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IR3638SPBF Datasheet(PDF) 8 Page - International Rectifier |
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IR3638SPBF Datasheet(HTML) 8 Page - International Rectifier |
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8 / 15 page  IR3638SPbF 3/19/07 Soft-Start The IR3638 has programmable soft-start to control the output voltage rise and limit the inrush current during start-up. To ensure correct start-up, the soft-start sequence initiates when Vcc and Enable rise above their threshold and generate the Power On Ready (POR) signal. The soft-start function operates by sourcing current to charge an external capacitor to about 3V. Initially, the soft-start function clamps the output of error amplifier by injecting a current (64uA) into the Fb pin and generates a voltage about 1.6V (64ux25K) across the negative input of error amplifier (see figure 6). The magnitude of the injected current is inversely proportional to the voltage at the soft-start pin. As the soft-start voltage ramps up, the injected current decreases linearly and so does the voltage at negative input of error amplifier. When the soft-start capacitor is around 1V, the voltage at the negative input of the error amplifier is approximately 0.8V (32uAx25K). As soon as the Fb reaches the Vp voltage, the output of error amplifier will start increasing and generating the first PWM signal. As the soft-start capacitor voltage continues to ramp up, the current flowing into the Fb pin will keep decreasing. The feedback voltage increases linearly as the soft start voltage ramps up. When soft-start voltage is around 2V the output voltage is reached the steady state and the injected current is zero. Figure 7 shows the theoretical operational waveforms during soft-start. The output voltage start-up time is the time period when soft-start capacitor voltage increases from 1V to 2V. The start-up time will be dependent on the size of the external soft-start capacitor and can be estimate by: Fig. 6: Soft-Start circuit for IR3638 Fig. 7: Theoretical operation waveforms during soft-start 22uA 64uA POR Error Amp SS/SD Fb Comp 25K Vp 25K 3V Soft-Start Voltage Voltage at negative input of Error Amp Voltage at Fb pin Current flowing into Fb pin 64uA 0uA 0V Vp=0.8V ≅1.6V 0.8V 0V 3V ≅2V ≅1V Output of UVLO POR 8 For a given start-up time, the soft-start capacitor (nF) can be estimated as: V 1 V 2 C T μA 22 ss start − = ∗ ) 1 --( ms T A 22 C start SS ) ( * μ ≅ |
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