	ECU-V1H224ZFX

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