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LM5007 Datasheet(PDF) 7 Page - Texas Instruments |
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LM5007 Datasheet(HTML) 7 Page - Texas Instruments |
7 / 18 page VIN VIN RON RON 7V SERIES REGULATOR ON TIMER VIN START COMPLETE RON STOP RUN LM5007 LM5007 www.ti.com SNVS252E – MAY 2004 – REVISED APRIL 2011 Over-Voltage Comparator The over-voltage comparator is provided to protect the output from overvoltage conditions due to sudden input line voltage changes or output loading changes. The over-voltage comparator monitors the FB pin versus an internal 2.875V reference (OV_REF). If the voltage at FB rises above OV_REF the comparator immediately terminates the buck switch on time pulse. ON Time generator and Shutdown The on time of the LM5007 is set inversely proportional to the input voltage by an external resistor connected between Ron and Vin. The Ron terminal is a low impedance input biased at approximately 1.5V. Thus the current through the resistor and into the Ron terminal is approximately proportional to Vin and used internally to control the on timer. This scheme of input voltage feed-forward hysteretic operation achieves nearly constant operational frequency over varying line and load conditions. The on time equation for the LM5007 is : Ton = 1.42 x 10 -10 x R ON / VIN (4) The RON pin of the LM5007 also provides a shutdown function which disables the regulator and significantly decreases quiescent power dissipation. By pulling the RON pin to below 0.7V logic threshold activates the low power shutdown mode. The VIN quiescent current in the shutdown mode is approximately 100µA internal to the LM5007 plus the current in the RON resistor. Figure 4. Shutdown Implementation Current Limit The LM5007 contains an intelligent current limit off timer intended to reduce the foldback characteristic inherent with fixed off-time over-current protection. If the current in the Buck switch exceeds 725mA the present cycle on time is immediately terminated (cycle by cycle current limit). Following the termination of the cycle a non- resetable current limit off timer is initiated. The duration of the off time is a function of the external resistor (RCI) and the FB pin voltage. When the FB pin voltage equals zero, the current limit off time is internally preset to 17uS. This condition occurs in short circuit operation when a maximum amount of off time is required. In cases of overload (not complete short circuit) the current limit off time can be reduced as a function of the output voltage (measured at the FB pin). Reducing the off time with smaller overloads reduces the amount of foldback and also reduces the initial start-up time. The current limit off time for a given FB pin voltage and RCI resistor can be calculated by the following equation: Copyright © 2004–2011, Texas Instruments Incorporated Submit Documentation Feedback 7 Product Folder Links: LM5007 |
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