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LM3100 Arkusz danych(PDF) 11 Page - Texas Instruments |
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LM3100 Arkusz danych(HTML) 11 Page - Texas Instruments |
11 / 21 page 1.3 x 10 -10 200 ns x VIN(MAX) RON t RFB2 RFB1 0.8V VOUT = -1 LM3100 STOP RUN VIN VIN SS + LM3100 www.ti.com SNVS421F – JANUARY 2006 – REVISED DECEMBER 2009 Soft-Start The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing start-up stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold, an internal 8µA current source charges up the external capacitor at the SS pin. The ramping voltage at SS (and the non- inverting input of the regulation comparator) ramps up the output voltage in a controlled manner. An internal switch grounds the SS pin if any of the following cases happen: (i) VCC falls below the under-voltage lock-out threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the converter can be disabled by connecting the SS pin to ground using an external switch. Releasing the switch allows the SS pin return to pull high and the output voltage returns to normal. The shut-down configuration is shown in Figure 4 . Figure 4. Alternate Shutdown Implementation Thermal Protection The LM3100 should be operated so the junction temperature does not exceed the maximum limit. An internal Thermal Shutdown circuit, which activates (typically) at 165°C, takes the controller to a low power reset state by disabling the buck switch and the on-timer, and grounding the SS pin. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal operation resumes. Applications Information EXTERNAL COMPONENTS The following guidelines can be used to select the external components. RFB1 and RFB2 : The ratio of these resistors is calculated from: (7) RFB1 and RFB2 should be chosen from standard value resistors in the range of 1.0 kΩ - 10 kΩ which satisfy the above ratio. For VOUT = 0.8V, the FB pin can be connected to the output directly. However, the converter operation needs a minimum inductor current ripple to maintain good regulation when no load is connected. This minimum load is about 10 µA and can be implemented by adding a pre-load resistor to the output. RON: The minimum value for RON is calculated from: (8) The equation 2 in Control Overview section can be used to select RON if a specific frequency is desired as long as the above limitation is met. L: The main parameter affected by the inductor is the output current ripple amplitude (IOR). The maximum allowable (IOR must be determined at both the minimum and maximum nominal load currents. At minimum load current, the lower peak must not reach 0A. At maximum load current, the upper peak must not exceed the current limit threshold (1.9A). The allowable ripple current is calculated from the following equations: Copyright © 2006–2009, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3100 |
Podobny numer części - LM3100_14 |
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Podobny opis - LM3100_14 |
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