Datasheet LTC3547B (Analog Devices) - 7

LTC3547B
U OPERATIO (Refer to Functional Diagram )
The LTC3547B 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 LTC3547B 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 LTC3547B slowly ramps up the output reverse (sensed by the IRCMP comparator). voltage during start-up. Whenever the RUN1 or RUN2 pin is The peak inductor current is controlled by the internally pulled high, the corresponding output will ramp from zero compensated ITH voltage, which is the output of the er- to full-scale over a time period of approximately 650μs. This ror amplifi er. This amplifi er regulates the VFB pin to the prevents the LTC3547B from having to quickly charge the internal 0.6V reference by adjusting the peak inductor output capacitor and thus supplying an excessive amount current accordingly. of instantaneous current. At very low load currents, the LTC3547B automatically
Short-Circuit Protection
transitions from constant frequency operation to discon- tinuous operation, where the regulator begins skipping When either regulator output is shorted to ground, the pulses. Even though the total power loss decreases with corresponding internal N-channel switch is forced on for load, the effi ciency of the switching regulator will drop, a longer time period for each cycle in order to allow the because the power delivered to the output becomes very inductor to discharge, thus preventing current runaway. small. For applications where light load effi ciency is a This technique has the effect of decreasing switching priority, consider using the LTC3547 instead. frequency. Once the short is removed, normal operation resumes and the regulator output will return to its nominal voltage. 3547bfb 7