LTC3409A OPERATION Main Control LoopBurst Mode Operation The LTC3409A uses a constant frequency, current mode step- The LTC3409A is capable of Burst Mode operation in down architecture. Both the main (P-channel MOSFET) and which the internal power MOSFETs operate intermittently synchronous (N-channel MOSFET) switches are internal. based on load demand. To enable Burst Mode operation, During normal operation, the internal top power MOSFET simply connect the MODE pin to GND. To disable Burst is turned on each cycle when the oscillator sets the RS Mode operation and enable PWM pulse-skipping mode, latch, and turned off when the current comparator, ICOMP, connect the MODE pin to VIN or drive it with a logic high resets the RS latch. The peak inductor current at which (VMODE >1.1V). In this mode, the effi ciency is lower at ICOMP resets the RS latch is controlled by the output of error light loads, but becomes comparable to Burst Mode opera- amplifi er EA. The VFB pin, described in the Pin Functions tion when the output load exceeds 30mA. The advantage section, allows EA to receive an output feedback voltage of pulse-skipping mode is lower output ripple and less from an external resistive divider. When the load current interference to audio circuitry. When the converter is in increases, it causes a slight decrease in the feedback voltage Burst Mode operation, the minimum peak current of the relative to the 0.612V reference, which in turn, causes the inductor is set to approximately 200mA regardless of the EA amplifi er’s output voltage to increase until the average output load. Each burst event can last from a few cycles inductor current matches the new load current. While the at light loads to almost continuously cycling with short top MOSFET is off, the bottom MOSFET is turned on until sleep intervals at moderate loads. In between these burst either the inductor current starts to reverse, as indicated events, the power MOSFETs and any unneeded circuitry by the current reversal comparator IRCMP, or the beginning are turned off, reducing the quiescent current to 65μA. In of the next clock cycle. this sleep state, the load current is being supplied solely Comparator OVDET guards against transient overshoots from the output capacitor. As the output voltage droops, >10% by turning the main switch off and keeping it off the EA amplifi er’s output rises above the sleep threshold until the transient has ended. signaling the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is depen- dent on the load demand. 3409af 8
