IRU3034CSTR International Rectifier, IRU3034CSTR Datasheet - Page 9

• Manufacturer Part Number: IRU3034CSTR
• Description: IC CNTRL PWM SYNC SWITCH 8-SOIC
• Manufacturer: International Rectifier

Specifications of IRU3034CSTR

• 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

The peak power dissipated in the CS resistor is:
However, the average power dissipated is much lower
than 2W due to the long off time caused by the hiccup
circuit of IRU3034. The average power is in fact the short
circuit period divided by the short circuit period plus the
off time or "Hiccup" period. For example, if the short
circuit lasts for Tsc=100µs before the IRU3034 enters
hiccup, the average power is calculated as:
For example:
Without "Hiccup" technique, the power dissipation of the
resistor is 2W.
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 hys-
teresis resistor:
Assuming, Rt = 1KΩ, 1%:
Assuming, ∆Vo=44mV:
Ppk = I
P
Where:
Dsc = Tsc / T
T
M = 200ms/µF
Css = The soft-start capacitor
For Css=0.1µF and Tsc=500µs=0.5ms
T
P
R
Where:
Rt = Top resistor of the resistor divider
R
and 4 of the IRU3034
∆Vo = Selected output ripple (typically 1% to 2% of
output voltage)
R
Select R
HCP
HCP
AVG
AVG
H
H
H
= (11×Rt) / ∆Vo
= (11×1000) / 0.044 = 250KΩ
= Hysteresis resistor connected between pins 3
= Css×M
= 0.1×200 = 20ms
= 0.5×Ppk×Dsc
= 0.5×2×(0.5/20) = 25mW
CL
H
2
×Rs = 20
= 249KΩ, 1%
HCP
2
×0.005 = 2W
www.irf.com
The bottom resistor of the divider is then calculated us-
ing the following equations:
Select R
Frequency Calculation
The IRU3034 frequency of operation is calculated using
the following formula:
For our example:
The ESR=18mΩ
6MV1500GX caps. If L=3.5µH then, Fs is calculated as
follows:
R
Where:
R
V
X = (Vo + (∆Vo/2)) / V
X= (3.38+ (0.044/2)) / 1.25 - 1 = 1.72
R
Fs = (Vo×(1-D)×ESR) / (L×∆Vo)
Where:
Vo = Output voltage (V)
D = Duty cycle
ESR = Output capacitor ESR (V)
L = Output inductance (µH)
∆Vo = Output ripple voltage (V)
D
Where:
Vf = Forward voltage drop of the Schottky diode.
D = (3.38 + 0.5) / 5 = 0.78
Fs =
Fs = 87KHz
REF
B
B
B
= 1000 / 1.72 = 580Ω
= Rt / X
= Bottom resistor of the divider
(Vo + Vf) / V
B
= 1.25V Typical
= 576Ω, 1%
(3.38 × (1-0.78) × 0.018)
(3.5 × 0.044)
IN
for 2 of the Sanyo 1500µF,
IRU3034 & (PbF)
REF
- 1
= 0.087
(MHz)
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