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LM22678 Scheda tecnica(PDF) 11 Page - Texas Instruments |
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LM22678 Scheda tecnica(HTML) 11 Page - Texas Instruments |
11 / 26 page LM22678, LM22678-Q1 www.ti.com SNVS585L – SEPTEMBER 2008 – REVISED NOVEMBER 2014 Feature Description (continued) In general, hand calculations or simulations can only aid in selecting good power stage components. Good design practice dictates that load and line transient testing should be done to verify the stability of the application. Also, Bode plot measurements should be made to determine stability margins. AN-1889 How to Measure the Loop Transfer Function of Power Supplies (SNVA364) shows how to perform a loop transfer function measurement with only an oscilloscope and function generator. 7.4 Device Functional Modes 7.4.1 Shutdown Mode The EN pin provides electrical ON and OFF control for the LM22678 device. When VEN is below 1.6 V, the device is in shutdown mode. The current drain from the input supply, in this state, is 25 µA (typ) at an input voltage of 12 V. The EN input has an internal pullup of about 6 µA. The LM22678 also incorporates an input undervoltage lock-out (UVLO) feature. This prevents the regulator from turning on when the input voltage is not great enough to properly bias the internal circuitry. The rising threshold is 4.3 V (typ) while the falling threshold is 3.9 V (typ). 7.4.2 Active Mode The LM22678 device is in active mode when VEN is above the precision enable threshold and its input voltage is above its UVLO level. The simplest way to enable the LM22678 is to connect the EN pin to VIN through a resistor. A resistor of 470 k Ω will limit the current to a safe value for input voltages as high 42 V. 7.4.3 Current Limit The LM22678 device 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 Equation 5. (5) Where: L is the value of the power inductor. When the LM22678 device 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. Equation 6 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. Vin is the maximum for the application. Copyright © 2008–2014, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM22678 LM22678-Q1 |
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