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IRU3037 Datasheet(PDF) 5 Page - International Rectifier |
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IRU3037 Datasheet(HTML) 5 Page - International Rectifier |
5 / 21 page IRU3037 / IRU3037A 5 Rev. 2.8 03/10/03 www.irf.com Figure 3 - Typical application of the IRU3037 for programming the output voltage. APPLICATION INFORMATION Design Example: The following example is a typical application for IRU3037, the schematic is Figure 18 on page 14. Output Voltage Programming Output voltage is programmed by reference voltage and external voltage divider. The Fb pin is the inverting input of the error amplifier, which is internally referenced to 1.25V (0.8V for IRU3037A). The divider is ratioed to pro- vide 1.25V at the Fb pin when the output is at its desired value. The output voltage is defined by using the follow- ing equation: When an external resistor divider is connected to the output as shown in Figure 3. Equation (1) can be rewritten as: Choose R5 = 1K V This will result to R6 = 1.65K V If the high value feedback resistors are used, the input bias current of the Fb pin could cause a slight increase in output voltage. The output voltage set point can be more accurate by using precision resistor. Soft-Start Programming The soft-start timing can be programmed by selecting the soft start capacitance value. The start up time of the converter can be calculated by using: Where: CSS is the soft-start capacitor ( mF) For a start-up time of 7.5ms, the soft-start capacitor will be 0.1 mF. Choose a ceramic capacitor at 0.1mF. Shutdown The converter can be shutdown by pulling the soft-start pin below 0.5V. The control MOSFET turns off and the synchronous MOSFET turns on during shutdown. Boost Supply Vc To drive the high-side switch it is necessary to supply a gate voltage at least 4V greater than the bus voltage. This is achieved by using a charge pump configuration as shown in Figure 18. The capacitor is charged up to approximately twice the bus voltage. A capacitor in the range of 0.1 mF to 1mF is generally adequate for most applications. In application, when a separate voltage source is available the boost circuit can be avoided as shown in Figure 1. Input Capacitor Selection The input filter capacitor should be based on how much ripple the supply can tolerate on the DC input line. The larger capacitor, the less ripple expected but consider should be taken for the higher surge current during the power-up. The IRU3037 provides the soft-start function which controls and limits the current surge. The value of the input capacitor can be calculated by the following formula: Where: CIN is the input capacitance ( mF) IIN is the input current (A) Dt is the turn on time of the high-side switch (ms) DV is the allowable peak to peak voltage ripple (V) Fb IRU3037 VOUT R5 R6 tSTART = 753Css (ms) ---(2) VIN = 5V VOUT = 3.3V IOUT = 4A DVOUT = 100mV fS = 200KHz R6 = R5 3 - 1 VOUT VREF ( ) VOUT = VREF 3 1 + ---(1) R6 R5 ( ) CIN = ---(3) IIN 3 Dt DV |
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