Datasheet LTC3809 (Analog Devices) - 8

LTC3809
OPERATION (Refer to Functional Diagram) Main Control Loop Light Load Operation (Burst Mode Operation, Continuous Conduction or Pulse-Skipping Mode)
The LTC3809 uses a constant frequency, current mode
(SYNC/MODE Pin)
architecture. During normal operation, the top external P-channel power MOSFET is turned on when the clock The LTC3809 can be programmed for either high effi ciency sets the RS latch, and is turned off when the current Burst Mode operation, forced continuous conduction comparator (ICMP) resets the latch. The peak inductor mode or pulse-skipping mode at low load currents. To current at which ICMP resets the RS latch is determined select Burst Mode operation, tie the SYNC/MODE pin by the voltage on the ITH pin, which is driven by the output to VIN. To select forced continuous operation, tie the of the error amplifi er (EAMP). The VFB pin receives the SYNC/MODE pin to a DC voltage below 0.4V (e.g., GND). output voltage feedback signal from an external resistor Tying the SYNC/MODE to a DC voltage above 0.4V and divider. This feedback signal is compared to the internal below 1.2V (e.g., VFB) enables pulse-skipping mode. The 0.6V reference voltage by the EAMP. When the load cur- 0.4V threshold between forced continuous operation and rent increases, it causes a slight decrease in VFB relative pulse-skipping mode can be used in secondary winding to the 0.6V reference, which in turn causes the ITH voltage regulation as described in the Auxiliary Winding Control to increase until the average inductor current matches the Using SYNC/MODE Pin discussion in the Applications new load current. While the top P-channel MOSFET is off, Information section. the bottom N-channel MOSFET is turned on until either When the LTC3809 is in Burst Mode operation, the peak the inductor current starts to reverse, as indicated by the current in the inductor is set to approximately one-fourth current reversal comparator IRCMP, or the beginning of of the maximum sense voltage even though the voltage on the next cycle. the ITH pin indicates a lower value. If the average inductor current is higher than the load current, the EAMP will
Shutdown and Soft-Start (RUN Pin)
decrease the voltage on the ITH pin. When the ITH voltage The LTC3809 is shut down by pulling the RUN pin low. drops below 0.85V, the internal SLEEP signal goes high In shutdown, all controller functions are disabled and the and the external MOSFET is turned off. chip draws only 9μA. The TG output is held high (off) and In sleep mode, much of the internal circuitry is turned off, the BG output low (off) in shutdown. Releasing the RUN reducing the quiescent current that the LTC3809 draws. pin allows an internal 0.7μA current source to pull up the The load current is supplied by the output capacitor. As RUN pin to VIN. The controller is enabled when the RUN the output voltage decreases, the EAMP increases the pin reaches 1.1V. ITH voltage. When the ITH voltage reaches 0.925V, the The start-up of VOUT is controlled by the LTC3809’s inter- SLEEP signal goes low and the controller resumes normal nal soft-start. During soft-start, the error amplifi er EAMP operation by turning on the external P-channel MOSFET compares the feedback signal VFB to the internal soft-start on the next cycle of the internal oscillator. ramp (instead of the 0.6V reference), which rises linearly When the controller is enabled for Burst Mode or pulse- from 0V to 0.6V in about 1ms. This allows the output skipping operation, the inductor current is not allowed to voltage to rise smoothly from 0V to its fi nal value while reverse. Hence, the controller operates discontinuously. maintaining control of the inductor current. 3809fc 8