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LM51551DSSR Datasheet(PDF) 23 Page - Texas Instruments |
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LM51551DSSR Datasheet(HTML) 23 Page - Texas Instruments |
23 / 49 page G@ VCC SW Q f 35mA u MAX2 SW D 1 100ns f u SYNC MAX1 RT f D 1 0.1 f u SUPPLY(MIN) LOAD F MAX SUPPLY(MAX) DCR SUPPLY(MAX) DS(ON) S MAX V V V 1 D I R I R R D | u u u u 64 cycles of current limit 32768 hiccup mode off cycles 60 cycles of current limit 7 normal switching cycles 4 cycles of current limit Time 32768 hiccup mode off cycles Inductor Current 23 LM5155 www.ti.com SNVSB75 – DECEMBER 2018 Product Folder Links: LM5155 Submit Documentation Feedback Copyright © 2018, Texas Instruments Incorporated 9.3.10 Hiccup Mode Overload Protection (LM51551 Version Only) To further protect the converter during prolonged current limit conditions, the LM51551 device option provides a hiccup mode overload protection. The internal hiccup mode fault timer of the LM51551 counts the PWM clock cycles when the cycle-by-cycle current limiting occurs. When the hiccup mode fault timer detects 64 cycles of current limiting, an internal hiccup mode off timer forces the device to stop switching and pulls down SS. Then, the device will restart after 32,768 cycles of hiccup mode off-time. The 64 cycle hiccup mode fault timer is reset if 8 consecutive switching cycles occur without exceeding the current limit threshold. The soft-start time must be long enough not to trigger the hiccup mode protection during soft-start time because the hiccup mode fault timer is enabled during the soft-start. Figure 36. Hiccup Mode Overload Protection To avoid an unexpected hiccup mode operation during a harsh load transient condition, it is recommended to have more margin when programming the peak-current limit. 9.3.11 Maximum Duty Cycle Limit and Minimum Input Supply Voltage When designing boost converters, the maximum duty cycle should be reviewed at the minimum supply voltage. The minimum input supply voltage that can achieve the target output voltage is limited by the maximum duty cycle limit, and it can be estimated as follows. where • ISUPPLY(MAX) is the maximum input current. • RDCR is the DC resistance of the inductor. • RDS(ON) is the on-resistance of the MOSFET. (15) (16) (17) The minimum input supply voltage can be further decreased by supplying fSYNC which is less than fRT. DMAX is DMAX1 or DMAX2, whichever is lower. 9.3.12 MOSFET Driver (GATE Pin) The device provides an N-channel MOSFET driver that can source or sink a peak current of 1.5 A. The peak sourcing current is larger when supplying an external VCC that is higher than 6.75 V VCC regulation target. During start-up especially when the input voltage range is below the VCC regulation target , the VCC voltage must be sufficient to completely enhance the MOSFET. If the MOSFET drive voltage is lower than the MOSFET gate plateau voltage during start-up, the boost converter may not start up properly and it may stick at the maximum duty cycle in a high power dissipation state. This condition can be avoided by selecting a lower threshold N-channel MOSFET switch and setting the VSUPPLY(ON) greater than 6 to 7 V. Because the internal VCC regulator has a limited sourcing capability, the MOSFET gate charge should satisfy the following inequality. (18) |
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