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IR3621MPBF Datasheet(PDF) 11 Page - International Rectifier |
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IR3621MPBF Datasheet(HTML) 11 Page - International Rectifier |
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11 / 29 page  IR3621&(PbF) 11 www.irf.com APPLICATION INFORMATION Design Example: The following example is a typical application for the IR3621, the schematic is Figure19 on page18. Output Voltage Programming Output voltage is programmed by the reference voltage and an external voltage divider. The Fb1 pin is the invert- ing input of the error amplifier, which is referenced to the voltage on the non-inverting pin of error amplifier. For this application, this pin (VP2) is connected to the reference voltage (VREF). The output voltage is defined by using the following equation: When an external resistor divider is connected to the output as shown in Figure 11. Figure 11 - Typical application of the IR3621 for pro- gramming the output voltage. Equation (4) can be rewritten as: Will result to: VOUT(2.5V) = 2.5V VREF = 0.8V R9= 2.15K, R5= 1K 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 can be set more accurately by using low value, precision resistors. For a start-up time of 4ms for both output, the soft-start capacitor will be 0.1 µF. Connect two 0.1 µFceramic capacitors from SS1 pin and SS2 pin to GND. Supply VcH1 and VcH2 To drive the high side MOSFET, it is necessary to sup- ply a gate voltage at least 4V greater than the bus volt- age. This is achieved by using a charge pump configu- ration as shown in Figure 12. This method is simple and inexpensive. The operation of the circuit is as fol- lows: when the lower MOSFET is turned on, the ca- pacitor (C1) charges up to VOUT3, through the diode (D1). The bus voltage will be added to this voltage when upper MOSFET turns on in next cycle, and providing supply voltage (VcH1) through diode (D2). VcH1 is ap- proximately: Capacitor in the range of 0.1 µF is generally adequate for most applications. The diode must be a fast recov- ery device to minimize the amount of charge fed back from the charge pump capacitor into VOUT3. The diodes need to be able to block the full power rail voltage, which is seen when the high side MOSFET is switched on. For low voltage application, Schottky diodes can be used to minimize forward drop across the diodes at start up. Figure 12 - Charge pump circuit. R6 = R5 × - 1 VOUT VP ( ) Fb IR3621 VOUT R5 R6 VREF VP2 VCH1 ≅ VOUT3 + VBUS - (VD1 + VD2) VIN = 12V VOUT(2.5V) = 2.5V @ 10A VOUT(1.8V) = 1.8V @ 10A ∆VOUT = Output voltage ripple ≅ 3% of VOUT FS = 400kHz L2 IR3621 D1 C1 VcH1 HDrv Regulator Q1 Q2 VOUT3 C2 VBUS D2 C3 Css ≅ 28×tSTART (µF) ---(5) Where tSTART is the desired start-up time (ms) VOUT = VP2 × 1 + ---(4) R6 R5 VP2 = VREF = 0.8V ( ) VOUT(1.8V) = 1.8V VREF = 0.8 R7= 1.24K, R8 = 1K 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: |
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