Datasheet LTC3854 (Analog Devices) - 9

LTC3854
OPERATION Main Control Loop Shutdown and Start-Up (RUN/SS)
The LTC3854 is a constant-frequency, peak current mode The LTC3854 is shut down using the RUN/SS pin. Pulling step-down controller. During normal operation, the top this pin below 1.2V disables the controller and most of the MOSFET is turned on when the clock sets the RS latch, internal circuitry, including the INTVCC regulator. and is turned off when the main current comparator, ICMP, However, for RUN/SS>0.8V the internal bandgap is resets the RS latch. The peak inductor current at which functional and the input current will be greater than the ICMP resets the RS latch is controlled by the voltage on minimum shutdown current. To keep the part in a true the ITH pin, which is the output of the error amplifier EA. shutdown mode the RUN/SS pin should be held below 0.4V. The VFB pin receives the voltage feedback signal, which is Releasing RUN/SS pin allows an internal 1.25µA current to compared to the internal reference voltage by the EA. When pull up the pin and enable the controller. Alternatively, the the load current increases, it causes a slight decrease in RUN/SS pin may be externally pulled up or driven directly VFB relative to the 0.8V reference, which in turn causes by logic. Be careful not to exceed the Absolute Maximum the ITH voltage to increase until the average inductor cur- Rating of 6V on this pin. rent matches the new load current. After the top MOSFET has turned off, the bottom MOSFET is turned on until the The start-up of the controller’s output voltage VOUT beginning of the next cycle. is governed by the voltage on the RUN/SS pin until RUN/SS > 2V.
INTVCC Power
When the voltage on the RUN/SS pin is greater than 1.2V Power for the top and bottom MOSFET drivers and most and less than 2V the LTC3854 regulates the VFB voltage to other internal circuitry is derived from the INTVCC pin. An 1.2V below the RUN/SS pin voltage. The RUN/SS pin pro- internal 5V low dropout linear regulator supplies INTVCC grams the soft-start period through an external capacitor power from VIN. from the RUN/SS pin to GND. An internal 1.25µA pull-up The top MOSFET driver is biased from a floating bootstrap current charges this capacitor creating a voltage ramp on capacitor C the RUN/SS pin. As the RUN/SS voltage rises linearly from B , which recharges during each off cycle through an external Schottky diode when the top MOSFET turns 1.2V to 2V, VOUT rises smoothly from zero to the target off. If the input voltage V output voltage. When the LTC3854 is in undervoltage IN decreases to a voltage close to V lockout the external MOSFETs are held off. OUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout detector then
Frequency of Operation
forces the top MOSFET off for 1/10 of the clock period every fourth cycle to allow CB to recharge. 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 top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. 3854fb 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts