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LM22670MRX-ADJ Scheda tecnica(PDF) 9 Page - Texas Instruments |
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LM22670MRX-ADJ Scheda tecnica(HTML) 9 Page - Texas Instruments |
9 / 29 page LM22670 www.ti.com SNVS584O – SEPTEMBER 2008 – REVISED MARCH 2013 Where Fsw is the switching frequency and TON is the minimum on-time; both found in the Electrical Characteristics table. If the frequency adjust feature is used, that value should be used for Fsw. Nominal values should be used. The worst case is lowest output voltage, and highest switching frequency. If this input voltage is exceeded, the regulator will skip cycles, effectively lowering the switching frequency. The consequences of this are higher output voltage ripple and a degradation of the output voltage accuracy. The second limitation is the maximum duty cycle before the output voltage will "dropout" of regulation. The following equation can be used to approximate the minimum input voltage before dropout occurs: (4) The values of TOFF and RDS(ON) are found in the Electrical Characteristics table. The worst case here is highest switching frequency and highest load. In this equation, RL is the D.C. inductor resistance. Of course, the lowest input voltage to the regulator must not be less than 4.5V (typ.). Current Limit The LM22670 has current limiting to prevent the switch current from exceeding safe values during an accidental overload on the output. This peak current limit is found in the Electrical Characteristics table under the heading of ICL. The maximum load current that can be provided, before current limit is reached, is determined from the following equation: (5) Where L is the value of the power inductor. When the LM22670 enters current limit, the output voltage will drop and the peak inductor current will be fixed at ICL at the end of each cycle. The switching frequency will remain constant while the duty cycle drops. The load current will not remain constant, but will depend on the severity of the overload and the output voltage. For very severe overloads ("short-circuit"), the regulator changes to a low frequency current foldback mode of operation. The frequency foldback is about 1/5 of the nominal switching frequency. This will occur when the current limit trips before the minimum on-time has elapsed. This mode of operation is used to prevent inductor current "run-away", and is associated with very low output voltages when in overload. The following equation can be used to determine what level of output voltage will cause the part to change to low frequency current foldback: (6) Where Fsw is the normal switching frequency and Vin is the maximum for the application. If the overload drives the output voltage to less than or equal to Vx, the part will enter current foldback mode. If a given application can drive the output voltage to ≤Vx, during an overload, then a second criterion must be checked. The next equation gives the maximum input voltage, when in this mode, before damage occurs: (7) Where Vsc is the value of output voltage during the overload and Fsw is the normal switching frequency. If the input voltage should exceed this value, while in foldback mode, the regulator and/or the diode may be damaged. It is important to note that the voltages in these equations are measured at the inductor. Normal trace and wiring resistance will cause the voltage at the inductor to be higher than that at a remote load. Therefore, even if the load is shorted with zero volts across its terminals, the inductor will still see a finite voltage. It is this value that should be used for Vx and Vsc in the calculations. In order to return from foldback mode, the load must be reduced to a value much lower than that required to initiate foldback. This load "hysteresis" is a normal aspect of any type of current limit foldback associated with voltage regulators. If the frequency synchronization feature is used, the current limit frequency fold-back is not operational, and the system may not survive a hard short-circuit at the output. The safe operating areas, when in short circuit mode, are shown in Figure 15 through Figure 17 , for different switching frequencies. Operating points below and to the right of the curve represent safe operation. Note that these curves are not valid when the LM22670 is in frequency synchronization mode. Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM22670 |
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