Datasheet LTC3563 (Analog Devices) - 8

LTC3563
U OPERATIO
The LTC3563 uses a constant frequency, current mode, In Burst Mode operation, the peak current of the inductor step-down architecture. The operating frequency is set is set to approximately 60mA regardless of the output load. at 2.25MHz. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals The output voltage is set by an internal divider. An error at moderate loads. In between these burst events, the amplifi er compares the divided output voltage with a power MOSFETs and any unneeded circuitry are turned off, reference voltage of 0.6V and adjusts the peak inductor reducing the quiescent current to 26µA. In this sleep state, current accordingly. the load current is being supplied solely from the output
Main Control Loop
capacitor. As the output voltage drops, the EA amplifi er’s output rises above the sleep threshold and turns the top During normal operation, the top power switch (P-channel MOSFET on. This process repeats at a rate that is dependent MOSFET) is turned on at the beginning of a clock cycle on the load demand. By running cycles periodically, the when the VOUT voltage is below the regulated voltage. switching losses which are dominated by the gate charge The current fl ows into the inductor and the load increases losses of the power MOSFETs are minimized. until the current limit is reached. The switch turns off and energy stored in the inductor fl ows through the bottom
Low Supply Operation
switch (N-channel MOSFET) into the load until the next To prevent unstable operation, the LTC3563 incorporates clock cycle. The peak inductor current is controlled by an undervoltage lockout circuit which shuts down the part the internally compensated output of the error amplifi er. when the input voltage drops below 2V. When the load current increases, the feedback voltage decreases slightly below the reference. This decrease
Internal Soft-Start
causes the error amplifi er to increase its output voltage At start-up when the RUN pin is brought high, the internal until the average inductor current matches the new load reference is linearly ramped from 0V to 0.6V in about current. The main control loop is shut down by pulling 1ms. The regulated output voltage follows this ramp from the RUN pin to ground. 0% to 100% in 1ms. The current in the inductor during soft-start is defi ned by the combination of the current
Burst Mode Operation
needed to charge the output capacitance and the current During light load currents, the LTC3563 operates in Burst provided to the load as the output voltage ramps up. The Mode operation in which the internal power MOSFETs start-up waveform, shown in the Typical Performance operate intermittently based on load demand. Characteristics, shows the output voltage start-up from 0V to 1.87V with a 500mA load and VIN = 3.6V. 3563f 8