Datasheet LTC3707-SYNC (Analog Devices) - 10

LTC3707-SYNC
OPERATION (Refer to Functional Diagram)
turns off. As VIN decreases to a voltage close to VOUT, a variable “sleep” interval depending upon the load current. the loop may enter dropout and attempt to turn on the The resultant output voltage ripple is held to a very small top MOSFET continuously. The dropout detector detects value by having the hysteretic comparator after the error this and forces the top MOSFET off for about 400ns every amplifi er gain block. tenth cycle to allow CB to recharge.
Frequency Synchronization
The main control loop is shut down by pulling the RUN/ SS pin low. Releasing RUN/SS allows an internal 1.2μA The phase-locked loop allows the internal oscillator to current source to charge soft-start capacitor C be synchronized to an external source via the PLLIN pin. SS. When C The output of the phase detector at the PLLFLTR pin is SS reaches 1.5V, the main control loop is enabled with the I also the DC frequency control input of the oscillator that TH voltage clamped at approximately 30% of its maximum value. As C operates over a 140kHz to 310kHz range corresponding SS continues to charge, the ITH pin voltage is gradually released allowing normal, full-current to a DC voltage input from 0V to 2.4V. When locked, the operation. When both RUN/SS1 and RUN/SS2 are low, all PLL aligns the turn on of the top MOSFET to the rising controller functions are shut down, including the 5V and edge of the synchronizing signal. When PLLIN is left 3.3V regulators. open, the PLLFLTR pin goes low, forcing the oscillator to minimum frequency.
Low Current Operation Continuous Current (PWM) Operation
The FCB pin is a multifunction pin providing two functions: 1) to provide regulation for a secondary winding by Tying the FCB pin to ground will force continuous current temporarily forcing continuous PWM operation on both operation. This is the least effi cient operating mode, but controllers; and 2) select between two modes of low current may be desirable in certain applications. The output can operation. When the FCB pin voltage is below 0.8V, the source or sink current in this mode. When sinking current controller forces continuous PWM current mode operation. while in forced continuous operation, current will be forced In this mode, the top and bottom MOSFETs are alternately back into the main power supply potentially boosting the turned on to maintain the output voltage independent input supply to dangerous voltage levels—BEWARE! of direction of inductor current. When the FCB pin is below V
INTV
INTVCC – 2V but greater than 0.8V, the controller
CC/EXTVCC Power
enters Burst Mode operation. Burst Mode operation sets Power for the top and bottom MOSFET drivers and most a minimum output current level before inhibiting the top other internal circuitry is derived from the INTVCC pin. When switch and turns off the synchronous MOSFET(s) when the EXTVCC pin is left open, an internal 5V low dropout the inductor current goes negative. This combination of linear regulator supplies INTVCC power. If EXTVCC is taken requirements will, at low currents, force the ITH pin below above 4.7V, the 5V regulator is turned off and an internal a voltage threshold that will temporarily inhibit turn-on of switch is turned on connecting EXTVCC to INTVCC. This al- both output MOSFETs until the output voltage drops. There lows the INTVCC power to be derived from a high effi ciency is 60mV of hysteresis in the burst comparator B tied to external source such as the output of the regulator itself the ITH pin. This hysteresis produces output signals to the or a secondary winding, as described in the Applications MOSFETs that turn them on for several cycles, followed by Information section. 3707sfa 10