US3033

4-9

Rev. 1.5

1/14/99

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

hysterises resistor.

Assuming, Rt=1k

Ω , 1%

Rh=(11*Rt)/

∆Vo

Where:

Rt=Top resistor of the resistor divider

Rh=Hysterises resistor connected between pins 3 and

4 of the US3033

∆Vo=Selected output ripple (typically 1% to 2% of out-

put voltage)

Assuming,

∆Vo=44mV

Rh=(11*1000)/0.044=250 k

Ω

Select Rh=249 k

Ω , 1%

The bottom resistor of the divider is then calculated us-

ing the following equations:

Rb=Rt/X

Where:

Rb=Bottom resistor of the divider

X=

[(Vo + (∆Vo/2))/Vref] - 1

Vref=1.25 V typ.

X=

[(3.38+ (0.044/2))/1.25] - 1 = 1.72

Rb=1000/1.72=580

Ω

Select Rb=576

Ω , 1%

Frequency Calculation

The US3033 frequency of operation is calculated using

the following formula:

Fs=

[(Vo*(1-D)*ESR)]/(L*∆Vo) (MHz)

Where:

Vo=Output voltage (V)

D=Duty cycle

ESR=Output capacitor ESR (V)

L=Output inductance (uH)

∆Vo=Output ripple voltage (V)

For our example:

D

≈(Vo + Vf)/Vin

Where, Vf=Forward voltage drop of the Schotky diode

D=(3.38 + 0.5)/5=0.78

The ESR=18m

Ω for 2 of the Sanyo 1500uF, 6MV1500GX

caps. If L=3.5uH then, Fs is calculated as follows:

Fs=

[(3.38*(1-0.78)*0.018)]/(3.5*0.044)= 0.087 Mhz

= 87 kHz