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IR3800MTR1PBF Datasheet(PDF) 11 Page - International Rectifier |
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IR3800MTR1PBF Datasheet(HTML) 11 Page - International Rectifier |
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11 / 21 page  11/04/08 11 PD-60352 IR3800MPbF Application Information Design Example: The following example is a typical application for the IR3800. The application circuit is shown in page 17. 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 0.6V. The divider is ratioed to provide 0.6V at the Fb pin when the output is at its desired value. The output voltage is defined by using the following equation: When an external resistor divider is connected to the output as shown in figure 10. Equation (4) can be rewritten as: For the calculated values of R 8 and R 9 see feedback compensation section. kHz 600 F mV 30 ΔV A 12 I V 8 . 1 V ) max V, 2 . 13 V,( 12 V s o o o in = = = = ≤ ) 4 --( R R 1 V V 9 8 ref o ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + ∗ = Fig. 10: Typical application of the IR3800 for programming the output voltage ) 5 --( V V V R R ref O ref 8 9 ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − ∗ = 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 T start is the desired start-up time (ms) For a start-up time of 11ms, the soft-start capacitor will be 0.22uF. Vc supply for single input voltage 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 11. This method is simple and inexpensive. The operation of the circuit is as follows: when the lower MOSFET is turned on, the capacitor (C1) is pulled down to ground and charges, up to V BUS value, 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 (Vc) through diode (D2). Vc is approximately: Capacitors in the range of 0.1uF are generally adequate for most applications. The diodes must be a fast recovery device to minimize the amount of charge fed back from the charge pump capacitor into V BUS. 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. Fig. 11: Charge pump circuit to generate Vc voltage ( ) ) 6 --( V V V 2 V 2 D 1 D bus C + − ∗ ≅ ) 1 --( T A 20 C start SS * μ ≅ Fb IR3624 VOUT R9 R8 IR3800 |
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