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IR3871M Datasheet(PDF) 11 Page - International Rectifier |
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IR3871M Datasheet(HTML) 11 Page - International Rectifier |
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11 / 21 page  11 IR3871MPBF PWM COMPARATOR The PWM comparator initiates a SET signal (PWM pulse) when the FB pin falls below the reference (Vref) or the soft start (SS) voltage. ON-TIME GENERATOR The PWM on-time duration is programmed with an external resistor (R FF) from the input supply (VIN) to the FF pin. The simplified calculation for R FF is shown in equation 1. The FF pin is held to an internal reference after EN goes HIGH. A copy of the current in R FF charges a timing capacitor, which sets the on-time duration, as shown in equation 2. CONTROL LOGIC The control logic monitors input power sources, sequences the converter through the soft-start and protective modes, and initiates an internal RUN signal when all conditions are met. VCC and 3VCBP pins are continuously monitored, and the IR3871 will be disabled if the voltage of either pin drops below the falling thresholds. EN_DELAY will become HIGH when VCC and 3VCBP are in the normal operating range and the EN pin = HIGH. SOFT START With EN = HIGH, an internal 10µA current source charges the external capacitor (C SS) on the SS pin to set the output voltage slew rate during the soft start interval. The soft start time (t SS) can be calculated from equation 3. The feedback voltage tracks the SS pin until SS reaches the 0.5V reference voltage (Vref), then feedback is regulated to Vref. C SS will continue to be charged, and when SS pin reaches V SS (see Electrical Specification), SS_DELAY goes HIGH. With EN_DELAY = LOW, the capacitor voltage and SS pin is held to the FB pin voltage. A normal startup sequence is shown in Figure 18. CIRCUIT DESCRIPTION (2) V 20 1 R T IN FF ON pF V (1) F 20 1 V R SW OUT FF pF V (3) A 10 5 . 0 t SS V C SS Figure 18. Normal Startup PGOOD The PGOOD pin is open drain and it needs to be externally pulled high. High state indicates that output is in regulation. The PGOOD logic monitors EN_DELAY, SS_DELAY, and under/over voltage fault signals. PGOOD is released only when EN_DELAY and SS_DELAY = HIGH and output voltage is within the OV and UV thresholds. PRE-BIAS STARTUP IR3871 is able to start up into pre-charged output, which prevents oscillation and disturbances of the output voltage. With constant on-time control, the output voltage is compared with the soft start voltage (SS) or Vref, depending on which one is lower, and will not start switching unless the output voltage drops below the reference. This scheme prevents discharge of a pre-biased output voltage. SHUTDOWN The IR3871 will shutdown if VCC is below its UVLO limit. The IR3871 can be shutdown by pulling the EN pin below its lower threshold. Alternatively, the output can be shutdown by pulling the soft start pin below 0.3V. |
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