Switching regulator controller operates at 4.5V to 40V

25th January 2023
Mick Elliott

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 device operates from a wide 4.5V to 40V input supply range, when biased from the output of the boost converter or another auxiliary supply, and it is capable of operating from an input supply as low as 1V after start-up.

Synchronous boost PassThru capability minimises 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 optimised over a wide range of output capacitance and ESR values.

This 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 device draws only 1.5μA of quiescent current.

It 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 controller 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.

It is available from distributor Anglia Components with free samples.

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