Datasheet LTC3834 (Analog Devices) - 10

LTC3834
OPERATION (Refer to Functional Diagram) Main Control Loop
The top MOSFET driver is biased from the floating bootstrap The LTC3834 uses a constant-frequency, current mode capacitor, CB, which normally recharges during each off step-down architecture. During normal operation, the ex- cycle through an external diode when the top MOSFET ternal top MOSFET is turned on when the clock sets the RS turns off. If the input voltage VIN decreases to a voltage latch, and is turned off when the main current comparator, close to VOUT , the loop may enter dropout and attempt ICMP, resets the RS latch. The peak inductor current at to turn on the top MOSFET continuously. The dropout which ICMP trips and resets the latch is controlled by the detector detects this and forces the top MOSFET off for voltage on the I about one twelfth of the clock period every tenth cycle to TH pin, which is the output of the error am- plifier EA. The error amplifier compares the output voltage allow CB to recharge. feedback signal at the VFB pin, (which is generated with
Shutdown and Start-Up (RUN and TRACK/SS Pins)
an external resistor divider connected across the output voltage, V The LTC3834 can be shut down using the RUN pin. Pulling OUT , to ground) to the internal 0.800V reference voltage. When the load current increases, it causes a slight this pin below 0.7V shuts down the main control loop decrease in V of the controller. A low disables the controller and most FB relative to the reference, which cause the EA to increase the I internal circuits, including the INTV TH voltage until the average inductor CC regulator, at which current matches the new load current. time the LTC3834 draws only 4µA of quiescent current. After the top MOSFET is turned off each cycle, the bottom Releasing the RUN pin allows an internal 0.5µA current MOSFET is turned on until either the inductor current starts to pull up the pin and enable that controller. Alternatively, to reverse, as indicated by the current comparator IR, or the RUN pin may be externally pulled up or driven directly the beginning of the next clock cycle. by logic. Be careful not to exceed the Absolute Maximum rating of 7V on this pin.
INTVCC/EXTVCC Power
The start-up of the output voltage VOUT is controlled by Power for the top and bottom MOSFET drivers and most the voltage on the TRACK/SS pin. When the voltage on other internal circuitry is derived from the INTVCC pin. the TRACK/SS pin is less than the 0.8V internal reference, When the EXTVCC pin is left open or tied to a voltage less the LTC3834 regulates the VFB voltage to the TRACK/SS than 4.7V, an internal 5.25V low dropout linear regulator pin voltage instead of the 0.8V reference. This allows supplies INTVCC power from VIN. If EXTVCC is taken above the TRACK/SS pin to be used to program a soft-start by 4.7V, the 5.25V regulator is turned off and a 7.5V low connecting an external capacitor from the TRACK/SS pin dropout linear regulator is enabled that supplies INTVCC to SGND. An internal 1µA pull-up current charges this power from EXTVCC. If EXTVCC is less than 7.5V (but capacitor creating a voltage ramp on the TRACK/SS pin. greater than 4.7V), the 7.5V regulator is in dropout and As the TRACK/SS voltage rises linearly from 0V to 0.8V INTVCC is approximately equal to EXTVCC. When EXTVCC (and beyond), the output voltage VOUT rises smoothly is greater than 7.5V (up to an absolute maximum rating from zero to its final value. of 10V), INTVCC is regulated to 7.5V. Using the EXTVCC Alternatively the TRACK/SS pin can be used to cause the pin allows the INTVCC power to be derived from a high start-up of V efficiency external source such as one of the LTC3834 OUT to “track” that of another supply. Typ- ically, this requires connecting to the TRACK/SS pin an switching regulator outputs. external resistor divider from the other supply to ground (see Applications Information section). 3834fc 10 For more information www.linear.com/LTC3834 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Operation Package Description Typical Application Related Parts