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LM2854 Datasheet(PDF) 11 Page - Texas Instruments |
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LM2854 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 33 page 11 LM2854 www.ti.com SNVS560D – MARCH 2008 – REVISED JANUARY 2016 Product Folder Links: LM2854 Submit Documentation Feedback Copyright © 2008–2016, Texas Instruments Incorporated Feature Description (continued) 7.3.8 Negative Current Limit The LM2854 implements negative current limit detection circuitry to prevent large negative current in the inductor. When the negative current sensed in the low-side NFET is below approximately –0.4 A, the present switching cycle is immediately terminated and both FETs are turned off. When both FETs are off, the negative inductor current originally flowing in the low-side NFET and into the SW pin commutates to the high-side PFET’s body diode and ramps back to zero. At this point, the SW pin becomes a high impedance node and ringing can be observed on the SW node as the stored energy in the inductor is dissipated while resonating with the parasitic nodal capacitance. 7.3.9 Overtemperature Protection When the LM2854 senses a junction temperature greater than 165°C, both switching FETs are turned off and the part enters a sleep state. Upon sensing a junction temperature below 155°C, the part will re-initiate the soft-start sequence and begin switching once again. This feature is provided to prevent catastrophic failure due to excessive thermal dissipation. 7.3.10 Loop Compensation The LM2854 preserves flexibility by integrating the control components around the error amplifier while using three small external compensation components from VOUT to FB. An integrated type II (two pole, one zero) voltage-mode compensation network is featured. To ensure stability, an external resistor and small value capacitor can be added across the upper feedback resistor as a pole-zero pair to complete a type III (three pole, two zero) compensation network. For correct selection of these components, see Detailed Design Procedure. 7.4 Device Functional Modes 7.4.1 Shutdown Mode If EN is less than VIH – VEN(HSY), the LM2854 shuts down. Most internal circuitry is shut down, and the SW output is high-impedance. Once EN voltage exceeds VIH, the LM2854 enters soft-start mode. 7.4.2 Soft-Start and Track Mode Once operation is initiated, the LM2854 starts charging its SS node. If a voltage is already present on the LM2854 circuit output, the LM2854 does not attempt to pull the circuit output low. If only a capacitor is connected to the SS pin, voltage on this pin rises, and once voltage exceeds 0.8 V, normal operating mode commences. If the high-side current limit is exceeded, the voltage on the SS pin is reduced, prolonging soft-start and track mode. If a resistor divider is used to connect the SS pin to another power supply, Soft Start and Track mode can be used to cause the output of the LM2854 Buck to track this supply. 7.4.3 Normal Operating Mode If the EN input of the LM2854 is above VIH and the SS pin is above 0.8 V, output is regulated normally. If EN is reduced to less than VIH – VEN(HSY), the LM2854 enters shutdown mode. If the high-side current limit is activated or SS is pulled to below 0.8 V, the LM2854 enters soft-start and track mode. |
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