IRU3033CS International Rectifier, IRU3033CS Datasheet - Page 10

• Manufacturer Part Number: IRU3033CS
• Description: IC CTRL/REG PWM SWITCH 8-SOIC
• Manufacturer: International Rectifier

Specifications of IRU3033CS

• Applications: Controller, Intel Pentium®, II, P55C
• Voltage - Input: 12V
• Number Of Outputs: 1
• Voltage - Output: 2 ~ 3.5 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *IRU3033CS; IRU3033-CS; IRU3033-CS

IRU3033(PbF)
For example, if Rt=1K and R
ripple is:
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 hys-
teresis resistor.
Assuming, Rt=1KΩ, 1%:
Assuming, ∆Vo=44mV:
The bottom resistor of the divider is then calculated us-
ing the following equations:
10
∆Vo = (1/422)×11 = 26mV
R
Where:
Rt = Top resistor of the resistor divider.
R
and 4 of the IRU3033.
∆Vo = Selected output ripple (typically 1% to 2% of
output voltage).
R
Select R
R
Where:
R
V
X = (Vo +
X = (3.38 +
R
Select R
REF
H
H
H
B
B
B
= Bottom resistor of the divider
= Hysteresis resistor connected between pins 3
= (11×1000) / 0.044 = 250KΩ
= Rt / X
= 1000 / 1.72 = 580Ω
= (11×Rt) / ∆V
= 1.25V typical
H
B
= 576Ω, 1%
= 249KΩ, 1%
∆Vo
0.044
2
2
)/V
)/1.25 - 1 = 1.72
REF
- 1
B
=422K, then the output
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Frequency Calculation
The IRU3033 frequency of operation is calculated using
the following formula:
For our example:
The ESR=18mΩ for 2 of the Sanyo 1500µF, 6MV1500GX
caps. If L=3.5µH then, Fs is calculated as follows:
Fs = (Vo×(1 - D)×ESR) / (L×∆Vo)
D (Vo + Vf) / V
D = (3.38 + 0.5) / 5 = 0.78
Fs =
Fs = 87KHz
Where:
Vo = Output voltage (V)
D = Duty cycle
ESR = Output capacitor ESR
L = Output inductance (µH)
∆Vo = Output ripple voltage (V)
Where:
Vf = Forward voltage drop of the Schottky diode.
(3.38 × (1 - 0.78) × 0.018)
(3.5 × 0.044)
IN
(V)
= 0.087MHz
(MHz)