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IR3621MPBF Datasheet(PDF) 8 Page - International Rectifier |
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IR3621MPBF Datasheet(HTML) 8 Page - International Rectifier |
8 / 29 page 8 IR3621&(PbF) www.irf.com Dual Soft-Start The IR3621 has programmable soft-start to control the output voltage rise and limit the inrush current during start-up. It provides a separate Soft-Start function for each outputs. This will enable to sequence the outputs by controlling the rise time of each output through selection of different value soft-start capacitors. The soft-start pins will be connected together for applications where, both outputs are required to ramp-up at the same time. To ensure correct start-up, the soft-start sequence ini- tiates when the Vcc, VcH1 and VcH2 rise above their threshold and generate the Power On Reset (POR) sig- nal. Soft-start function operates by sourcing an internal current to charge an external capacitor to about 3V. Ini- tially, the soft-start function clamps the E/A’s output of the PWM converter. During power up, the converter out- put starts at zero and thus the voltage at Fb is about 0V. A current (64 µA) injects into the Fb pin and generates a voltage about 1.6V (64 µA×25K) across the negative input of E/A and (see Figure6). The magnitude of this current is inversely proportional to the voltage at soft-start pin. The 28 µA current source starts to charge up the external capacitor. In the mean time, the soft-start voltage ramps up, the current flowing into Fb pin starts to decrease linearly and so does the voltage at negative input of E/A. SS1 / SD Comp2 Error Amp2 POR 28uA Fb1 Comp1 Error Amp1 64uA Max Fb2 0.8V SS2 / SD 20 22 21 6 VP2 26 8 7 28uA 64uA Figure 6 -Soft-start circuit for IR3621 When the soft-start capacitor is around 1V, the current flowing into the Fb pin is approximately 32 µA. The volt- age at the positive input of the E/A is approximately: The E/A will start to operate and the output voltage starts to increase. As the soft-start capacitor voltage contin- ues to go up, the current flowing into the Fb pin will keep decreasing. Because the voltage at pin of E/A is regu- lated to reference voltage 0.8V, the voltage at the Fb is: VFB = 0.8-(25K ×Injected Current) The feedback voltage increases linearly as the injecting current goes down. The injecting current drops to zero when soft-start voltage is around 1.8V and the output voltage goes into steady state. Figure 7 shows the theo- retical operational waveforms during soft-start. 32 µA×25K = 0.8V Soft-Start Voltage Voltage at negative input of Error Amp Voltage at Fb pin Current flowing into Fb pin 64uA 0uA 0V 0.8V ≅1.6V 0.8V 0V 3V ≅1.8V ≅1V Output of POR Figure 7 - Theoretical operational waveforms during soft-start. The output start-up time is the time period when soft- start capacitor voltage increases from 1V to 1.8V. The start-up time will be dependent on the size of the exter- nal soft-start capacitor. The start-up time can be esti- mated by: 28 µA×TSTART/CSS = 1.8V-1V Low Temperature Start-Up The controller is capable of starting at -40 !C ambient temperature. |
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