Datasheet LTC3406AB (Analog Devices) - 8

LTC3406AB
operaTion (Refer to Functional Diagram) Main Control Loop Dropout Operation
The LTC3406AB uses a constant frequency, current As the input supply voltage decreases to a value approach- mode step-down architecture. Both the main (P-channel ing the output voltage, the duty cycle increases toward the MOSFET) and synchronous (N-channel MOSFET) switches maximum on-time. Further reduction of the supply voltage are internal. During normal operation, the internal top power forces the main switch to remain on for more than one MOSFET is turned on each cycle when the oscillator sets cycle until it reaches 100% duty cycle. The output voltage the RS latch, and turned off when the current comparator, will then be determined by the input voltage minus the volt- ICOMP, resets the RS latch. The peak inductor current at age drop across the P-channel MOSFET and the inductor. which ICOMP resets the RS latch, is controlled by the output An important detail to remember is that at low input supply of error amplifier EA. When the load current increases, voltages, the R it causes a slight decrease in the feedback voltage, FB, DS(ON) of the P-channel switch increases (see Typical Performance Characteristics). Therefore, relative to the 0.6V reference, which in turn, causes the the user should calculate the power dissipation when the EA amplifier’s output voltage to increase until the average LTC3406AB is used at 100% duty cycle with low input inductor current matches the new load current. While the voltage (See Thermal Considerations in the Applications top MOSFET is off, the bottom MOSFET is turned on until Information section). either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning
Slope Compensation and Inductor Peak Current
of the next clock cycle. Slope compensation provides stability in constant fre- The main control loop is shut down by grounding RUN, quency architectures by preventing subharmonic oscilla- resetting the internal soft-start. Re-enabling the main tions at high duty cycles. It is accomplished internally by control loop by pulling RUN high activates the internal adding a compensating ramp to the inductor current signal soft-start, which slowly ramps the output voltage over at duty cycles in excess of 40%. Normally, this results in approximately 0.9ms until it reaches regulation. a reduction of maximum inductor peak current for duty cycles >40%. However, the LTC3406AB uses a patented
Pulse Skipping Mode Operation
scheme that counteracts this compensating ramp, which At light loads, the inductor current may reach zero or allows the maximum inductor peak current to remain reverse on each pulse. The bottom MOSFET is turned off unaffected throughout all duty cycles. by the current reversal comparator, IRCMP, and the switch voltage will ring. This is discontinuous mode operation, and is normal behavior for the switching regulator. At very light loads, the LTC3406AB will automatically skip pulses in pulse skipping mode operation to maintain output regu- lation. Refer to the LTC3406A data sheet if Burst Mode operation is preferred. 3406abfb 8 For more information www.linear.com/LTC3406AB