Datasheet LTC3835-1 (Analog Devices) - 9

LTC3835-1
OPERATION (Refer to Functional Diagram) Main Control Loop Shutdown and Start-Up (RUN and TRACK/SS Pins)
The LTC3835-1 uses a constant-frequency, current mode The LTC3835-1 can be shut down using the RUN pin. Pulling step-down architecture. During normal operation, each this pin below 0.7V shuts down the main control loop for the external top MOSFET is turned on when the clock sets the controller. A low disables the controller and most internal RS latch, and is turned off when the main current compara- circuits, including the INTVCC regulator, at which time the tor, ICMP, resets the RS latch. The peak inductor current LTC3835-1 draws only 10μA of quiescent current. at which ICMP trips and resets the latch is controlled by Releasing the RUN pin allows an internal 0.5μA current the voltage on the ITH pin, which is the output of the error to pull up the pin and enable that controller. Alternatively, amplifi er EA. The error amplifi er compares the output volt- the RUN pin may be externally pulled up or driven directly age feedback signal at the VFB pin, (which is generated with by logic. Be careful not to exceed the Absolute Maximum an external resistor divider connected across the output rating of 7V on this pin. voltage, VOUT , to ground) to the internal 0.800V reference voltage. When the load current increases, it causes a slight The start-up of the output voltage VOUT is controlled by decrease in V the voltage on the TRACK/SS pin. When the voltage on FB relative to the reference, which cause the EA to increase the I the TRACK/SS pin is less than the 0.8V internal reference, TH voltage until the average inductor current matches the new load current. the LTC3835-1 regulates the VFB voltage to the TRACK/SS pin voltage instead of the 0.8V reference. This allows After the top MOSFET is turned off each cycle, the bottom the TRACK/SS pin to be used to program a soft-start by MOSFET is turned on until either the inductor current starts connecting an external capacitor from the TRACK/SS pin to reverse, as indicated by the current comparator IR, or to SGND. An internal 1μA pull-up current charges this the beginning of the next clock cycle. capacitor creating a voltage ramp on the TRACK/SS pin. As the TRACK/SS voltage rises linearly from 0V to 0.8V
INTVCC Power
(and beyond), the output voltage VOUT rises smoothly Power for the top and bottom MOSFET drivers and most from zero to its fi nal value. other internal circuitry is derived from the INTVCC pin. Alternatively the TRACK/SS pin can be used to cause the An internal 5.25V low dropout linear regulator supplies start-up of V INTV OUT to “track” that of another supply. Typi- CC power from VIN. cally, this requires connecting to the TRACK/SS pin an The top MOSFET driver is biased from the fl oating bootstrap external resistor divider from the other supply to ground capacitor CB, which normally recharges during each off (see Applications Information section). cycle through an external diode when the top MOSFET When the RUN pin is pulled low to disable the LTC3835-1, turns off. If the input voltage VIN decreases to a voltage or when V close to V IN drops below its undervoltage lockout thresh- OUT , the loop may enter dropout and attempt old of 3.5V, the TRACK/SS pin is pulled low by an internal to turn on the top MOSFET continuously. The dropout MOSFET. When in undervoltage lockout, the controller is detector detects this and forces the top MOSFET off for disabled and the external MOSFETs are held off. about one twelfth of the clock period every tenth cycle to allow CB to recharge. 38351fc 9