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LTC3409AIDD Datasheet(PDF) 8 Page - Linear Technology |
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LTC3409AIDD Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LTC3409A 8 3409af Main Control Loop TheLTC3409Ausesaconstantfrequency,currentmodestep- down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resetstheRSlatchiscontrolledbytheoutputoferror amplifier EA. The VFB pin, described in the Pin Functions section, allows EA to receive an output feedback voltage from an external resistive divider. When the load current increases, it causes a slight decrease in the feedback voltage relative to the 0.612V reference, which in turn, causes the EA amplifier’s output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. Comparator OVDET guards against transient overshoots >10% by turning the main switch off and keeping it off until the transient has ended. Burst Mode Operation The LTC3409A is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. To enable Burst Mode operation, simply connect the MODE pin to GND. To disable Burst Mode operation and enable PWM pulse-skipping mode, connect the MODE pin to VIN or drive it with a logic high (VMODE >1.1V). In this mode, the efficiency is lower at light loads, but becomes comparable to Burst Mode opera- tion when the output load exceeds 30mA. The advantage of pulse-skipping mode is lower output ripple and less interference to audio circuitry. When the converter is in Burst Mode operation, the minimum peak current of the inductor is set to approximately 200mA regardless of the output load. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 65μA. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA amplifier’s output rises above the sleep threshold 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. OPERATION |
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