Datasheet LTC3827-1 (Analog Devices) - 10

LTC3827-1
OPERATION (Refer to Functional Diagram) INTVCC/EXTVCC Power
The start-up of each controller’s output voltage VOUT is con- trolled by the voltage on the TRACK/SS1 and TRACK/SS2 Power for the top and bottom MOSFET drivers and most pin. When the voltage on the TRACK/SS pin is less than other internal circuitry is derived from the INTVCC pin. the 0.8V internal reference, the LTC3827-1 regulates the When the EXTVCC pin is left open or tied to a voltage less V than 4.7V, an internal 5.25V low dropout linear regulator FB voltage to the TRACK/SS pin voltage instead of the 0.8V reference. This allows the TRACK/SS pin to be used supplies INTVCC power from VIN. If EXTVCC is taken above to program a soft-start by connecting an external capacitor 4.7V, the 5.25V regulator is turned off and a 7.5V low from the TRACK/SS pin to SGND. An internal 1μA pull-up dropout linear regulator is enabled that supplies INTVCC current charges this capacitor creating a voltage ramp on power from EXTVCC. If EXTVCC is less than 7.5V (but the TRACK/SS pin. As the TRACK/SS voltage rises linearly greater than 4.7V), the 7.5V regulator is in dropout and from 0V to 0.8V (and beyond), the output voltage V INTV OUT CC is approximately equal to EXTVCC. When EXTVCC rises smoothly from zero to its fi nal value. is greater than 7.5V (up to an absolute maximum rating 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 startup of VOUT to “track” that of another supply. Typically, effi ciency external source such as one of the LTC3827-1 this requires connecting to the TRACK/SS pin an external switching regulator outputs. resistor divider from the other supply to ground (see Ap- plications Information section). Each top MOSFET driver is biased from the fl oating boot- strap capacitor CB, which normally recharges during each When the corresponding RUN pin is pulled low to disable off cycle through an external diode when the top MOSFET a controller, or when VIN drops below its undervoltage turns off. If the input voltage V lockout threshold of 3.5V, the TRACK/SS pin is pulled low IN decreases to a voltage close to V by an internal MOSFET. When in undervoltage lockout, OUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout both controllers are disabled and the external MOSFETs detector detects this and forces the top MOSFET off for are held off. about one twelfth of the clock period every tenth cycle to allow C
Light Load Current Operation (Burst Mode Operation,
B to recharge.
Pulse-Skipping or Continuous Conduction) Shutdown and Start-Up (RUN1, RUN2 and (PLLIN/MODE Pin) TRACK/SS1, TRACK/SS2 Pins)
The LTC3827-1 can be enabled to enter high effi ciency The two channels of the LTC3827-1 can be independently Burst Mode operation, constant frequency pulse-skipping shut down using the RUN1 and RUN2 pins. Pulling either mode, or forced continuous conduction mode at low load of these pins below 0.7V shuts down the main control currents. To select Burst Mode operation, tie the PLLIN/ loop for that controller. Pulling both pins low disables MODE pin to a DC voltage below 0.7V (e.g., SGND). To both controllers and most internal circuits, including the select forced continuous operation, tie the PLLIN/MODE INTVCC regulator, and the LTC3827-1 draws only 8μA of pin to INTVCC. To select pulse-skipping mode, tie the quiescent current. PLLIN/MODE pin to a DC voltage greater than 0.9V and less than INTV Releasing either RUN pin allows an internal 0.5μA current CC – 1.2V. to pull up the pin and enable that controller. Alternatively, When a controller is enabled for Burst Mode operation, the RUN pin may be externally pulled up or driven directly the peak current in the inductor is set to approximately by logic. Be careful not to exceed the Absolute Maximum one-tenth of the maximum sense voltage even though the rating of 7V on this pin. voltage on the ITH pin indicates a lower value. If the aver- age inductor current is lower than the load current, the error amplifi er EA will decrease the voltage on the ITH pin. 38271fe 10