US3033 UNISEM, US3033 Datasheet - Page 9

• Manufacturer Part Number: US3033
• Description: PWM SWITCHER & LINEAR CONTROLLER IC
• Manufacturer: UNISEM

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
are derived that selects the resistor divider and the
hysterises resistor.
Assuming, Rt=1k , 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
put voltage)
Assuming, Vo=44mV
Rh=(11*1000)/0.044=250 k
Select Rh=249 k , 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
Select Rb=576
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)
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
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
Rev. 1.5
1/14/99
= 87 kHz
Vo=Selected output ripple (typically 1% to 2% of out-
Vo=Output ripple voltage (V)
1.3%(44mV) of the output voltage, a set of equations
for 2 of the Sanyo 1500uF, 6MV1500GX
, 1%
US3033
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