Datasheet LTC3565 (Analog Devices) - 9

LTC3565
OPERATION
The LTC3565 uses a constant frequency, current mode switch from continuous operation to the selected mode architecture. The operating frequency is determined by when the load current is low. the value of the RT resistor or can be synchronized to an To optimize efficiency, the Burst Mode operation can be external oscillator. To suit a variety of applications, the selected. When the load is relatively light, the LTC3565 selectable MODE pin allows the user to trade-off noise automatically switches into Burst Mode operation in which for efficiency. the PMOS switch operates intermittently based on load The output voltage is set by an external divider returned demand. By running cycles periodically, the switching to the VFB pin. An error amplifier compares the divided losses which are dominated by the gate charge losses output voltage with the reference voltage of 0.6V and ad- of the power MOSFETs are minimized. The main control justs the peak inductor current accordingly. Overvoltage loop is interrupted when the output voltage reaches the and undervoltage comparators will pull the PGOOD output desired regulated value. The burst comparator trips when low if the output voltage is not within ±7% of its regulated ITH is below approximately 0.5V, shutting off the switch value. A tripping delay of 40µs and untripping delay of and reducing the power. The output capacitor and the in- 105µs ensures PGOOD will not glitch due to transient ductor supply the power to the load until ITH rises above spikes on VOUT. approximately 0.5V, turning on the switch and the main control loop which starts another cycle.
Main Control Loop
For lower output voltage ripple at low currents, pulse During normal operation, the top power switch (P-channel skipping mode can be used. In this mode, the LTC3565 MOSFET) is turned on at the beginning of a clock cycle. continues to switch at a constant frequency down to Current flows through this switch into the inductor and very low currents, where it will eventually begin skipping the load, increasing until the peak inductor current reaches pulses. the limit set by the voltage on the ITH pin. Then, the top switch is turned off, the bottom switch is turned on, and Finally, in forced continuous mode, the inductor current the energy stored in the inductor forces the current to flow is constantly cycled which creates a fixed output voltage through the bottom switch and the inductor out into the ripple at all output current levels. This feature is desirable load until the next clock cycle. in telecommunications since the noise is at a constant fre- quency and is thus easy to filter out. Another advantage of The peak inductor current is controlled by the voltage this mode is that the regulator is capable of both sourcing on the ITH pin, which is the output of the error amplifier. current into a load and sinking current from the output. The output is developed by the error amplifier comparing the feedback voltage, VFB, to the 0.6V reference voltage.
Dropout Operation
When the load current increases, the output voltage and When the input supply voltage decreases toward the VFB decrease slightly. This decrease in VFB causes the er- output voltage, the duty cycle increases to 100% which ror amplifier to increase the ITH voltage until the average is the dropout condition. In dropout, the PMOS switch is inductor current matches the new load current. turned on continuously with the output voltage being equal The main control loop is shut down by grounding the RUN to the input voltage minus the voltage drops across the pin, resetting the internal soft-start. Re-enabling the main internal P-channel MOSFET and the inductor. control loop by pulling RUN high activates the internal soft-start, which slowly ramps the output voltage over
Low Supply Operation
approximately 0.9ms until it reaches regulation. The LTC3565 incorporates an undervoltage lockout circuit which shuts down the part when the input voltage drops
Low Current Operation
below about 1.9V to prevent unstable operation. Three modes are available to control the operation of the LTC3565 at low currents. All three modes automatically 3565fc For more information www.linear.com/LTC3565 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Revision History Typical Application Related Parts