Datasheet LTC3707 (Analog Devices) - 10

LTC3707
OPERATION (Refer to Functional Diagram) Main Control Loop
secondary winding by temporarily forcing continuous PWM operation on both controllers and 2) a logic input The LTC3707 uses a constant frequency, current mode to select between two modes of low current operation. step-down architecture with the two controller channels When the FCB pin voltage is below 0.800V, the controller operating 180 degrees out of phase. During normal forces continuous PWM current mode operation. In operation, each top MOSFET is turned on when the clock this mode, the top and bottom MOSFETs are alternately for that channel sets the RS latch, and turned off when turned on to maintain the output voltage independent the main current comparator, I1, resets the RS latch. of direction of inductor current. When the FCB pin is The peak inductor current at which I1 resets the RS below V latch is controlled by the voltage on the I INTVCC – 2V but greater than 0.80V, the controller TH pin, which enters Burst Mode operation. Burst Mode operation sets is the output of each error amplifi er EA. The VOSENSE pin a minimum output current level before inhibiting the top receives the voltage feedback signal, which is compared switch and turns off the synchronous MOSFET(s) when to the internal reference voltage by the EA. When the load the inductor current goes negative. This combination of current increases, it causes a slight decrease in VOSENSE requirements will, at low currents, force the I relative to the 0.8V reference, which in turn causes the TH pin below a voltage threshold that will temporarily inhibit turn-on of ITH voltage to increase until the average inductor current both output MOSFETs until the output voltage drops. There matches the new load current. After the top MOSFET has is 60mV of hysteresis in the burst comparator B tied to turned off, the bottom MOSFET is turned on until either the the I inductor current starts to reverse, as indicated by current TH pin. This hysteresis produces output signals to the MOSFETs that turn them on for several cycles, followed comparator I2, or the beginning of the next cycle. by a variable “sleep” interval depending upon the load The top MOSFET drivers are biased from fl oating bootstrap current. The resultant output voltage ripple is held to a capacitor CB, which normally is recharged during each off very small value by having the hysteretic comparator cycle through an external diode when the top MOSFET after the error amplifi er gain block. turns off. As VIN decreases to a voltage close to VOUT, the loop may enter dropout and attempt to turn on the
Constant Frequency Operation
top MOSFET continuously. The dropout detector detects When the FCB pin is tied to INTV this and forces the top MOSFET off for about 500ns every CC, Burst Mode operation is disabled and the forced minimum output current tenth cycle to allow CB to recharge. requirement is removed. This provides constant frequency, The main control loop is shut down by pulling the RUN/ discontinuous (preventing reverse inductor current) SS pin low. Releasing RUN/SS allows an internal 1.2μA current operation over the widest possible output current current source to charge soft-start capacitor CSS. When range. This constant frequency operation is not as effi cient CSS reaches 1.5V, the main control loop is enabled with the as Burst Mode operation, but does provide a lower noise, ITH voltage clamped at approximately 30% of its maximum constant frequency operating mode down to approximately value. As CSS continues to charge, the ITH pin voltage is 1% of designed maximum output current. Voltage should gradually released allowing normal, full-current operation. not be applied to the FCB pin prior to the application of When both RUN/SS1 and RUN/SS2 are low, all LTC3707 voltage to the VIN pin. controller functions are shut down, and the STBYMD pin determines if the standby 5V and 3.3V regulators are
Continuous Current (PWM) Operation
kept alive. Tying the FCB pin to ground will force continuous current operation. This is the least effi cient operating mode,
Low Current Operation
but may be desirable in certain applications. The output The FCB pin is a multifunction pin providing two can source or sink current in this mode. When sinking functions: 1) an analog input to provide regulation for a current while in forced continuous operation, current will 3707fb 10