説明

The LTC3245 is a switched capacitor buck-boost DC/DC converter that produces a regulated output (3.3V, 5V or adjustable) from a 2.7V to 38V input. The device uses switched capacitor fractional conversion to maintain regulation over a wide range of input voltage. Internal circuitry automatically selects the conversion ratio to optimize efficiency as input voltage and load conditions vary. No inductors are required.

The unique constant frequency architecture provides a lower noise output than conventional charge pump regulators. To optimize efficiency at the expense of slightly higher output ripple, the device has pin selectable Burst Mode operation.

Low operating current (20μA with no load, 4μA in shut down) and low external parts count (three small ceramic capacitors) make the LTC3245 ideally suited for low power, space constrained automotive/industrial applications. The device is short-circuit and overtemperature protected, and is available in thermally enhanced 12-pin MSOP and low profile 3mm × 4mm 12-pin DFN packages.

 

General Operation

The LTC3245 uses switched capacitor based DC/DC con version to provide the efficiency advantages associated with inductor based circuits as well as the cost and simplicity advantages of a linear regulator. The LTC3245’s unique constant frequency architecture provides a low noise regulated output as well as lower input noise than conventional switch capacitor charge pump regulators. The LTC3245uses an internal switch network and fractional conversion ratios to achieve high efficiency and regulation over widely varying VIN and output load conditions.

Internal control circuitry selects the appropriate conversion ratio based on VIN and load conditions. The device has three possible conversion modes: 2:1 step-down mode, 1:1 step-down mode and 1:2 step-up mode. Only one external flying capacitor is needed to operate in all three modes. 2:1 mode is chosen when VIN is greater than two times the desired VOUT. 1:1 mode is chosen when VIN falls between two times VOUT and VOUT. 1:2 mode is chosen when VIN falls below the desired VOUT. An internal load current sense circuit controls the switch point of the conversion ratio as needed to maintain output regulation over all load conditions.

Regulation is achieved by sensing the output voltage and  regulating the amount of charge transferred per cycle. This method of regulation provides much lower input and output ripple than that of conventional switched capacitor charge pumps. The constant frequency charge transfer also makes additional output or input filtering much less demanding than conventional switched capacitor charge pumps.

The LTC3245 has a Burst Mode operation pin that allows the user to trade output ripple for better efficiency/lower quiescent current. The device has two SEL pins that select the output regulation (fixed 5V, fixed 3.3V or adjustable) as well as shutdown. The device includes soft-start function to limit in-rush current at startup. The device is also short-circuit and overtemperature protected.

 

VOUT Regulation and Mode Selection

The device uses a control loop to adjust the strength of the charge pump to match the current required at the output. The error signal of this loop is stored directly on the output charge storage capacitor. As the load on VOUT increases, VOUT will drop slightly increasing the amount of charge transferred until the output current matches the output load. This method of regulation applies regardless of the conversion ratio.

The optimal conversion ratio is chosen based on VIN, VOUT and output load conditions. Two internal comparators are used to select the default conversion ratio. Each comparator has an adjustable offset built in that increases (decreases) in proportion to the increasing (decreasing) output load current. In this manner, the conversion ratio switch point is optimized to provide peak efficiency over all supply and load conditions while maintaining regulation. Each comparator also has built-in hysteresis to reduce the tendency of oscillating between modes when a transition point is reached.