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LM3150 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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LM3150 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
11 / 32 page IL = (VIN - VOUT) x tON L Ivalley = IOUT - IL 2 RLIM = ICL x RDS(ON)max ILIM-TH ICL = IOCL - IL 2 LM3150 www.ti.com SNVS561F – SEPTEMBER 2008 – REVISED DECEMBER 2014 Feature Description (continued) The RLIM value can be approximated as follows: (6) where • IOCL is the user-defined average output current limit value • RDS(ON)max is the resistance value of the low-side FET at the expected maximum FET junction temperature • ILIM-TH is an internal current supply of 85 µA typical (7) Figure 13 illustrates the inductor current waveform. During normal operation, the output current ripple is dictated by the switching of the FETs. The current through the low-side switch, Ivalley, is sampled at the end of each switching cycle and compared to the current limit, ICL, current. The valley current can be calculated as follows: where • IOUT is the average output current • ΔIL is the peak-to-peak inductor ripple current (8) If an overload condition occurs, the current through the low-side switch will increase which will cause the current limit comparator to trigger the logic to skip the next on-time cycle. The IC will then try to recover by checking the valley current during each off-time. If the valley current is greater than or equal to ICL, then the IC will keep the low-side FET on and allow the inductor current to further decay. Throughout the whole process, regardless of the load current, the on-time of the controller will stay constant and thereby the positive ripple current slope will remain constant. During each on-time the current ramps-up an amount equal to: (9) The valley current limit feature prevents current runaway conditions due to propagation delays or inductor saturation because the inductor current is forced to decay following any overload conditions. Current sensing is achieved by either a low value sense resistor in series with the low-side FET or by utilizing the RDS(ON) of the low-side FET. The RDS(ON) sensing method is the preferred choice for a more simplified design and lower costs. The RDS(ON) value of a FET has a positive temperature coefficient and will increase in value as the temperature of the FET increases. The LM3150 controller will maintain a more stable current limit that is closer to the original value that was set by the user, by positively adjusting the ILIM-TH value as the IC temperature increases. This does not provide an exact temperature compensation but allows for a more tightly controlled current limit when compared to traditional RDS(ON) sensing methods when the RDS(ON) value can change typically 140% from room to maximum temperature and cause other components to be over-designed. The temperature compensated ILIM-TH is shown below where TJ is the die temperature of the LM3150 controller in Celsius: ILIM-TH(TJ) = ILIM-TH x [1 + 3.3 x 10 -3 x (T J - 27)] (10) To calculate the RLIM value with temperature compensation, substitute Equation 10 into ILIM-TH in Equation 7. 8.3.5 Short-Circuit Protection The LM3150 controller will sense a short-circuit on the output by monitoring the output voltage. When the feedback voltage has fallen below 60% of the reference voltage, Vref x 0.6 (≈ 0.36 V), short-circuit mode of operation will start. During short-circuit operation, the SS pin is discharged and the output voltage will fall to 0 V. The SS pin voltage, VSS, is then ramped back up at the rate determined by the SS capacitor and ISS until VSS reaches 0.7 V. During this re-ramp phase, if the short-circuit fault is still present the output current will be equal to the set current limit. Once the soft-start voltage reaches 0.7 V, the output voltage is sensed again and if the VFB is still below Vref x 0.6 then the SS pin is discharged again and the cycle repeats until the short-circuit fault is removed. Copyright © 2008–2014, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3150 |
Số phần tương tự - LM3150_15 |
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Mô tả tương tự - LM3150_15 |
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