AND8079 datasheet(4/20 Pages) ONSEMI

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AND8079/D

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Component Selection

Input Inductor

The input inductor (LIN) is used to isolate the input power

supply from the switching portion of the buck regulator. LIN

also limits the inrush current into the bulk input capacitors

and limits the input current slew rate that results from the

transient load. The inductor blocks the ripple current and

transfers the transient current requirement to the bulk input

capacitor bank.

The design equations for LIN are listed below and for

connivance an inductance of 1.0

mH is chosen. The cut-off

frequency of the second order LC filter provides adequate

attenuation for the 200 kHz switching frequency of the

NCP1571.

LIN +

(dI dt)Max

+ 5V * 2.5 V

10 A

ms + 1.25 mH

* 3db +

LIN CIN

5400

+ 216 Hz

where:

LIN = input inductor

CIN = bulk input capacitor(s)

dI/dt = 10 A in 5.0

Input Capacitors

The input filter capacitors provide a charge reservoir that

minimizes the supply voltage variations due to the pulsating

current through the MOSFETs. The input capacitors are

chosen primarily to meet the ripple current rating of the

capacitors.

The design equation is listed below.

ICin(RMS) + D

* D)

Iout2

+ .5

* .5)

102

+ 5A

where:

D = duty cycle = VOUT/VIN = 2.5 V/5.0 V = 0.5

IOUT = maximum output current

The Rubycon 10 V 1800

mf capacitors have a ripple

current rating of 2.55 A. Thus only 2 of the capacitors are

needed to meet the ripple requirements; however, 3

capacitors were chosen to be conservative.

Output Inductance

The main criterion in selecting the output filter inductance

(LOUT) is to provide a satisfactory response to the load

transients. The inductance affects the output voltage ripple

by limiting the rate at which the current can either increase

or decrease. The design equation used for selecting LOUT is

listed below. A 2.2

mH inductor was chosen for the design.

LOUT +

(VIN * VOUT)

(5 V

* 2.5 V)

10 A

+ 2.5 mH

where:

tr = output transient load time

Output Capacitors

The output capacitors are selected to meet the desired

output ripple requirements. The key specifications for the

capacitors are their ESR (Equivalent Series Resistance) and

ESL (Equivalent Series Inductance). In order to obtain a

good transient response, a combination of low value/high

frequency ceramic

capacitors

and

bulk

electrolytic

capacitors are placed as close to the load as possible.

The voltage change during the load current transient is:

DVOUT + DIOUT

ESL

Dt )

ESR

)

COUT

^ DIOUT

ESR

Empirical data indicates that most of the output voltage

change that results from the load current transients is

determined by the capacitor ESR; therefore, the maximum

allowable ESR can be approximated from the following

equation.

ESR max ^

DVOUT

DIOUT

+ 75 mV

10 A

+ 7.5 mW

The number of capacitors is calculated by using the

equation listed below.

Number of capacitors

ESRCAP

ESR max

+ 19 mW

7.5 m

W +

2.5

The ESR of the Rubycon 6.3 V 1800

mF capacitors is

specified at 19 m

W; therefore, 3 capacitors are used in the

design.

MOSFET Selection

The output switch MOSFETs are chosen based on the gate

charge/gate-source threshold voltage, gate capacitance, on

resistance, current rating and the thermal capacity of the

package. In this DDR design, the MOSFETs were chosen for

economical reasons and have a current and power rating that

is much better than needed for this design. In addition, the

MOSFETs selected were verified by measuring the thermal

characteristics of the devices on the PCB.

The power dissipation design equation for selecting the

MOSFETs is given below.

+ IMAX2

RDS(ON)

)

IMAX

VDS

)

IMAX

VDS

where:

Tr = rise time or turn-on time of MOSFET

Tf = fall time or turn-off time of MOSFET

FS = switching frequency

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