Datasheet LTC3834-1 (Analog Devices) - 9

LTC3834-1
U OPERATIO (Refer to Functional Diagram) Main Control Loop Shutdown and Start-Up (RUN and TRACK/SS Pins)
The LTC3834-1 uses a constant-frequency, current mode The LTC3834-1 can be shut down using the RUN pin. step-down architecture. During normal operation, each Pulling this pin below 0.7V shuts down the main control external top MOSFET is turned on when the clock sets the loop for the controller. A low disables the controller and RS latch, and is turned off when the main current com- most internal circuits, including the INTVCC regulator, parator, ICMP, resets the RS latch. The peak inductor at which time the LTC3834-1 draws only 4μA of quies- current at which ICMP trips and resets the latch is con- cent current. trolled by the voltage on the ITH pin, which is the output of Releasing the RUN pin allows an internal 0.5μA current to the error amplifier EA. The error amplifier compares the pull up the pin and enable that controller. Alternatively, the output voltage feedback signal at the VFB pin, (which is RUN pin may be externally pulled up or driven directly by generated with an external resistor divider connected logic. Be careful not to exceed the Absolute Maximum across the output voltage, VOUT, to ground) to the internal rating of 7V on this pin. 0.800V reference voltage. When the load current increases, it causes a slight decrease in VFB relative to the reference, The start-up of the output voltage VOUT is controlled by the which cause the EA to increase the ITH voltage until the voltage on the TRACK/SS pin. When the voltage on the average inductor current matches the new load current. TRACK/SS pin is less than the 0.8V internal reference, the After the top MOSFET is turned off each cycle, the bottom LTC3834-1 regulates the VFB voltage to the TRACK/SS pin MOSFET is turned on until either the inductor current voltage instead of the 0.8V reference. This allows the starts to reverse, as indicated by the current comparator TRACK/SS pin to be used to program a soft-start by IR, or the beginning of the next clock cycle. connecting an external capacitor from the TRACK/SS pin to SGND. An internal 1μA pull-up current charges this
INTV
capacitor creating a voltage ramp on the TRACK/SS pin. As
CC Power
the TRACK/SS voltage rises linearly from 0V to 0.8V (and Power for the top and bottom MOSFET drivers and most beyond), the output voltage V other internal circuitry is derived from the INTV OUT rises smoothly from CC pin. An zero to its final value. internal 5.25V low dropout linear regulator supplies INTVCC power from VIN. Alternatively the TRACK/SS pin can be used to cause the start-up of V The top MOSFET driver is biased from the floating boot- OUT to “track” that of another supply. Typi- cally, this requires connecting to the TRACK/SS pin an strap capacitor CB, which normally recharges during each external resistor divider from the other supply to ground off cycle through an external diode when the top MOSFET (see Applications Information section). turns off. If the input voltage VIN decreases to a voltage close to VOUT, the loop may enter dropout and attempt to When the RUN pin is pulled low to disable the LTC3834- turn on the top MOSFET continuously. The dropout detec- 1, or when VIN drops below its undervoltage lockout tor detects this and forces the top MOSFET off for about threshold of 3.5V, the TRACK/SS pin is pulled low by an one twelfth of the clock period every tenth cycle to allow CB internal MOSFET. When in undervoltage lockout, the to recharge. controller is disabled and the external MOSFETs are held off. 38341f 9