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IRU3055CQ Datasheet(PDF) 10 Page - International Rectifier |
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IRU3055CQ Datasheet(HTML) 10 Page - International Rectifier |
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10 / 26 page  10 Rev. 1.4 08/13/02 IRU3055 www.irf.com When the inductor current is large enough, the voltage across the low side switch is low enough so that the voltage at OCSet node is below zero and the compara- tor will flip and trigger a switch to discharge the soft-start capacitor at a certain slope rate. The system enters into a hiccup mode. The over current threshold can be set by resistor RSET. Suppose the current sharing is perfect, then the current flowing into phase 1 will be one third of the total output current. The maximum allowed output current can be represented as: Where RDS(ON) is the ON resistance of low side MOSFET. In practice, the RDS(ON) of MOSFET is temperature de- pendent. The overhead has to be considered. For prac- tice, over current threshold has to be at least 50% higher than the nominal current plus ripple. In the demo-board, the maximum output current is set to be: For each phase, the maximum current is one third (33A), assuming good current sharing. The low side of MOSFET is IRF3711S. The On resistor at 150 degrees is given from the data sheet: The over current setting resistor can be set as IMAX = 160 mA 3 RSET / (RDS(ON)/3) RSET = IMAX 3RDS(ON)/3/160mA ---(3) IMAX = (1+50%) 3IOUT = 1.5360A = 90A Consider ripple current, select IMAX=100A RDS(ON) = 1.5 36mV = 9mV RSET = 33A 3 0.009/160mA = 1.86K Select RSET = 2.2K Fs ≅ 7500/Rt ---(5) Where Rt is in K V and Fs is in KHz. Rt ≅ 7500/150 = 50KV 10 mA3tSTART/Css = VSET+0.7V tSTART = (VSET+0.7V) 3Css/10mA ---(4) Where: Css is the soft-start capacitor ( mF). VSET is the voltage from DAC and equal to the de- sired output voltage. Over Voltage Protection The Fb pin is connected to the output voltage. An over- voltage condition is detected when the voltage at Fb pin is 15% higher than the programmed voltage by DAC. When the overvoltage occurs, the soft-start capacitor is discharged. The high side MOSFETs are turned off and the low side MOSFETs are turned on. As a result, the low side MOSFET of synchronous rectifier conduct and shunt the output voltage to ground and protect the load. In the meantime, the PGood pin is held to low. Soft-Start The IRU3055 has a soft-start function to limit the current surge at the start-up. An external capacitor which is charged by a 10 mA internal current source is used to program the soft-start timing. The voltage of the external capacitor linearly increases, which forces the output volt- age to go up linearly until the voltage at soft-start reaches the desired voltage. The following equation can be used to calculate the start up time. For a 7.5ms start-up time and 1.5V output, the required capacitor will be 33nF. Operation Frequency Selection The operation switching frequency is determined by an external resistor (Rt). The switching frequency is approxi- mately inversely proportioned to resistance (see Fig.10). The switching frequency can also be estimated by: For example, if the 150KHz switching frequency is se- lected, the required Rt is calculated as: Figure 10 - The operation frequency vs. Rt. Figure 9 - Operation waveforms at short circuit. (Hiccup mode) Ch1: Input current, 5A/div. Ch2: Phase 1 inductor current, 10A/div. Ch3: Soft-start capacitor voltage, 5V/div. Ch4: Output voltage, 2V/div. Frequency versus Rt 0 100 200 300 400 500 0 10 20 30 40 50 60 70 Rt(KV) frequency |
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