Analog Devices Inc. LTC3355IUF#TRPBF

Introduction to the LTC3355IUF#TRPBF Buck Converter and Supercapacitor Backup Solution

The LTC3355IUF#TRPBF, developed by Analog Devices Inc., is a highly integrated 20V input, 1A output buck DC/DC switching regulator featuring an on-board supercapacitor charger and powerful backup regulator capabilities. Housed in a compact 20-pin, 4mm × 4mm QFN package, the LTC3355IUF#TRPBF is purpose-built for applications demanding seamless ride-through or “dying gasp” power—where output continuity during input supply interruptions is critical. It is particularly suited for power meters, industrial alarms, solid-state drives, and embedded systems requiring supercapacitor energy storage for reliable backup power.

Main Attributes and Advantages of the LTC3355IUF#TRPBF

The LTC3355IUF#TRPBF stands out due to its high degree of functional integration, enabling simplified system design for engineers facing backup and ride-through power requirements. Key features include:

Wide VIN range: Operates from 3V to 20V, suitable for a broad spectrum of power sources.

Positive adjustable output: VOUT can be set from 2.7V to 5V, supporting legacy and modern logic rails.

Integrated charging and backup: Provides seamless charging of a single supercapacitor with a programmable charge current up to 1A; when input power fails, a 5A boost regulator utilizes the stored energy to maintain VOUT.

Extensive current limit and protection: Supports programmable VIN and boost current limits, in addition to charger current regulation and thermal protection.

Multiple system status indicators: Includes VIN Power Fail, VCAP Power Good, and VOUT Power-On Reset outputs for system monitoring.

Compact QFN package: High integration density reduces the required PCB area, enabling size-constrained designs.

These features together address procurement and engineering imperatives—cost, board space efficiency, reliability, and regulatory compliance for backup systems.

Description of LTC3355IUF#TRPBF Functional Blocks

At the heart of LTC3355IUF#TRPBF is a synchronous current-mode buck regulator, which efficiently converts varying input voltages to a stable output. A supercapacitor charger maintains the energy store when the input—VIN—is present, while multiple comparators and logic blocks ensure precise charge/discharge thresholds, current limits, and safe handover between normal and backup operation.

During brownout or blackout events, the PFI (power-fail input) circuitry detects the loss of VIN and instantaneously switches to the integrated boost regulator, powering VOUT from the supercapacitor. Both buck and boost stages are optimized for high efficiency thanks to selectable PWM or Burst Mode operation via the MODE pin, minimizing losses across the full load range.

Status monitoring is available through open-drain outputs for power fail (PFOB), output voltage ready (RSTB), and capacitor good (CPGOOD), supporting supervisory and host microcontroller interfacing. The seamless crossover and monitoring architecture ensure continuous system operation with minimal switchover delays.

Electrical Specifications and Performance Details

The LTC3355IUF#TRPBF’s electrical specifications underscore its suitability for demanding industrial and instrumentation applications:

Absolute maximum VIN: 22V

Output voltage (VOUT): Adjustable from 2.7V to 5V

Main buck regulator output: Up to 1A continuous current at high efficiency

Backup boost operation: Delivers up to 5A pulses from a single supercapacitor (backup supply 0.5V to 5V)

Supercapacitor charger: Linear, up to 1A programmable current, complete with overvoltage and thermal regulation

Integrated thermal regulation: Charger current reduced if die temperature exceeds 110°C; full thermal shutdown at 155°C

Fast switchover typified by minimal voltage droop and low switchover delay

Moisture Sensitivity Level (MSL): 1 (unlimited)

Operating junction temperature: –40°C to +125°C

These parameters ensure robustness in installations where environmental extremes and unpredictable line conditions are routine.

LTC3355IUF#TRPBF Pinout and Packaging Information

The LTC3355IUF#TRPBF is offered in a space-saving 20-lead, 4mm × 4mm QFN package, featuring an exposed thermal pad for optimal heat dissipation. Its pinout supports standard layout practices for both buck and boost configurations:

Critical functions are assigned dedicated pins: PFI (power-fail detection), EN_CHG (charger enable), VIN/VINS (input and current sense), FB and CFB (voltage feedback for output and capacitor), and analog ground (exposed pad) for improved EMC and thermal performance.

Analog I/Os for programming currents/voltage and digital I/Os for status output simplify system-level integration and configuration.

Layout guidelines emphasize minimal trace inductance at switching nodes (SW1, SW2), local high-frequency bypassing, and low-impedance ground returns for best results.

Common Applications of the LTC3355IUF#TRPBF

The versatile architecture of the LTC3355IUF#TRPBF leads to its adoption across several critical use cases:

Ride-through (dying gasp) supplies for industrial automation or communication modules, ensuring that microcontrollers or transceivers complete crucial tasks during sudden input power interruptions.

Power meters and data acquisition systems, maintaining data integrity by supporting microcontroller and memory rails long enough to store or communicate readings before power-down.

Solid-state drives and embedded platforms, preserving the output rail while cached data is flushed to nonvolatile storage in power fail events.

Tantalum or supercapacitor charger and backup circuits, balancing cost and performance for large-scale deployments.

Essential Design Guidelines for Implementing the LTC3355IUF#TRPBF

Engineering implementation with LTC3355IUF#TRPBF requires thorough attention to external network configuration, including:

Setting VOUT using a precision resistor divider from VOUT to FB; use 1% tolerance resistors to maintain regulation accuracy.

Programming the supercapacitor charge current with a resistor at ICHG, per the device formula (Charger Current (A) = 60400/RICHG).

Defining VIN Power Fail (PFI) threshold and boost peak current limit through their respective feedback and programming pins.

Ensuring the supercapacitor (VCAP) voltage is safely configured and monitored to prevent overcharge, using the provided networks and hysteretic comparators.

Adhering to application-specific input voltage and output current requirements, and calculating the minimum input voltage based on load and inductor parameters.

Suggestions for Power Path Design, Passive Components, and PCB Layout with the LTC3355IUF#TRPBF

Performance and system reliability hinge on selecting optimal passives and thoughtful PCB design:

Inductors: Use low DCR, high-frequency cores. Ensure inductor current ratings exceed maximum load plus a 30% margin.

Input/output capacitors: Prefer X5R or X7R ceramics; size and voltage-derating are important, especially for high ripple or ESD-prone environments.

Catch and rectifier diodes: Choose low-leakage, fast Schottky types with appropriate current and voltage ratings.

PCB layout: Minimize loop areas for high-current paths (VIN, SW1/SW2, GND), keep critical feedback nodes compact and shielded, and implement solid ground planes with multiple thermal vias beneath the QFN’s exposed pad.

Thermal Performance and Reliability Considerations for the LTC3355IUF#TRPBF

For high reliability and extended product life, the LTC3355IUF#TRPBF incorporates both thermal regulation and shutdown protections:

A thermal regulation loop reduces charging current if the die temperature exceeds 110°C, allowing safe operation in hot ambient conditions or with minimal board cooling.

Thermal shutdown disables all regulators if the junction temperature rises above 155°C, automatically restarting upon cooldown (~140°C).

Effective heat sinking via copper pours, thermal vias, and airflow ensures the junction temperature stays within specification, maximizing MTTF (mean time to failure) even in high-duty-cycle or elevated temperature installations.

Alternative or Replacement Options for the LTC3355IUF#TRPBF

Selecting alternatives for LTC3355IUF#TRPBF depends on system requirements for input range, output current, integration level, and package. Potential equivalents may include:

LTC3351 (Analog Devices): Enhanced supercapacitor backup controller with energy monitoring and uninterrupted backup regulation.

LTC3128 (Analog Devices): Synchronous buck-boost with integrated supercapacitor charger but lower output current capability.

Texas Instruments BQ25570: For ultra-low power backup systems with integrated battery management but lacking higher output current support.

Consult the manufacturer’s portfolio or established electronics partners to verify electrical compatibility, thermal/pin package equivalence, and necessary feature trade-offs when substituting.

Conclusion

The LTC3355IUF#TRPBF from Analog Devices integrates a versatile buck regulator, programmable supercapacitor charger, and powerful backup boost stage in a single compact QFN package. Its robust feature set—spanning comprehensive protection, current monitoring, and seamless switchover—equips engineers to meet demanding system uptime and data integrity requirements with minimal component count and design effort. Careful attention to programming networks, passive selection, layout, and thermal design ensures maximum performance and reliability. For backup power and ride-through applications in instrumentation, communications, and industrial electronics, the LTC3355IUF#TRPBF represents a benchmark solution with the flexibility and robustness that product selection teams and system designers prioritize.