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LTC1474 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1474 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  6 LTC3406 LTC3406-1.5/LTC3406-1.8 3406fa FU CTIO AL DIAGRA + – + – EA IRCMP + – ICOMP 5 1 RUN OSC SLOPE COMP OSC FREQ SHIFT 0.6V FB R1 LTC3406-1.5 R1 + R2 = 550k LTC3406-1.8 R1 + R2 = 540k R2 0.6V REF SHUTDOWN 0.4V 0.65V SLEEP VIN VFB/VOUT BURST VIN S R RS LATCH SWITCHING LOGIC AND BLANKING CIRCUIT ANTI- SHOOT- THRU Q Q 5Ω 4 SW 3 GND 3406 BD 2 OPERATIO (Refer to Functional Diagram) Main Control Loop The LTC3406 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. When the load current increases, it causes a slight decrease in the feedback voltage, FB, relative to the 0.6V reference, which in turn, causes the EA amplifier’s output voltage to increase until the average inductor current matches the new load cur- rent. 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. Burst Mode Operation The LTC3406 is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. In Burst Mode operation, the 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 20µA. 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 signal- ing the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is dependent on the load demand. |
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Similar Description - LTC1474 |
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