Datasheet LTC3736-2 (Analog Devices) - 10

LTC3736-2
OPERATION (Refer to Functional Diagram) Main Control Loop
The start-up of VOUT2 is controlled by the voltage on the TRACK pin. When the voltage on the TRACK pin is less The LTC3736-2 uses a constant-frequency, current mode than the 0.6V internal reference, the LTC3736-2 regulates architecture with the two controllers operating 180 degrees out-of-phase. During normal operation, the top external the VFB2 voltage to the TRACK pin instead of the 0.6V ref- P-channel power MOSFET is turned on when the clock erence. Typically, a resistor divider on VOUT1 is connected for that channel sets the RS latch, and turned off when to the TRACK pin to allow the start-up of VOUT2 to track the current comparator (I that of VOUT1. For one-to-one tracking during start-up, the CMP) resets the latch. The peak inductor current at which I resistor divider would have the same values as the divider CMP resets the RS latch is determined by the voltage on the I on VOUT2 that is connected to VFB2. TH pin, which is driven by the output of the error amplifi er (EAMP). The VFB pin
Light Load Operation (Pulse-Skipping or Continuous
receives the output voltage feedback signal from an ex-
Conduction) (SYNC/FCB Pin)
ternal resistor divider. This feedback signal is compared to the internal 0.6V reference voltage by the EAMP. When The LTC3736-2 can be enabled to enter high effi ciency the load current increases, it causes a slight decrease in pulse-skipping operation or forced continuous conduc- VFB relative to the 0.6V reference, which in turn causes the tion mode at low load currents. To select pulse-skipping ITH voltage to increase until the average inductor current operation, tie the SYNC/FCB pin to a DC voltage above matches the new load current. While the top P-channel 0.6V (e.g., VIN). To select forced continuous operation, tie MOSFET is off, the bottom N-channel MOSFET is turned the SYNC/FCB to a DC voltage below 0.6V (e.g., SGND). on until either the inductor current starts to reverse, as This 0.6V threshold between pulse-skipping operation indicated by the current reversal comparator, IRCMP, or the and forced continuous mode can be used in secondary beginning of the next cycle. winding regulation as described in the Auxiliary Winding Control Using SYNC/FCB Pin discussion in the Applications
Shutdown, Soft-Start and Tracking Start-Up (RUN/SS
Information section.
and TRACK Pins)
In forced continuous operation, the inductor current is The LTC3736-2 is shut down by pulling the RUN/SS pin allowed to reverse at light loads or under large transient low. In shutdown, all controller functions are disabled and conditions. The peak inductor current is determined by the the chip draws only 9μA. The TG outputs are held high voltage on the ITH pin. The P-channel MOSFET is turned (off) and the BG outputs low (off) in shutdown. Releasing on every cycle (constant frequency) regardless of the ITH RUN/SS allows an internal 0.7μA current source to charge pin voltage. In this mode, the effi ciency at light loads is up the RUN/SS pin. When the RUN/SS pin reaches 0.65V, lower than in pulse-skipping operation. However, continu- the LTC3736-2’s two controllers are enabled. ous mode has the advantages of lower output ripple and The start-up of V less interference with audio circuitry. OUT1 is controlled by the LTC3736-2’s internal soft-start. During soft-start, the error amplifi er When the SYNC/FCB pin is tied to a DC voltage above EAMP compares the feedback signal VFB1 to the internal 0.6V or when it is clocked by an external clock source to soft-start ramp (instead of the 0.6V reference), which rises use the phase-locked loop (see Frequency Selection and linearly from 0V to 0.6V in about 1ms. This allows the Phase-Locked Loop), the LTC3736-2 operates in PWM output voltage to rise smoothly from 0V to its fi nal value, pulse-skipping mode at light loads. In this mode, the while maintaining control of the inductor current. current comparator ICMP may remain tripped for several The 1ms soft-start time can be increased by connecting the cycles and force the external P-channel MOSFET to stay optional external soft-start capacitor CSS between the RUN/ off for the same number of cycles. The inductor current is SS and SGND pins. As the RUN/SS pin continues to rise not allowed to reverse, though (discontinuous operation). linearly from approximately 0.65V to 1.3V (being charged This mode, like forced continuous operation, exhibits low by the internal 0.7μA current source), the EAMP regulates output ripple as well as low audio noise and reduced RF the VFB1 proportionally linearly from 0V to 0.6V. interference. However, it provides low current effi ciency 37362fb 10