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34653 Datasheet(PDF) 11 Page - Freescale Semiconductor, Inc |
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34653 Datasheet(HTML) 11 Page - Freescale Semiconductor, Inc |
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11 / 24 page  Analog Integrated Circuit Device Data Freescale Semiconductor 11 34653 FUNCTIONAL DEVICE OPERATION OPERATIONAL MODES Figure 7. Power MOSFET Turn-On and the Gradual Increase in the Charging Current from 0 A to ICHG (2.0 ms in Example) The ICHG current charges up the load capacitor relatively slowly. When the load capacitor is fully charged, the Power MOSFET reaches its full enhancement, which triggers the current limit detection to change from ICHG to ILIM and the load current to decrease (Point C in Figure 6, page 10). The current spike at Point C in Figure 6 is better displayed in Figure 8. We can see that when the ⎜V OUT - VIN ⎜< 0.5 V, the Power MOSFET fully turns on to reach its full enhancement, charging the capacitor an additional 0.5 V with a higher current value that quickly ramps down. This eliminates the need for a current slew rate control because the hazard for a voltage change is less than 0.5 V. The power good output signals activate after a 20 ms delay (Point D in Figure 6), which in turn enables the load. The 34653 is now in normal operation mode and the retry counter resets. Figure 8. Full Power MOSFET Turn-On and Current Spike Associated with It. End of Charging Process NORMAL MODE If one of the start-up conditions (list on page 10) is violated any time from the start of the Power MOSFET enhancement process and thereafter during normal operation, the Power MOSFET turns off and the power good output signals deactivate, disabling the load, and a new timer cycle starts as explained previously. The 34653 also monitors the load current to prevent any overload or short circuit conditions from happening to protect the load from damage. LOAD CURRENT CONTROL When in normal operation mode, the 34653 monitors the load and provides two modes of current control as explained in the paragraphs below. Overcurrent Mode The 34653 monitors the load for overcurrent conditions. If the current going through the load becomes larger than the overcurrent limit for longer than the overcurrent limit filter timer of 100 µs, the overcurrent signal is asserted and the gate of the Power MOSFET is discharged to try to regulate the current at the ILIM value (Point A in Figure 9). The 34653 is in overcurrent mode for 3.0 ms . If after a 3.0 ms filter timer the device is still in overcurrent mode, the device turns off the Power MOSFET and deactivates the power good output signals (Point B in Figure 9). The 34653 then initiates another start-up timer and goes back through the enhancement process. If during the 3.0 ms timer the fault was cleared, then the 34653 goes back to the normal operation mode and the power good output signals stay activated as shown in Figure 10. This way the device overcomes temporary overcurrent situations and at the same time protects the load from a more severe overcurrent situation. Short Circuit Mode If the current going through the load becomes > 5.0 A, the Power MOSFET is discharged very fast (in less than 10 µs) to try to regulate the current at the ILIM value, and the 34653 is in the overcurrent mode for 3.0 ms. Then it follows the pattern outlined in the Overcurrent Mode paragraph above. |
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