Datasheet LTC3560 (Analog Devices) - 7

LTC3560
OPERATION (Refer to Functional Diagram) Main Control Loop
When the converter is in Burst Mode operation, the peak current of the inductor is set to approximately 150mA The LTC3560 uses a constant frequency, current mode step- regardless of the output load. Each burst event can last down architecture. Both the main (P-channel MOSFET) from a few cycles at light loads to almost continuously and synchronous (N-channel MOSFET) switches are cycling with short sleep intervals at moderate loads. In internal. During normal operation, the internal top power between these burst events, the power MOSFETs and any MOSFET is turned on each cycle when the oscillator sets unneeded circuitry are turned off, reducing the quiescent the RS latch, and turned off when the current comparator, current to 16μA. In this sleep state, the load current is being ICOMP, resets the RS latch. The peak inductor current supplied solely from the output capacitor. As the output at which ICOMP resets the RS latch, is controlled by the voltage droops, the EA amplifi er’s output rises above the output of error amplifi er EA. The VFB pin, described in sleep threshold signaling the BURST comparator to trip the Pin Functions section, allows EA to receive an output and turn the top MOSFET on. This process repeats at a feedback voltage from an external resistive divider. When rate that is dependent on the load demand. the load current increases, it causes a slight decrease in the feedback voltage relative to the 0.6V reference, which
Frequency Synchronization
in turn, causes the EA amplifi er’s output voltage to increase until the average inductor current matches the new load When the LTC3560 is clocked by an external source, Burst current. While the top MOSFET is off, the bottom MOSFET Mode operation is disabled; the LTC3560 then operates in is turned on until either the inductor current starts to PWM pulse-skipping mode. In this mode, when the output reverse, as indicated by the current reversal comparator load is very low, current comparator ICOMP may remain I tripped for several cycles and force the main switch to stay RCMP, or the beginning of the next clock cycle. off for the same number of cycles. Increasing the output The main control loop is shut down by grounding RUN, load slightly allows constant frequency PWM operation resetting the internal soft-start. Re-enabling the main to resume. This mode exhibits low output ripple as well control loop by pulling RUN high activates the internal as low audio noise and reduced RF interference while soft-start, which slowly ramps the output voltage over providing reasonable low current effi ciency. approximately 0.9ms until it reaches regulation.
Dropout Operation Burst Mode Operation
As the input supply voltage decreases to a value approaching The LTC3560 is capable of Burst Mode operation in which the output voltage, the duty cycle increases toward the the internal power MOSFETs operate intermittently based maximum on time. Further reduction of the supply voltage on load demand. To enable Burst Mode operation, simply forces the main switch to remain on for more than one cycle connect the SYNC/MODE pin to GND. To disable Burst until it reaches 100% duty cycle. The output voltage will Mode operation and enable PWM pulse-skipping mode, then be determined by the input voltage minus the voltage connect the SYNC/MODE pin to VIN or drive it with a logic drop across the P-channel MOSFET and the inductor. high (VMODE > 1.5V). In this mode, the effi ciency is lower at light loads, but becomes comparable to Burst Mode Another important detail to remember is that at low input operation when the output load exceeds 100mA. The supply voltages, the RDS(ON) of the P-channel switch advantage of pulse-skipping mode is lower output ripple increases (see Typical Performance Characteristics). and less interference to audio circuitry. Therefore, the user should calculate the power dissipation when the LTC3560 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applica- tions Information section). 3560fb 7