US3036

4-7

Rev. 1.1

12/4/98

Switcher Output Voltage Setting

The output voltage 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 US3034

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

output 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

using 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 US3034 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