Datasheet LTC3822-1 (Analog Devices) - 9

LTC3822-1
OPERATION (Refer to Functional Diagram) Main Control Loop
by releasing the RUN pin, the TRACK/SS pin is charged up by an internal 1µA current source and rises linearly from The LTC3822-1 uses a constant frequency, current mode 0V to above 0.6V. The error amplifi er EAMP compares the architecture. During normal operation, the top external feedback signal V N-channel power MOSFET is turned on when the clock FB to this ramp instead, and regulates V sets the RS latch, and is turned off when the current FB linearly from 0V to 0.6V. 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 0.6V internal reference, the LTC3822-1 regulates the VFB by the voltage on the ITH pin, which is driven by the output voltage to the TRACK/SS pin instead of the 0.6V reference. of the error amplifi er (EAMP). The VFB pin receives the Therefore VOUT of the LTC3822-1 can track an external output voltage feedback signal from an external resistor voltage VX during start-up. Typically, a resistor divider on divider. This feedback signal is compared to the internal VX is connected to the TRACK/SS pin to allow the start-up 0.6V reference voltage by the EAMP. When the load cur- of VOUT to “track” that of VX. For coincident tracking during rent increases, it causes a slight decrease in VFB relative start-up, the regulated fi nal value of VX should be larger to the 0.6V reference, which in turn causes the ITH voltage than that of VOUT, and the resistor divider on VX should to increase until the average inductor current matches the have the same ratio as the divider on VOUT that is connected new load current. While the top N-channel MOSFET is off, to VFB. See detailed discussions in the Run and Soft-Start/ the bottom N-channel MOSFET is turned on until either Tracking Functions in the Applications Information the inductor current starts to reverse, as indicated by the section. current reversal comparator RICMP, 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 LTC3822-1 can be programmed for either high effi - The LTC3822-1 is shut down by pulling the RUN pin low. ciency Burst Mode operation, forced continuous conduc- In shutdown, all controller functions are disabled and the tion mode or pulse skipping mode at low load currents. To chip draws only 7.5µA. The TG and BG outputs are held select Burst Mode operation, tie the SYNC/MODE pin to low (off) in shutdown. Releasing the RUN pin allows an VIN. To select forced continuous operation, tie the SYNC/ internal 0.7µA current source to pull up the RUN pin to MODE pin to a DC voltage below 0.4V (e.g., GND). Tying VIN. The controller is enabled when the RUN pin reaches the SYNC/MODE to a DC voltage above 0.4V and below 1.1V. 1.2V (e.g., VFB) enables pulse skipping mode. The start-up of VOUT is based on the three different con- When the LTC3822-1 is in Burst Mode operation, the peak nections on the TRACK/SS pin. The start-up of VOUT is current in the inductor is set to approximate one-fourth of controlled by the LTC3822-1’s internal soft-start when the maximum sense voltage even though the voltage on TRACK/SS is connected to VIN. During soft-start, the error the ITH pin indicates a lower value. If the average induc- amplifi er EAMP compares the feedback signal VFB to the tor current is higher than the load current, the EAMP will internal soft-start ramp (instead of the 0.6V reference), decrease the voltage on the ITH pin. When the ITH voltage which rises linearly from 0V to 0.6V in about 1ms. This drops below 0.85V, the internal SLEEP signal goes high allows the output voltage to rise smoothly from 0V to and the external MOSFETs are turned off. its fi nal value while maintaining control of the inductor In sleep mode, much of the internal circuitry is turned off, current. reducing the quiescent current that the LTC3822-1 draws. The 1ms soft-start time can be changed by connecting The load current is supplied by the output capacitor. As the optional external soft-start capacitor CSS between the the output voltage decreases, the EAMP increases the ITH TRACK/SS and GND pins. When the controller is enabled voltage. When the ITH voltage reaches 0.925V, the SLEEP 38221f 9