# Fairchild SEMICONDUCTOR RC5051 User Manual

## Page 15

**APPLICATION NOTE**

**AN50**

**15**

** **

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:

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:

where

∆

V is the maximum voltage deviation due to load

transients,

∆

T is the reaction time of the power source (loop

response time for the RC5050 and RC5051 isapproximately

2µ

s), and I

O

is the output load current.

For I

O

= 12.2A (0-13A load step) and

∆

V = 100mV, the bulk

capacitance required can be approximated as follows:

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 2.5

µ

H is rec-

ommended, as illustrated in Figure 14.

**Figure 14. Input Filter**

**Bill of Material**

Table 11 is the Bill of Material for the Application Circuits

of Figure 3 and Figure 4.

ESR

DF

2

π

fC

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

=

C

µ

F

( )

I

O

∆

T

×

∆

V

I

O

ESR

×

–

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

=

C

µ

F

( )

I

O

∆

T

×

∆

V

I

O

ESR

×

–

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

12.2A

2

µ

s

×

100mV

12.2A

7.5m

Ω

×

–

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

2870

µ

F

=

=

=

1000

µ

F, 10V

Electrolytic

0.1

µ

F

65-AP42-17

2.5

µ

H

5V

Vin

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

**Quantity**

**Reference**

**Manufacturer Part **

**Order #**

**Description**

**Requirements and**

**Comments**

7

C4, C5, C7,

C8, C9, C10,

C11

Panasonic

ECU-V1H104ZFX

0.1

µ

F 50V capacitor

1

C6

Panasonic

ECSH1CY475R

4.7

µ

F 16V capacitor

1

Cext

Panasonic

ECU-V1H121JCG

120pF capacitor

1

C12

Panasonic

ECSH1CY105R

1

µ

F 16V capacitor

3

C1, C2, C3

United Chemi-con

LXF16VB102M

1000

µ

F 6.3V electrolytic

capacitor 10mm x 20mm

ESR < 0.047

Ω

4

C13, C14,

C15, C16

Sanyo

6MV1500GX

1500

µ

F 6.3V electrolytic

capacitor 10mm x 20mm

ESR < 0.047

Ω

1

DS1

(note 1)

Motorola

MBR2015CT

Shottky diode, 15A

Vf < 0.52V @ I

f

= 10A

1

D1

Motorola 1N4691

6.2V Zener Diode