US3033 查看數據表（PDF） - Unisem
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US3033 Datasheet PDF : 9 Pages
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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=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 US3033
∆Vo=Selected output ripple (typically 1% to 2% of out-
put 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 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=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. 1.5
1/14/99
US3033
4-9
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