ML4863

7

DESIGN CONSIDERATIONS

DESIGN PROCEDURE

A typical design can be implemented by using the

following procedure.

1.

Specify the application by defining:

The maximum output ripple (

As a general design rule, the output ripple should be kept

below 100mV to ensure stability.

2.

3a.

Determine the inductance required for the

optimum output ripple using equation 2.

3b.

Determine the minimum inductor current rating

required. The peak inductor current is calculated

using the following formula:

I

mV

R

V

L

LPEAK

SENSE

IN MAX

P

=+

™™

-

235

25 10

6

()

(.

)

(4)

3c.

Specify the inductor's DC winding resistance. A

good rule of thumb is to allow 5m

W, or less, of

resistance per µH of inductance. For minimum

core loss, choose a high frequency core material

such as Kool-Mu, ferrite, or MPP.

3d.

Specify the coupled inductor's turns ratio:

Np : Ns = 1:1

4a.

Calculate the minimum output capacitance

required using:

CI

VV

VV

OUT MAX

OUT

IN MAX

OUT

OUT

=™

+







™

-

()

()

.

25 10

6

D

(5)

4b.

Establish the maximum allowable ESR for the

ouput capacitor:

R

VR

mV

ESR

OUT

SENSE

<

™

D

150

(6)

5.

As a final design check, evaluate the system

stability (no compensation, single pole response)

by using the following equation:

∆V

RV

V

L

OUT

SENSE

OUT

IN MIN

P

≤×

×

×+



!

"

$#

−

()

()

()

610

6

(7)

used.

See Table 1 for suggested component manufacturers.

Table 1. Component Suppliers

Part

Component Manufacturer

Number

Phone

Sense

Dale

LRC Series

(402) 563-6506

Resistors

IRC

WSL Series

(512) 992-7900

Inductors

Coilcraft

R4999

(847) 639-6400

Coiltronics

OCTA-PAC Series (561)241-7876

Dale

LPE-6562 Series (605) 665-9301

LPT-4545 series

Capacitors

AVX

TPS series

(207) 282-5111

Sprague

593D Series

(207) 324-4140

MOSFETs

National

NDS94XX

(800) 272-9954

NDS99XX

Motorola

MMDF Series

(602) 897-5056

MMSF Series

Siliconix

Littlefoot Series (408) 988-8000

DESIGN EXAMPLE

1.

Specify the application by defining:

2.

R

mV

mA

V

+

×+

××



4

54

150

500

4

2060 5

085

.

.

(1a)

3a.

Determine the inductance required using

equation 2.

× ×

−

()

.

25 10

6 0 12

6

= 18µH

(2a)

3b.

Determine the minimum inductor current rating

required.

I

mV

m

A

××

×

=

−

235

120

625 10

18 10

279

6

Ω

(.

)

.

–6

(4a)