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FAN4868 Datasheet(PDF) 8 Page - ON Semiconductor |
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FAN4868 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 12 page ![]() FAN4868 www.onsemi.com 8 To ensure that VOUT does not pump significantly above the regulation point, the boost switch remains off as long as FB > VREF. Fault State The regulator enters the FAULT state under any of the following conditions: • VOUT fails to achieve the voltage required to advance from LIN state to SS state • VOUT fails to achieve the voltage required to advance from SS state to BST state • Sustained (32 CLK counts) pulse−by−pulse current limit during the BST state • The regulator moves from BST to LIN state due to a short circuit or output overload (VOUT < VIN−1 V) Once a fault is triggered, the regulator stops switching and presents a high−impedance path between VIN and VOUT. After waiting 480 CLK counts, a restart is attempted. Soft−Start and Fault Timing The soft−start timing for each state, and the fault times, are determined by the fault clock, whose period is inversely proportional to VIN. This allows the regulator more time to charge larger values of COUT when VIN is lower. With higher VIN, this also reduces power delivered to VOUT during each cycle in current limit. Figure 15. Fault Response into Short Circuit ILIN2 VOUT 0 64 16 ILIN1 EN 0 V 480 8 The fault clock period as a function of VIN is shown in Figure 16. Figure 16. Fault Clock Period vs. VIN The VIN−dependent LIN Mode charging current is illustrated in Figure 17 . Figure 17. LIN Mode Current vs. VIN Over−Temperature Protection (OTP) The regulator shuts down when the thermal shutdown threshold is reached. Restart, with soft−start, occurs when the IC has cooled by about 30 °C. Over−Current Protection (OCP) During Boost Mode, the FAN4868 employs a cycle−by−cycle peak current limit to protect switching elements. Sustained current limit, for 32 consecutive fault clock counts, initiates a fault condition. During an overload condition, as VOUT collapses to approximately VIN-1 V, the synchronous rectifier is immediately switched off and a fault condition is declared. Automatic restart occurs once the overload/short is removed and the fault timer completes counting. |
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