Fairchild RC5051, RC5050 specifications Input Filter, Bill of Material

APPLICATION NOTE AN50

FET. Low Equivalent Series Resistance (ESR) capacitors are best suited for this type of application. Incorrect selection can hinder the converter's overall performance. 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.

The ESR rating of a capacitor is a difficult number to quantify. ESR is defined as the resonant impedance of the capacitor. Since the capacitor is actually a complex imped- ance device having resistance, inductance, and capacitance, it is natural for this device 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 capacitor manufacturers do not supply ESR data. A useful estimate of the ESR can be 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.

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(µF)	IO × ΔT
	= Δ------------------------------------V – IO × ESR-

where ΔV is the maximum voltage deviation due to load transients, ΔT is the reaction time of the power source (loop response time for the RC5050 and RC5051 isapproximately 2µs), and IO is the output load current.

For IO = 12.2A (0-13A load step) and ΔV = 100mV, the bulk capacitance required can be approximated as follows:

C(µF)	IO × ΔT	12.2A × 2µs	= 2870µF
	= Δ------------------------------------V – IO × ESR-=	100mV---------------------------------------------------------------– 12.2A × 7.5mΩ

Because the control loop response of the controller is not instantaneous, the initial load transient must be supplied entirely by the output capacitors. The initial voltage deviation is determined by the total ESR of the capacitors used and the parasitic resistance of the output traces. For a detailed analysis of capacitor requirements in a high-end microprocessor system, please refer to Application Bulletin 5.

Input Filter

The DC-DC converter should include an input inductor between the system +5V supply and the converter input as described below. This inductor serves to isolate the +5V supply from the noise in the switching portion of the DC-DC converter, and to limit the inrush current into the input capacitors during power up. A value of 2.5µH is rec- ommended, as illustrated in Figure 14.

5V		2.5∝H	Vin
	0.1∝F		1000∝F, 10V
			Electrolytic
			65-AP42-17

Figure 14. Input Filter

Bill of Material

Table 11 is the Bill of Material for the Application Circuits of Figure 3 and Figure 4.

Table 11. Bill of Materials for a 13A Pentium Pro Klamath Application

Quantity	Reference	Manufacturer Part	Description	Requirements and
		Order #		Comments
7	C4, C5, C7,	Panasonic	0.1µF 50V capacitor
	C8, C9, C10,	ECU-V1H104ZFX
	C11
1	C6	Panasonic	4.7µF 16V capacitor
		ECSH1CY475R
1	Cext	Panasonic	120pF capacitor
		ECU-V1H121JCG
1	C12	Panasonic	1µF 16V capacitor
		ECSH1CY105R
3	C1, C2, C3	United Chemi-con	1000µF 6.3V electrolytic	ESR < 0.047 Ω
		LXF16VB102M	capacitor 10mm x 20mm
4	C13, C14,	Sanyo	1500µF 6.3V electrolytic	ESR < 0.047 Ω
	C15, C16	6MV1500GX	capacitor 10mm x 20mm
1	DS1	Motorola	Shottky diode, 15A	Vf < 0.52V @ If = 10A
	(note 1)	MBR2015CT
1	D1	Motorola 1N4691	6.2V Zener Diode

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