Analog Devices Inc. LTC3118IUFD#PBF

Product Summary Analog Devices LTC3118IUFD#PBF Buck-Boost Converter

The LTC3118IUFD#PBF is a highly versatile, high-efficiency synchronous buck-boost DC/DC converter offered by Analog Devices. Housed in a 24-lead, thermally enhanced QFN package (4mm × 5mm), this device integrates a dual-input PowerPath™ controller and delivers up to 2A of output current with a programmable output voltage ranging from 2V to 18V. Its unique feature set targets applications requiring seamless power source transition, high efficiency across a broad voltage range, and robust system-level protection. This regulator is particularly well-suited for systems such as backup power supplies, industrial automation equipment, portable instrumentation, and solutions replacing diode-OR schemes for higher efficiency.

Main Attributes and Functional Overview of the LTC3118IUFD#PBF

At its core, the LTC3118IUFD#PBF combines a low loss, fully integrated buck-boost converter with a sophisticated dual-input multiplexer. Fundamental operational highlights include:

Dual input voltage capability: 2.2V to 18V range on each VIN

Output voltage adjustable from 2V to 18V, delivering up to 2A

Integrated PowerPath™ Controller: Seamless selection and transition between two input sources

Pin-selectable operation modes: Forced PWM or automatic Burst Mode® for light load efficiency (Iq as low as 50μA in sleep)

Integrated precision undervoltage lockout (UVLO) and RUN pin thresholds per input—enabling flexible, programmable startup sequencing and custom UVLO design

Current mode control topology for fast transient response, inherent line regulation, and simplified frequency compensation

Fixed 1.2MHz switching frequency for compact, low-noise designs

Up to 94% peak efficiency, built-in short-circuit, thermal, and overload protection

Power good indicators for both inputs and output, facilitating robust system monitoring and fault management

These attributes make the LTC3118IUFD#PBF an especially compelling choice for designers seeking to efficiently manage backup/primary source transitions in battery-powered and industrial systems.

Advanced Dual-Source PowerPath™ Design

A standout aspect of the LTC3118IUFD#PBF is its sophisticated PowerPath™ switch network, which offers true dual input capability. Two independent paths are provided from VIN1 and VIN2 to the power stage, each with its own set of low RDS(ON) N-channel MOSFETs. This design minimizes conduction loss and ensures maximum efficiency regardless of which input is active. Isolation is guaranteed between the selected and inactive input, further reducing unwanted leakage currents—a crucial consideration in backup battery or supercapacitor applications.

The primary system designer benefit is the flexibility to accept power from either source, regardless of whether VIN1/2 is above, below, or equal to VOUT. For example, a system can draw from a wall adapter by default and instantaneously fall back to a backup battery in case of input brownout, with smooth, disturbance-free output.

Input Selection Methods Preferred Source and Ideal Diode Modes

Input selection is configured via the SEL pin:

Priority Mode (SEL = low): LTC3118IUFD#PBF prioritizes VIN1, shifting to VIN2 only if VIN1 or its RUN pin drops below the programmable UVLO thresholds. This is recommended when a preferred supply must always be used, falling back to an alternate only as necessary.

Ideal Diode Mode (SEL = high): Automatic selection of the higher valid input voltage. The internal comparator circuit includes 800mV (typ) hysteresis and 250μs filtering, minimizing chattering and ensuring stable transitions—even during brownout events or cable insertions.

Both VIN paths include open-drain “good” indicators (V1GD, V2GD), allowing supervision and control logic to detect undervoltage situations and activate or deactivate system components accordingly.

Integrated Power Stage, Current-Mode Regulation, and Protection Features

The LTC3118IUFD#PBF utilizes a fully integrated current mode control loop, merging the benefits of both peak and average current mode approaches. These features improve line/load transient stability, ease compensation, and provide robust analog performance, especially important in systems subject to input/output disturbances.

The device clamps the average inductor current to a nominal 3.6A, with the output current rating in boost mode somewhat reduced by duty-cycle effects. Integrated protection includes:

Overload current limiting (~1.6x over average), with inductor disconnect and cycle-by-cycle recovery

Output short-circuit protection triggering automatic soft-start, thereby minimizing thermal stress during faults

Reverse current limit (–200mA typ), preventing output capacitor discharge in edge-case operating scenarios

Soft-start on initial power-up, and in response to brownouts or short-circuit events, reducing system stress

The fixed 1.2MHz operation allows the use of compact external components, enabling small PCB footprints—a necessity in modern, dense system-in-package solutions.

Thermal Management and PCB Layout Guidelines for LTC3118IUFD#PBF

Thermal design is critical; at full 2A output and especially at higher input voltages, heat dissipation can become substantial. The QFN package features an exposed power pad that must be solidly soldered to an underlying PCB copper plane and connected with multiple thermal vias to increase heat spreading. Board designers are encouraged to optimize copper area and provide adequate airflow or heat sinking, as the device initiates thermal shutdown at ~165°C internal die temperature, re-enabling after a 10°C cool-down and soft-start event.

Recommendations for Choosing External Parts for LTC3118IUFD#PBF

Proper external component selection is essential for unlocking the full performance of the LTC3118IUFD#PBF:

Inductor: Low DCR, saturation current above 4A, typically 3.3μH–10μH depending on VOUT (for example, 3.3μH for 5V output).

Input capacitors: ≥22μF ceramic per VIN; higher bulk capacitance recommended if powered via cables or high ESR sources.

Output capacitor: ≥47μF low ESR multilayer ceramic preferred for low voltage ripple; actual capacitance should ensure DC bias effects are considered.

Compensation network: Tailored with external RC components at VC pin, with recommended values calculated based on system load, output capacitance, and expected transient requirements. See datasheet worked examples for detailed guidance.

Bootstrapping: For applications with 5V output, connect VOUT to VCC via a Schottky diode to improve efficiency by reducing power to the internal LDO.

Practical Applications and Common Implementations for LTC3118IUFD#PBF

The combination of wide input/output flexibility and dual-input switching control makes the LTC3118IUFD#PBF suited for scenarios such as:

Battery/adapter backup, e.g., 3-cell Li-Ion (VIN1) and wall adapter (VIN2) to 5V VOUT systems; automatic switchover in power loss events.

Portable industrial or medical applications where power source reliability is critical, with seamless failover from primary to secondary supply.

Embedded systems replacing legacy Schottky or ideal-diode ORing with higher efficiency and integrated intelligence.

Uninterruptible power modules, industrial sensor systems, and communication base stations where system uptime is paramount.

For example, typical circuits can efficiently generate 5V from a voltage range as low as a 2-cell lithium battery or as high as an 18V wall supply, or automatically fall back to a supercapacitor source when the primary fails.

Alternative or Compatible Models to Replace the LTC3118IUFD#PBF

Engineers considering alternates or generational upgrades to the LTC3118IUFD#PBF can evaluate related devices in the Linear Technology/Analog Devices wide-input buck-boost family, including:

LTC3115-1: A similar buck-boost with lower current capability (1.2A) in a 4mm × 4mm QFN, with a lower input voltage range up to 40V.

LTC3112: 15V/2.5A buck-boost with a single input, fixed/preset output, and lower minimum voltage capabilities.

LTC3129: Ultra-low quiescent current buck-boost solutions (15V, 200mA) for ultra-low power applications with unique self-regulation features.

LTC3780: High-power, 60V, 10A buck-boost controller for higher voltage and current needs (external switches required).

Selection should be based on required input/output range, current, packaging, input source architecture, and power path requirements.

Conclusion

: Suitability and Recommendations for System Design with LTC3118IUFD#PBF

The LTC3118IUFD#PBF represents a highly integrated, robust solution for demanding buck-boost power conversion requirements in modern systems. Its dual-input, low loss integrated PowerPath™ controller, excellent efficiency, synchronized switching, and advanced protection features enable engineers to design resilient, efficient, and compact power rails, particularly where seamless primary/backup source selection is required.

For system architects and procurement teams, the LTC3118IUFD#PBF offers a compelling blend of functionality, ease of application-level design, and industry-leading power density—making it a primary choice in applications spanning industrial, instrumentation, portable, and critical infrastructure electronics. Careful attention to external component selection and PCB thermal design will ensure optimal real-world performance and long-term system reliability.