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LTC3409 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3409 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LTC3409 3409f FU CTIO AL DIAGRA + – + – EA + – OVDET IRCMP + – ICOMP 1 7 RUN PLL SLOPE COMP OSC 0.613V REFERENCE SHUTDOWN OV 0.675 0.4V 0.65V SLEEP EN VIN VFB 5 MODE 8 SYNC BURST VIN S R RS LATCH SWITCHING LOGIC AND BLANKING CIRCUIT ANTI- SHOOT- THRU SOFT- START Q Q 5Ω 3, 4 SW 6 GND 3409 FD 2 OPERATIO Main Control Loop The LTC3409 uses a constant frequency, current mode step-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 com- parator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch is controlled by the output of error 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 0.613V 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 LTC3409 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 operation when the output load exceeds 30mA. The advantage of pulse |
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