IRU3033

10

Rev. 1.7

07/17/02

www.irf.com

Frequency Calculation

The IRU3033 frequency of operation is calculated using

the following formula:

Where:

Vo = Output voltage

(V)

D = Duty cycle

ESR = Output capacitor ESR

(V)

L = Output inductance

(

mH)

DVo = Output ripple voltage

(V)

For our example:

Where:

Vf = Forward voltage drop of the Schottky diode.

The ESR=18m

V for 2 of the Sanyo 1500mF, 6MV1500GX

caps. If L=3.5

mH then, Fs is calculated as follows:

D = (3.38 + 0.5) / 5 = 0.78

Fs =

[(Vo3(1 - D)3ESR)] / (L3DVo)

(MHz)

ripple is:

The advantage of fixed output ripple is that when the

output voltage changes from 2V to 3.5V, the ripple volt-

age remains the same which is important in meeting the

Intel maximum tolerance specification.

Switcher Output Voltage Setting

The output voltage of the switcher can be set using the

following equations:

Assuming, Vo=3.38V and the selected output ripple is

≈

1.3%(44mV) of the output voltage, a set of equations

are derived that selects the resistor divider and the hys-

teresis resistor.

Assuming, Rt=1K

V , 1%:

Where:

Rt = Top resistor of the resistor divider.

and 4 of the IRU3033.

DVo = Selected output ripple (typically 1% to 2% of

output voltage).

Assuming,

DVo=44mV:

V, 1%

The bottom resistor of the divider is then calculated us-

ing the following equations:

Where:

V, 1%

IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, California 90245, USA Tel: (310) 252-7105

TAC Fax: (310) 252-7903

Visit us at www.irf.com for sales contact information

Data and specifications subject to change without notice. 02/01

DVo = (1/422)311 = 26mV

3Rt) / DV

31000) / 0.044 = 250KV

X =

[(Vo +

] - 1

X =

[(3.38 +

)/1.25

] - 1 = 1.72

V

DVo

2

0.044

2

D

Fs =

= 0.087MHz

Fs = 87KHz

[(3.38 3 (1 - 0.78) 3 0.018)]

(3.5

3 0.044)