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IR3628MPBF Datasheet(PDF) 9 Page - International Rectifier |
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IR3628MPBF Datasheet(HTML) 9 Page - International Rectifier |
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9 / 22 page  IR3628MPbF 08/10/2007 Soft-Start The IR3628 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 Vc 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 (40uA) into the Fb pin and generates a voltage about 0.96V (40ux24K) across the negative input of error amplifier (see figure 5). 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 positive input of the error amplifier is approximately 0.6V. The output of error amplifier will start increasing and generating the first PWM signal. As the soft- start capacitor voltage continues to go 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 6 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. 5: Soft-Start circuit for IR3628 Fig. 6: Theoretical operation waveforms during soft-start 20uA 40uA POR Error Amp SS/SD Fb Comp 24K 0.6V 24K 3V Soft-Start Voltage Voltage at negative input of Error Amp Voltage at Fb pin Current flowing into Fb pin 40uA 0uA 0V 0.6V ≅0.96V 0.6V 0V 3V ≅2V ≅1V Output of UVLO POR For a given start-up time, the soft-start capacitor (nF) can be estimated as: V 1 V 2 C T μA 20 ss start − = ∗ ) 1 --( ms T A 20 C start SS ) ( * μ ≅ 9 |
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