Datasheet LTC3406AB-2 (Analog Devices) - 8

LTC3406AB-2
operaTion (Refer to Functional Diagram) Main Control Loop
pulses in pulse skipping mode operation to maintain out- The LTC3406AB-2 uses a constant frequency, current put regulation. Refer to the LTC3406A data sheet if Burst mode step-down architecture. Both the main (P-channel Mode operation is preferred. MOSFET) and synchronous (N-channel MOSFET) switches
Dropout Operation
are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets As the input supply voltage decreases to a value approach- the RS latch, and turned off when the current comparator, ing the output voltage, the duty cycle increases toward the ICOMP, resets the RS latch. The peak inductor current at maximum on-time. Further reduction of the supply voltage which ICOMP resets the RS latch, is controlled by the output forces the main switch to remain on for more than one of error amplifier EA. When the load current increases, cycle until it reaches 100% duty cycle. The output voltage it causes a slight decrease in the feedback voltage, VFB, will then be determined by the input voltage minus the volt- relative to the 0.6V reference, which in turn, causes the age drop across the P-channel MOSFET and the inductor. EA amplifier’s output voltage to increase until the average An important detail to remember is that at low input supply inductor current matches the new load current. While the voltages, the R top MOSFET is off, the bottom MOSFET is turned on until DS(ON) of the P-channel switch increases (see Typical Performance Characteristics). Therefore, either the inductor current starts to reverse, as indicated the user should calculate the power dissipation when the by the current reversal comparator IRCMP, or the beginning LTC3406AB-2 is used at 100% duty cycle with low input of the next clock cycle. voltage (See Thermal Considerations in the Applications The main control loop is shut down by grounding RUN, Information section). resetting the internal soft-start. Re-enabling the main control loop by pulling RUN high activates the internal
Slope Compensation and Inductor Peak Current
soft-start, which slowly ramps the output voltage over Slope compensation provides stability in constant fre- approximately 0.9ms until it reaches regulation. quency architectures by preventing subharmonic oscil- lations at high duty cycles. It is accomplished internally
Pulse Skipping Mode Operation
by adding a compensating ramp to the inductor current At light loads, the inductor current may reach zero or signal at duty cycles in excess of 40%. Normally, this reverse on each pulse. The bottom MOSFET is turned off results in a reduction of maximum inductor peak current by the current reversal comparator, IRCMP, and the switch for duty cycles >40%. However, the LTC3406AB-2 uses voltage will ring. This is discontinuous mode operation, a patented scheme that counteracts this compensating and is normal behavior for the switching regulator. At ramp, which allows the maximum inductor peak current very light loads, the LTC3406AB-2 will automatically skip to remain unaffected throughout all duty cycles. 3406ab2fa 8 For more information www.linear.com/LTC3406AB-2