IRU3034CS datasheet(8/12 Pages) IRF

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IRU3034 & (PbF)

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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 (VFB) exceeds 1.25V causing

the output drive to switch Low and the VHYST pin to switch

High which jumps the feedback pin higher than 1.25V

resulting in a fixed output ripple which is given by the

following equation:

Where:

Rt = Resistor connected from VOUT to the VFB pin of

IRU3034.

Rh = Resistor connected from VFB pin to VHYST pin.

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

slowly ramps up the inverting input of the PWM com-

parator, VFB. This insures the output to ramp up at the

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:

Where:

K = 30ms/µF

For example for Css=0.1

µF,

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 (RCL) placed

between the CS- terminal of the IC and VOUT. Once the

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, RCL is calculated as:

Where:

IB = 20µA is the internal current source of IRU3034

IIN(pk) = Css×(∆V/∆t) = 6000µF×(0.1V/ms) = 0.6A

∆Vo = (Rt/Rh)×11

∆Vo = (1/422)×11 = 26mV

Vcs = ICL×Rs = 20×0.005 = 0.1V

RCL = Vcs/IB = (0.1V)/(20

µA) = 5KΩ

TD = Css×K

TD = 0.1×30 = 3ms

(∆V/∆t) = I/Css = 10/0.1 = 100V/s or 0.1V/ms

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