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LTC3809EMSE Datasheet(PDF) 8 Page - Linear Technology |
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LTC3809EMSE Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  LTC3809 8 3809fc OPERATION (Refer to Functional Diagram) Main Control Loop The LTC3809 uses a constant frequency, current mode architecture. During normal operation, the top external P-channel power MOSFET is turned on when the clock sets the RS latch, and is turned off when the current comparator (ICMP) resets the latch. The peak inductor current at which ICMP resets the RS latch is determined by the voltage on the ITH pin, which is driven by the output of the error amplifier (EAMP). The VFB pin receives the output voltage feedback signal from an external resistor divider. This feedback signal is compared to the internal 0.6V reference voltage by the EAMP. When the load cur- rent increases, it causes a slight decrease in VFB relative to the 0.6V reference, which in turn causes the ITH voltage to increase until the average inductor current matches the new load current. While the top P-channel MOSFET is off, the bottom N-channel MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next cycle. Shutdown and Soft-Start (RUN Pin) The LTC3809 is shut down by pulling the RUN pin low. In shutdown, all controller functions are disabled and the chip draws only 9μA. The TG output is held high (off) and the BG output low (off) in shutdown. Releasing the RUN pin allows an internal 0.7μA current source to pull up the RUN pin to VIN. The controller is enabled when the RUN pin reaches 1.1V. The start-up of VOUT is controlled by the LTC3809’s inter- nal soft-start. During soft-start, the error amplifier EAMP compares the feedback signal VFB to the internal soft-start ramp (instead of the 0.6V reference), which rises linearly from 0V to 0.6V in about 1ms. This allows the output voltage to rise smoothly from 0V to its final value while maintaining control of the inductor current. Light Load Operation (Burst Mode Operation, Continuous Conduction or Pulse-Skipping Mode) (SYNC/MODE Pin) The LTC3809 can be programmed for either high efficiency Burst Mode operation, forced continuous conduction mode or pulse-skipping mode at low load currents. To select Burst Mode operation, tie the SYNC/MODE pin to VIN. To select forced continuous operation, tie the SYNC/MODE pin to a DC voltage below 0.4V (e.g., GND). Tying the SYNC/MODE to a DC voltage above 0.4V and below 1.2V (e.g., VFB) enables pulse-skipping mode. The 0.4V threshold between forced continuous operation and pulse-skipping mode can be used in secondary winding regulation as described in the Auxiliary Winding Control Using SYNC/MODE Pin discussion in the Applications Information section. When the LTC3809 is in Burst Mode operation, the peak current in the inductor is set to approximately one-fourth of the maximum sense voltage even though the voltage on the ITH pin indicates a lower value. If the average inductor current is higher than the load current, the EAMP will decrease the voltage on the ITH pin. When the ITH voltage drops below 0.85V, the internal SLEEP signal goes high and the external MOSFET is turned off. In sleep mode, much of the internal circuitry is turned off, reducing the quiescent current that the LTC3809 draws. The load current is supplied by the output capacitor. As the output voltage decreases, the EAMP increases the ITH voltage. When the ITH voltage reaches 0.925V, the SLEEP signal goes low and the controller resumes normal operation by turning on the external P-channel MOSFET on the next cycle of the internal oscillator. When the controller is enabled for Burst Mode or pulse- skipping operation, the inductor current is not allowed to reverse. Hence, the controller operates discontinuously. |
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