The LTC7818 from Analog Devices is a high-performance triple output (buck/buck/boost) synchronous DC/DC switching regulator controller that drives all N-channel power MOSFET stages. Its constant-frequency current mode architecture allows a phase-lockable switching frequency of up to 3MHz.
The LTC7818 operates from a wide 4.5V to 40V input supply range, when biased from the output of the boost converter or another auxiliary supply, the LTC7818 is capable of operating from an input supply as low as 1V after start-up.
The LTC7818 synchronous boost PassThru capability minimizes losses, for example in automotive start-stop applications, whilst the very low no-load quiescent current extends operating run time in battery powered systems and the OPTI-LOOP® compensation allows the transient response to be optimized over a wide range of output capacitance and ESR values.
Application Circuit
The LTC7818 synchronous three-channel controller utilises a constant-frequency, peak current mode architecture. The two step-down (buck) controllers, channels 1 and 2, operate 180° out-of-phase with each other and the step-up (boost) controller, channel 3, operates in phase with channel 1. During normal operation, the main switch (external top MOSFET for the buck channels or the external bottom MOSFET for the boost channel) is turned on when the clock for that channel sets the SR latch, causing the inductor current to increase. The main switch is turned off when the main current comparator, ICMP, resets the SR latch. After the main switch is turned off each cycle, the synchronous switch (the bottom MOSFET for the buck channels or the top MOSFET for the boost channel) is turned on which causes the inductor current to decrease until either the inductor current starts to reverse, as indicated by the current comparator IR, or the beginning of the next clock cycle.
The peak inductor current at which ICMP trips and resets the latch is controlled by the voltage on the ITH pin, which is the output of the error amplifier EA. The error amplifier compares the output voltage feedback signal at the VFB pin, (which is generated with an external resistor divider connected across the output voltage, VOUT, to ground) to the internal reference voltage (0.8V for the bucks or 1.2V for the boost). When the load current increases, it causes a slight decrease in VFB relative to the reference, which causes the EA to increase the ITH voltage until the average inductor current matches the new load current.
The three channels of the LTC7818 can be independently shut down using the RUN1/2/3 pins. Pulling a RUN pin below 1.1V shuts down the main control loop for that channel and pulling all three pins below 0.7V disables all controllers and most internal circuits, including the INTVCC LDOs. In this state, the LTC7818 draws only 1.5μA of quiescent current. The LTC7818 features a precision 0.8V reference for the bucks, 1.2V reference for the boost and a power good output indicator.
The LTC7818 additionally features spread spectrum operation which significantly reduces the peak radiated and conducted noise on both the input and output supplies, making it easier to comply with electromagnetic interference (EMI) standards. The LTC7818 can also be set to enter high efficiency Burst Mode operation, constant-frequency pulse-skipping mode or forced continuous conduction mode at low load currents by simply tying the MODE pin to ground, INTVCC or to a DC voltage respectively, full details can be found in the datasheet.
The device is housed in a compact 6mm x 6mm 40-lead Plastic QFN package and is available in a choice of operating temperature ranges of -40°C to +125°C or -40°C to +150°C.
The LTC7818 buck and buck/boost synchronous controller is suitable for a range of applications including Automotive, Transport Systems, Military, Avionics, and Industrial equipment.
Evaluation Board
he LTC7818 is supported by demonstration circuit DC2855A, a triple output synchronous step-up/dual step-down supply featuring the LTC7818. The demonstration circuit is designed for two buck outputs 5V/10A, 3.3V/10A supplied by a boosted 10V output. The buck outputs can maintain regulation over a wide input voltage range of 4.5V to 36V which is suitable for automotive or other battery fed applications. The demonstration circuit uses a drop-in layout whereas the main buck circuit components fit in an area of ¾” by 1½”, while the main boost circuit area is ¾” by 1¾”. The package style for the LTC7818 is a 40-pin exposed pad QFN.
The main features of the board include rail tracking (Buck channels only), an internal 5V linear regulator for bias, separated RUN pins for each output, a PGOOD signal (CH1 only), an overvoltage indicator for CH3 and a Mode selector that allow the converter to run in CCM, Pulseskipping or Burst Mode operation. Spread Spectrum Mode is available for EMI improvement. Synchronization to an external clock is also possible.
FREE samples for UK and Ireland customers only, subject to availability.
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