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IRU3034PBF Datasheet(PDF) 8 Page - International Rectifier |
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IRU3034PBF Datasheet(HTML) 8 Page - International Rectifier |
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8 / 12 page  IRU3034 & (PbF) 8 www.irf.com 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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