US3036
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=1kΩ , 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 kΩ
Select Rh=249 kΩ , 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=18mΩ 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
Rev. 1.1
12/4/98
4-7
UNISEM [ UNISEM ]
