Datasheet LTC3547 (Analog Devices) - 7

LTC3547
U OPERATIO (Refer to Functional Diagram )
The LTC3547 uses a constant-frequency current mode
Dropout Operation
architecture. The operating frequency is set at 2.25MHz. When the input supply voltage decreases toward the out- Both channels share the same clock and run in-phase. put voltage the duty cycle increases to 100%, which is the The output voltage is set by an external resistor divider dropout condition. In dropout, the PMOS switch is turned returned to the VFB pins. An error amplifi er compares the on continuously with the output voltage being equal to the divided output voltage with a reference voltage of 0.6V and input voltage minus the voltage drops across the internal regulates the peak inductor current accordingly. P-channel MOSFET and the inductor. An important design consideration is that the R
Main Control Loop
DS(ON) of the P-channel switch increases with decreasing input During normal operation, the top power switch (P-channel supply voltage (see Typical Performance Characteristics). MOSFET) is turned on at the beginning of a clock cycle Therefore, the user should calculate the worst-case power when the VFB voltage is below the reference voltage. The dissipation when the LTC3547 is used at 100% duty cycle current into the inductor and the load increases until the with low input voltage (see Thermal Considerations in the peak inductor current (controlled by ITH) is reached. The Applications Information Section). RS latch turns off the synchronous switch and energy stored in the inductor is discharged through the bottom
Soft-Start
switch (N-channel MOSFET) into the load until the next In order to minimize the inrush current on the input by- clock cycle begins, or until the inductor current begins to pass capacitor, the LTC3547 slowly ramps up the output reverse (sensed by the IRCMP comparator). voltage during start-up. Whenever the RUN1 or RUN2 pin The peak inductor current is controlled by the internally is pulled high, the corresponding output will ramp from compensated ITH voltage, which is the output of the er- zero to full-scale over a time period of approximately ror amplifi er. This amplifi er regulates the VFB pin to the 650µs. This prevents the LTC3547 from having to quickly internal 0.6V reference by adjusting the peak inductor charge the output capacitor and thus supplying an exces- current accordingly. sive amount of instantaneous current.
Burst Mode Operation Short-Circuit Protection
To optimize effi ciency, the LTC3547 automatically switches When either regulator output is shorted to ground, the from continuous operation to Burst Mode operation when corresponding internal N-channel switch is forced on for the load current is relatively light. During Burst Mode op- a longer time period for each cycle in order to allow the eration, the peak inductor current (as set by ITH) remains inductor to discharge, thus preventing current runaway. fi xed at approximately 60mA and the PMOS switch operates This technique has the effect of decreasing switching intermittently based on load demand. By running cycles frequency. Once the short is removed, normal operation periodically, the switching losses are minimized. resumes and the regulator output will return to its nominal voltage. The duration of each burst event can range from a few cycles at light load to almost continuous cycling with short sleep intervals at moderate loads. During the sleep intervals, the load current is being supplied solely from the output capacitor. As the output voltage droops, the error amplifi er output rises above the sleep threshold, signaling the burst comparator to trip and turn the top MOSFET on. This cycle repeats at a rate that is dependent on load demand. 3547fa 7