IRU3034CSTR International Rectifier, IRU3034CSTR Datasheet - Page 8

• 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

IRU3034 & (PbF)
APPLICATION INFORMATION
Introduction
The IRU3034 device is an application specific product
designed to provide an on-board switching supply for the
new generation of microprocessors requiring separate
Core and I/O supplies where the load current demand
from the I/O supply requires this regulator to also be a
switching regulator such as the motherboard applica-
tions with AGP slot or the Pentium II with on-board 5V
to 3.3V converter. The IRU3034 provides an easy and
low cost switching regulator solution for Vcore and 3.3V
supplies with true short circuit protection.
Switching Controller Operation
The operation of the switching controller is as follows:
After the power is applied, the output drive pin (Drv) goes
to 100% duty cycle and the current in the inductor
charges the output capacitor causing the output voltage
to increase. When output reaches a pre-programmed
set point the feedback pin (V
the output drive to switch Low and the V
High which jumps the feedback pin higher than 1.25V
resulting in a fixed output ripple which is given by the
following equation:
For example, if Rt=1K and Rh=422K, then the output
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.
Soft-Start
The soft-start capacitor must be selected such that dur-
ing the start-up when the output capacitors are charging
up, the peak inductor current does not reach the current
limit threshold. A minimum of 0.1µF capacitor insures
this for most applications. During start-up the soft-start
capacitor is charged up to approximately 6V keeping
the output shutdown before an internal 10µA current
source start discharging the soft-start capacitor which
8
∆Vo = (Rt/Rh)×11
Where:
Rt = Resistor connected from V
IRU3034.
Rh = Resistor connected from V
∆Vo = (1/422)×11 = 26mV
FB
) exceeds 1.25V causing
OUT
FB
HYST
pin to V
to the V
pin to switch
HYST
FB
pin of
www.irf.com
pin.
slowly ramps up the inverting input of the PWM com-
parator, V
same rate as the soft-start cap thereby limiting the input
current. For example, with 0.1µF and the 10µA internal
current source the ramp up rate is:
Assuming that the output capacitance is 6000µF, the
peak input current will be:
The soft start capacitor also provides a delay in the turn
on of the output which is given by:
For example for Css=0.1 F,
Switcher Current Limit Protection
The IRU3034 uses an external current sensing resistor
and compares the voltage drop across it to a programmed
voltage which is set externally via a resistor (R
between the CS- terminal of the IC and V
voltage across the sense resistor exceeds the thresh-
old, the soft-start capacitor pulls up to 12V, pulling up
the inverting pin of the error comparator higher than non-
inverting which causes the external MOSFET to shut
off. At this point the CS comparator changes its state
and pulls the soft-start capacitor to Vcc which is 12V
and shutting the PWM drive. After the output drive is
turned off, an internal 10µA current source slowly dis-
charges the soft-start capacitor to approximately 5.7V,
before the output starts to turn back on causing a long
delay before the MOSFET turns back on. This delay
causes the catch diode to cool off between the current
limit cycles allowing the converter to survive a short cir-
cuit condition. An example is given below as how to
select the current limiting components. Assuming the
desired current limit point is set to be 20A and the cur-
rent sense resistor Rs=5mΩ, then the current limit pro-
gramming resistor, R
(∆V/∆t) = I/Css = 10/0.1 = 100V/s or 0.1V/ms
I
T
T
Vcs = I
R
Where:
K = 30ms/µF
Where:
I
IN(pk)
B
D
D
CL
= 20µA is the internal current source of IRU3034
= Css×K
= 0.1×30 = 3ms
= Vcs/I
= Css×(∆V/∆t) = 6000µF×(0.1V/ms) = 0.6A
FB
CL
. This insures the output to ramp up at the
×Rs = 20×0.005 = 0.1V
B
= (0.1V)/(20 A) = 5KΩ
CL
is calculated as:
OUT
. Once the
CL
) placed