Datasheet LTC3851A (Analog Devices) - 10

LTC3851A
operaTion Main Control Loop
pull up the pin and enable that control ler. Alternatively, The LTC3851A is a constant frequency, current mode the RUN pin may be externally pulled up or driven directly step-down controller. During normal operation, the top by logic. Be careful not to exceed the absolute maximum MOSFET is turned on when the clock sets the RS latch, rating of 6V on this pin. and is turned off when the main current comparator, ICMP , The start-up of the controller’s output voltage, VOUT , is resets the RS latch. The peak inductor current at which controlled by the voltage on the TK/SS pin. When the ICMP resets the RS latch is controlled by the voltage on voltage on the TK/SS pin is less than the 0.8V internal the ITH pin, which is the output of the error amplifier, EA. reference, the LTC3851A regulates the VFB voltage to the The VFB pin receives the voltage feedback signal, which is TK/SS pin voltage instead of the 0.8V reference. This al- compared to the internal reference voltage by the EA. When lows the TK/SS pin to be used to program a soft-start by the load current increases, it causes a slight decrease in connecting an external capacitor from the TK/SS pin to VFB relative to the 0.8V reference, which in turn causes the GND. An internal 1µA pul -up current charges this capacitor ITH voltage to increase until the average inductor current creating a voltage ramp on the TK/SS pin. As the TK/SS matches the new load current. After the top MOSFET has voltage rises linearly from 0V to 0.8V (and beyond), the turned off, the bottom MOSFET is turned on until either output voltage VOUT rises smoothly from zero to its final the inductor current starts to reverse, as indicated by the value. Alternatively, the TK/SS pin can be used to cause reverse current comparator, IREV, or the beginning of the the start-up of VOUT to track another supply. Typically, next cycle. this requires connecting to the TK/SS pin an external resistor divider from the other supply to ground (see the
INTVCC Power
Applica tions Information section). When the RUN pin Power for the top and bottom MOSFET drivers and most is pulled low to disable the controller, or when INTVCC other internal circuitry is derived from the INTV drops below its undervoltage lockout threshold of 3.2V, CC pin. An internal 5V low dropout linear regulator supplies INTV the TK/SS pin is pulled low by an internal MOSFET. When CC power from V in undervoltage lockout, the controller is disabled and the IN. external MOSFETs are held off. The top MOSFET driver is biased from the floating boot- strap capacitor, CB, which normally recharges during each
Light Load Current Operation (Burst Mode Operation,
off cycle through an external diode when the top MOSFET
Pulse-Skipping or Continuous Conduction)
turns off. If the input voltage, VIN, decreases to a voltage close to V The LTC3851A can be enabled to enter high efficiency OUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout Burst Mode operation, constant frequency pulse-skipping detec tor detects this and forces the top MOSFET off for mode or forced continuous conduction mode. To select about 1/10 of the clock period every tenth cycle to allow forced continuous operation, tie the MODE/PLLIN pin to C INTVCC. To select pulse-skipping mode of operation, float B to recharge. However, it is recommended that there is always a load present during the drop-out transition to the MODE/PLLIN pin or tie it to GND. To select Burst Mode ensure C operation, tie MODE/PLLIN to INTVCC through a resistor B is recharged. no less than 50k, but no greater than 250k.
Shutdown and Start-Up (RUN and TK/SS)
When the controller is enabled for Burst Mode operation, The LTC3851A can be shut down using the RUN pin. Pull- the peak current in the inductor is set to approximately ing this pin below 1.1V disables the controller and most one-forth of the maximum sense voltage even though of the internal circuitry, including the INTV the voltage on the I CC regulator. TH pin indicates a lower value. If the Releasing the RUN pin allows an internal 2µA current to average inductor current is higher than the load current, 3851afa 10 Document Outline Features Applications Description 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 Typical Application Related Parts