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LTC3854EDDB Datasheet(PDF) 9 Page - Linear Technology |
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LTC3854EDDB Datasheet(HTML) 9 Page - Linear Technology |
9 / 28 page LTC3854 3854fa operaTion Main Control Loop The LTC3854 is a constant-frequency, peak current mode step-down controller. During normal operation, the top MOSFET is turned on when the clock sets the RS latch, and is turned off when the main current comparator, ICMP, resets the RS latch. The peak inductor current at which ICMP resets the RS latch is controlled by the voltage on the ITH pin, which is the output of the error amplifier EA. The VFB pin receives the voltage feedback signal, which is comparedtotheinternalreferencevoltagebytheEA.When the load current increases, it causes a slight decrease in VFB relative to the 0.8V reference, which in turn causes the ITH voltage to increase until the average inductor cur- rent matches the new load current. After the top MOSFET has turned off, the bottom MOSFET is turned on until the beginning of the next cycle. INTVCC Power Power for the top and bottom MOSFET drivers and most other internal circuitry is derived from the INTVCC pin. An internal 5V low dropout linear regulator supplies INTVCC power from VIN. The top MOSFET driver is biased from a floating bootstrap capacitorCB,whichrechargesduringeachoffcyclethrough an external Schottky diode when the top MOSFET turns off. If the input voltage VIN decreases to a voltage close to VOUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout detector then forces the top MOSFET off for 1/10 of the clock period every fourth cycle to allow CB to recharge. Shutdown and Start-Up (RUN/SS) The LTC3854 is shut down using the RUN/SS pin. Pulling this pin below 1.2V disables the controller and most of the internal circuitry, including the INTVCC regulator. However, for RUN/SS>0.8V the internal bandgap is functional and the input current will be greater than the minimum shutdown current. To keep the part in a true shutdownmodetheRUN/SSpinshouldbeheldbelow0.4V. ReleasingRUN/SSpinallowsaninternal1.25µAcurrentto pull up the pin and enable the controller. Alternatively, the RUN/SS pin may be externally pulled up or driven directly by logic. Be careful not to exceed the Absolute Maximum Rating of 6V on this pin. The start-up of the controller’s output voltage VOUT is governed by the voltage on the RUN/SS pin until RUN/SS > 2V. When the voltage on the RUN/SS pin is greater than 1.2V and less than 2V the LTC3854 regulates the VFB voltage to 1.2V below the RUN/SS pin voltage. The RUN/SS pin pro- grams the soft-start period through an external capacitor from the RUN/SS pin to GND. An internal 1.25µA pull-up current charges this capacitor creating a voltage ramp on the RUN/SS pin. As the RUN/SS voltage rises linearly from 1.2V to 2V, VOUT rises smoothly from zero to the target output voltage. When the LTC3854 is in undervoltage lockout the external MOSFETs are held off. Frequency of Operation The LTC3854 operates at a fixed frequency of 400kHz. Output Overvoltage Protection An overvoltage comparator, OV, guards against transient overshoots (>10%) as well as other more serious condi- tions that may overvoltage the output. In such cases, the topMOSFETisturnedoffandthebottomMOSFETisturned on until the overvoltage condition is cleared. |
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