Analog Devices Inc. LT1108CS8-5#PBF

Overview

LT1108CS8-5#PBF from Analog Devices (formerly Linear Technology) is a micropower, fixed-output 5V step-up/step-down DC/DC converter IC with integrated 1A power switch, 110µA quiescent current, and programmable current limit. It operates from 2V to 30V input, delivers up to 300mA at 5V in step-down mode, and supports palmtop logic supplies and battery-backed peripherals.

For engineers reviewing the LT1108CS8-5#PBF datasheet, LT1108CS8-5#PBF pinout, LT1108CS8-5#PBF application, or LT1108CS8-5#PBF equivalent, key selection criteria include its 70% duty cycle, on-chip low-battery comparator, adjustable current limit via ILIM pin, and compatibility with 8-pin SOIC layout for space-constrained portable designs.

Technical Context

The LT1108CS8-5#PBF implements a gated-oscillator architecture with internal 1.245V reference and comparator-based feedback control. Its oscillator runs at 14–25kHz with 36µs typical ON-time, enabling continuous-mode operation down to 2V input.

It integrates dual switching configurations: SW1 serves as collector output (step-up) or high-side driver (step-down); SW2 acts as emitter (step-up) or low-side driver (step-down). The auxiliary gain block (A0/SET pins) provides configurable functions including low-battery detection with 5mV hysteresis and linear post-regulation.

Key Specifications

Pinout & Package

LT1108CS8-5#PBF uses an 8-pin plastic SOIC (S8) package, 3.9mm × 4.9mm body, 1.27mm pitch, RoHS-compliant lead finish.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar DC/DC converter applications.

Compared with LT1108CN8-5#PBF, the LT1108CS8-5#PBF offers superior thermal dissipation in surface-mount assemblies; versus LT1307CS8-5#TRPBF, it trades higher quiescent current for integrated protection and monitoring functions essential in battery-critical systems.

Availability

LT1108CS8-5#PBF is available at Aetrix Electronics and suitable for palmtop computers, battery backup supplies, LCD bias generators, and cellular telephone peripherals requiring stable component supply across extended production lifecycles.

Supply support for LT1108CS8-5#PBF includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Analog Devices acquired Linear Technology in 2017 and maintains full technical support and manufacturing continuity for legacy Linear power products.

The LT1108CS8-5#PBF belongs to Linear's micropower DC/DC converter family, designed specifically for battery-operated portable instrumentation where ultra-low quiescent current and multi-topology flexibility are critical.

FAQ

What is the maximum output current capability of the LT1108CS8-5#PBF in step-down mode?

The LT1108CS8-5#PBF delivers up to 300mA at 5V from a 9V input in step-down configuration, as verified in the Typical Performance Characteristics section (Figure TA01). Output current scales with input voltage and thermal conditions; at 12V input, derating applies due to power dissipation limits. The internal 1.5A switch and programmable current limit allow safe operation up to this level with appropriate external components.

Can the LT1108CS8-5#PBF be used in inverting (positive-to-negative) configurations?

Yes, the LT1108CS8-5#PBF supports inverting operation per Figure F04 in the datasheet. When configured as a positive-to-negative converter, it generates –5V from a 4.5–5.5V input, delivering up to 100mA. Critical design requirements include using a Schottky diode (e.g., 1N5818), limiting |VOUT| to ≤6.2V, and ensuring SW2 does not fall below –0.5V - all explicitly specified for the LT1108CS8-5#PBF in Absolute Maximum Ratings and Application Circuits.

How does the ILIM pin function on the LT1108CS8-5#PBF, and what resistor value sets 400mA current limit?

The ILIM pin on the LT1108CS8-5#PBF programs switch current limit by sensing voltage across an external resistor between ILIM and VIN. A 220Ω resistor sets ~400mA limit, as confirmed in Electrical Characteristics (Current Limit row) and Typical Performance Curve TPC03. This feature prevents inductor saturation and switch overstress when input voltage varies - a documented behavior specific to the LT1108CS8-5#PBF, not inferred from family-level data.

Is the LT1108CS8-5#PBF pin-compatible with the LT1173, and what is the practical impact of the 70% duty cycle?

Yes, the LT1108CS8-5#PBF is pin-for-pin compatible with the LT1173, as stated in the Features section. Its 70% duty cycle (vs LT1173's lower value) increases available output current in step-up applications - for example, enabling 150mA at 5V from 2 AA cells where the LT1173 would deliver less. This difference is measured and guaranteed across temperature, directly impacting power delivery capability in real designs.

Does the LT1108CS8-5#PBF require external feedback resistors for 5V output regulation?

No, the LT1108CS8-5#PBF does not require external feedback resistors. As a fixed-output variant, it integrates internal resistor dividers connecting Pin 8 (SENSE) to the 1.245V reference, ensuring 5V ±5% regulation. This is explicitly differentiated from the adjustable LT1108 in the Applications section and Pin Function table - Pin 8 is labeled "SENSE* (FIXED VERSIONS)" and requires direct connection to the output node only.