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IR3637ASTRPBF Datasheet(PDF) 6 Page - International Rectifier |
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IR3637ASTRPBF Datasheet(HTML) 6 Page - International Rectifier |
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6 / 19 page  6 IR3637ASPbF www.irf.com Figure 5 - Typical application of the IR3637A for programming the output voltage. APPLICATION INFORMATION Design Example: The following example is a typical application for IR3637A. Appliaction circuit is shown in page 12. 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 0.8V. The divider is ratioed to provide 0.8V at the Fb pin when the output is at its desired value. The output volt- age is defined by using the following equation: When an external resistor divider is connected to the output as shown in Figure 5. Equation (1) can be rewritten as: Choose R5 = 1KΩ This will result to R6 = 1.25KΩ 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: Fb IR3637A VOUT R5 R6 VIN = Vcc = 5V Vc=12V VOUT = 1.8V IOUT = 6A ∆VOUT = 50mV FS = 600KHz R6 = R5 × - 1 VOUT VREF ( ) VOUT = VREF × 1 + ---(1) R6 R5 ( ) For a start-up time of 4ms, the soft-start capacitor will be 0.1µF. Choose a ceramic capacitor at 0.1µF. Boost Supply for Single 5V appliaction To drive the high side switch, it is necessary to supply a gate voltage at least 4V grater than the bus voltage. This is achieved by using a charge pump configuration as shown in Figure 6. This method is simple and inexpen- sive. 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 VBUS 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 0.1µF and 1µF are generally adequate for most applications. The diode must be a fast recovery device to minimize the amount of charge fed back from the charge pump capacitor into VBUS. The diodes need to be able to block the full power rail volt- age, which is seen when the high side MOSFET is switched on. For low voltage application, schottky di- odes can be used to minimize forward drop across the diodes at start up. Figure 6 - Charge pump circuit. Input Capacitor Selection The input filter capacitor should be based on how much ripple the supply can tolerate on the DC input line. The ripple current generated during the on time of upper MOSFET should be provided by input capacitor. The RMS value of this ripple is expressed by: Css ≅ 25×tSTART (µF) ---(2) Where tSTART is the desired start-up time (ms) L IR3637A D1 C1 Vc HDrv Q1 Q2 C2 VBUS D2 C3 VBUS Vc ≅ 2VBUS - (VD1 + VD2) |
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