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SP6128AEY Datasheet(PDF) 6 Page - Exar Corporation |
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SP6128AEY Datasheet(HTML) 6 Page - Exar Corporation |
6 / 10 page 6 Rev. 08/19/05 SP6128A Low Voltage, Synchronous Step Down PWM Controller © Copyright 2005 Sipex Corporation As the figure shows, the SS voltage controls a variety of signals. First, provided all the exter- nal fault conditions are removed, an internal 5 µA pull-up at the COMP pin brings the SP6128A out of shutdown mode. The internal timing circuit is then activated and controls the ramp-up of the error amp reference voltage. The COMP pin is pulled to 0.7V by the internal clamp and then gradually charges preventing the error amplifier from forcing the loop to maximum duty cycle. As the COMP voltage crosses about 1V (valley voltage of the PWM ramp), the driver begins to switch the high side MOSFET with narrow pulses in an effort to keep the converter output regulated . The SP6128A operates at low duty cycle as the COMP voltage increases above 1V. As the error amp reference ramps upward, the driver pulses widen until a steady state value is reached and the output voltage is regulated to the final value ending the soft start charge cycle. Hiccup Mode When the converter enters a fault mode, the SP6128A holds the high side and low side MOSFETs off for a finite period of time. Provided that the SP6128A is enabled, this time is set by the internal charge of the soft-start capacitor. In the event of an overcurrent condition, the current sense comparator sets the fault latch, which in turn discharge the internal SS capacitor, the COMP pin and holds the output drivers off. During this con- dition, the SP6128A stays off for the time it takes to discharge the COMP pin down to the 0.29V shutdown threshold. At this point, the fault latch is reset, but before the SP6128A is allowed to attempt restart, the COMP pin has to charge back to 1V before any output switching can be initiated. Then, the regulator attempts to restart normally by delivering short gate pulses and if the overcurrent condition is still present, the cycle will repeat itself. However, if upon restart, the overcurrent condi- tion is still present, the SP6128A will detect the fault and remain in a fault state until the internal soft start voltage reaches about VCC-1V thereby increasing the MOSFET off-time. This protection scheme minimizes thermal stress to the regulator components as the overcurrent condition persists. OPERATION: continued 0.8 V I(L) TIME 0.3 V 0.7 V 1 V 0 V COMP Internal SS 0 V SWN Voltage Voltage FAULT Current Inductor 0 A Voltage Reference Error Amp 0 V V(VCC) 0 V V(VCC) |
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