Datasheet LTC3736 (Analog Devices) - 10

LTC3736
U OPERATIO (Refer to Functional Diagram) Main Control Loop
rise linearly from approximately 0.65V to 1.3V (being charged by the internal 0.7 The LTC3736 uses a constant frequency, current mode µA current source), the EAMP regulates the V architecture with the two controllers operating 180 de- FB1 proportionally linearly from 0V to 0.6V. grees out of phase. During normal operation, the top The start-up of VOUT2 is controlled by the voltage on the external P-channel power MOSFET is turned on when the TRACK pin. When the voltage on the TRACK pin is less clock for that channel sets the RS latch, and turned off than the 0.6V internal reference, the LTC3736 regulates when the current comparator (ICMP) resets the latch. The the VFB2 voltage to the TRACK pin instead of the 0.6V peak inductor current at which ICMP resets the RS latch is reference. Typically, a resistor divider on VOUT1 is con- determined by the voltage on the ITH pin, which is driven nected to the TRACK pin to allow the start-up of VOUT2 to by the output of the error amplifier (EAMP). The VFB pin “track” that of VOUT1. For one-to-one tracking during start- receives the output voltage feedback signal from an exter- up, the resistor divider would have the same values as the nal resistor divider. This feedback signal is compared to divider on VOUT2 that is connected to VFB2. the internal 0.6V reference voltage by the EAMP. When the load current increases, it causes a slight decrease in V
Light Load Operation (Burst Mode or Continuous
FB relative to the 0.6V reference, which in turn causes the I
Conduction) (SYNC/FCB Pin)
TH voltage to increase until the average inductor current The LTC3736 can be enabled to enter high efficiency Burst matches the new load current. While the top P-channel Mode operation or forced continuous conduction mode at MOSFET is off, the bottom N-channel MOSFET is turned low load currents. To select Burst Mode operation, tie the on until either the inductor current starts to reverse, as SYNC/FCB pin to a DC voltage above 0.6V (e.g., VIN). To indicated by the current reversal comparator, IRCMP, or the select forced continuous operation, tie the SYNC/FCB to a beginning of the next cycle. DC voltage below 0.6V (e.g., SGND). This 0.6V threshold between Burst Mode operation and forced continuous mode
Shutdown, Soft-Start and Tracking Start-Up
can be used in secondary winding regulation as described
(RUN/SS and TRACK Pins)
in the Auxiliary Winding Control Using SYNC/FCB Pin dis- The LTC3736 is shut down by pulling the RUN/SS pin low. cussion in the Applications Information section. In shutdown, all controller functions are disabled and the When a controller is in Burst Mode operation, the peak chip draws only 9µA. The TG outputs are held high (off) current in the inductor is set to approximate one-fourth of and the BG outputs low (off) in shutdown. Releasing the maximum sense voltage even though the voltage on RUN/SS allows an internal 0.7µA current source to charge the I up the RUN/SS pin. When the RUN/SS pin reaches 0.65V, TH pin indicates a lower value. If the average inductor current is lower than the load current, the EAMP will the LTC3736’s two controllers are enabled. decrease the voltage on the ITH pin. When the ITH voltage The start-up of VOUT1 is controlled by the LTC3736’s drops below 0.85V, the internal SLEEP signal goes high internal soft-start. During soft-start, the error amplifier and both external MOSFETs are turned off. EAMP compares the feedback signal VFB1 to the internal In sleep mode, much of the internal circuitry is turned off, soft-start ramp (instead of the 0.6V reference), which rises reducing the quiescent current that the LTC3736 draws. linearly from 0V to 0.6V in about 1ms. This allows the The load current is supplied by the output capacitor. As output voltage to rise smoothly from 0V to its final value, the output voltage decreases, the EAMP increases the I while maintaining control of the inductor current. TH voltage. When the ITH voltage reaches 0.925V, the SLEEP The 1ms soft-start time can be increased by connecting signal goes low and the controller resumes normal the optional external soft-start capacitor CSS between the operation by turning on the external P-channel MOSFET RUN/SS and SGND pins. As the RUN/SS pin continues to on the next cycle of the internal oscillator. 3736fa 10