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LTC3446IDE-PBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3446IDE-PBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC3446 10 3446fd The LTC3446 combines a constant frequency, current mode synchronous buck converter with two very low dropout (VLDO) linear DC regulators to provide up to three high efficiency, low voltage outputs from a single higher voltage input source. Each output can be independently enabled and disabled, and has its own independent soft-start cir- cuit to help reduce inrush current. A power good circuit monitors all three supplies. The LTC3446 incorporates an undervoltage lockout circuit that shuts down the IC when the input voltage drops below about 2.4V to prevent unstable operation. SYNCHRONOUS BUCK OPERATION A buck converter takes power from a high input voltage, VIN, and delivers it at a lower output voltage, VOUT. The buck converter inside the LTC3446 achieves over 80% efficient power conversion under a wide range of VIN, VOUT and load conditions, whereas a linear regulator is limited by physics to a maximum efficiency of (VOUT/VIN) × 100%. Main Control Loop During normal operation, the internal oscillator produces a constant frequency 2.25MHz clock. The top power switch (P-channel MOSFET) turns on at the beginning of a clock cycle. Inductor current increases to a peak value which is set by the voltage on the ITH pin. Then the top switch turns off and the energy stored in the inductor flows through the bottom switch (N-channel MOSFET) into the load until the next clock cycle. The peak inductor current is controlled by the voltage on the ITH pin, which is the output of the error amplifier. This amplifier compares the BUCKFB pin to the 0.8V reference. When the load current increases, the BUCKFB voltage de- creases slightly below the reference. This decrease causes the error amplifier to increase the ITH voltage until the average inductor current matches the new load current. The main control loop is shut down by pulling the ENBUCK pin to ground. A soft-start is enabled whenever ENBUCK is brought high. Soft-start limits the peak inductor current from reaching maximum for the first millisecond after ENBUCK is brought high. Overcurrent Protection To help avert inductor current runaway in case the buck output is accidentally shorted to ground, the LTC3446 features a bottom switch NMOS overcurrent limit, which works as follows. When the buck output is shorted to ground, inductor current will rise to its maximum peak level, IMAXP, such that on every oscillator cycle the PMOS top switch will turn on for only its minimum duty cycle, and the bottom switch NMOS turns on for the remainder of the cycle. Temporarily ignoring inductor, switch and parasitic resis- tance drops, which in most applications are designed to be small in order to maximize buck converter efficiency, it is to first order true that when the PMOS is on, the VIN supply voltage is placed across the inductor, increasing the inductor current, but when the NMOS is on, there is no output voltage to be placed across the inductor to reduce its current. Inductor current ratchets up each cycle and could lead to the destruction of the buck IC. The NMOS overcurrent limit helps prevent this by sensing the current through the NMOS bottom switch, and for as long as this current exceeds the overcurrent limit level, IMAXN, it: 1. Keeps the NMOS on, allowing the tiny voltage drops from parasitic resistances to reduce the inductor current. 2. Refuses to allow the PMOS to turn on, preventing any additional energy from being fed into the system. Low Current Operation The MODESEL pin controls the buck converter’s behavior at light load currents to help optimize efficiency, output ripple and noise. When the load is relatively light and MODESEL is grounded, the buck converter automatically switches into Burst Mode operation, which operates the PMOS switch intermittently based on load demand rather than at a constant frequency. Every switch cycle during Burst Mode operation delivers more energy than would occur in constant frequency operation, minimizing the switch- ing loss per unit of energy delivered. Since the dominant power loss at light loads is gate charge switching loss in the power MOSFETs, operating in Burst Mode operation OPERATION |
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Similar Description - LTC3446IDE-PBF |
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