Datasheet LTC3808 (Analog Devices) - 9

LTC3808
U OPERATIO (Refer to Functional Diagram) Main Control Loop
by releasing the RUN pin, the TRACK/SS pin is charged up by an internal 1 The LTC3808 uses a constant frequency, current mode µA current source and rises linearly from 0V to above 0.6V. The error amplifier EAMP compares the architecture. During normal operation, the top external feedback signal V P-channel power MOSFET is turned on when the clock FB to this ramp instead, and regulates VFB linearly from 0V to 0.6V. sets the RS latch, and is turned off when the current comparator (ICMP) resets the latch. The peak inductor When the voltage on the TRACK/SS pin is less than the current at which ICMP resets the RS latch is determined by 0.6V internal reference, the LTC3808 regulates the VFB the voltage on the ITH pin, which is driven by the output of voltage to the TRACK/SS pin instead of the 0.6V reference. the error amplifier (EAMP). The VFB pin receives the output Therefore VOUT of the LTC3808 can track an external voltage feedback signal from an external resistor divider. voltage VX during start-up. Typically, a resistor divider on This feedback signal is compared to the internal 0.6V VX is connected to the TRACK/SS pin to allow the start-up reference voltage by the EAMP. When the load current of VOUT to “track” that of VX. For coincident tracking during increases, it causes a slight decrease in VFB relative to the start-up, the regulated final value of VX should be larger 0.6V reference, which in turn causes the ITH voltage to than that of VOUT, and the resistor divider on VX has the increase until the average inductor current matches the same ratio as the divider on VOUT that is connected to VFB. new load current. While the top P-channel MOSFET is off, See detailed discussions in the Run and Soft-Start/ the bottom N-channel MOSFET is turned on until either the Tracking Functions in the Applications Information inductor current starts to reverse, as indicated by the Section. current reversal comparator IRCMP, or the beginning of the next cycle.
Light Load Operation (Burst Mode Operation, Continuous Conduction or Pulse Skipping Mode) Shutdown, Soft-Start and Tracking Start-Up (SYNC/MODE Pin) (RUN and TRACK/SS Pins)
The LTC3808 can be programmed for either high effi- The LTC3808 is shut down by pulling the RUN pin low. In ciency Burst Mode operation, forced continuous conduc- shutdown, all controller functions are disabled and the tion mode or pulse skipping mode at low load currents. To chip draws only 9µA. The TG output is held high (off) and select Burst Mode operation, tie the SYNC/MODE pin to the BG output low (off) in shutdown. Releasing the RUN VIN. To select forced continuous operation, tie the SYNC/ pin allows an internal 0.7µA current source to pull up the MODE pin to a DC voltage below 0.4V (e.g., GND). Tying RUN pin to VIN. The controller is enabled when the RUN pin the SYNC/MODE to a DC voltage above 0.4V and below reaches 1.1V. 1.2V (e.g., VFB) enables pulse skipping mode. The 0.4V threshold between forced continuous operation and pulse The start-up of VOUT is based on the three different skipping mode can be used in secondary winding regula- connections on the TRACK/SS pin. The start-up of VOUT tion as described in the Auxiliary Winding Control Using is controlled by the LTC3808’s internal soft-start when SYNC/MODE Pin discussion in the Applications Informa- TRACK/SS is connected to VIN. During soft-start, the error tion section. amplifier EAMP compares the feedback signal VFB to the internal soft-start ramp (instead of the 0.6V reference), When the LTC3808 is in Burst Mode operation, the peak which rises linearly from 0V to 0.6V in about 1ms. This current in the inductor is set to approximate one-fourth of allows the output voltage to rise smoothly from 0V to its the maximum sense voltage even though the voltage on final value while maintaining control of the inductor the ITH pin indicates a lower value. If the average inductor current. current is higher than the load current, the EAMP will decrease the voltage on the I The 1ms soft-start time can be changed by connecting the TH pin. When the ITH voltage drops below 0.85V, the internal SLEEP signal goes high optional external soft-start capacitor CSS between the and the external MOSFET is turned off. TRACK/SS and GND pins. When the controller is enabled 3808f 9