Analog Devices Inc. LT1108CS8#PBF

Overview

LT1108CS8#PBF from Analog Devices (formerly Linear Technology) is a micropower, gated-oscillator DC/DC converter IC capable of step-up, step-down, and inverting topologies. It delivers up to 150mA at 5V from 2 AA cells (2V–3V input), operates from 2V to 30V input, consumes only 110µA quiescent current, and integrates a 1A internal NPN power switch - ideal for battery-powered palmtop computers and portable instruments.

For engineers reviewing the LT1108CS8#PBF datasheet, LT1108CS8#PBF pinout, LT1108CS8#PBF application, or LT1108CS8#PBF equivalent, key selection considerations include its 70% duty cycle, programmable current limit via ILIM pin, on-chip low-battery comparator, adjustable output voltage (via FB/SENSE divider), and compatibility with 8-pin SOIC (S8) packaging for space-constrained designs.

Technical Context

The LT1108CS8#PBF implements a hysteretic, gated-oscillator control architecture: the internal 1.245V reference feeds a comparator that enables a ~19kHz oscillator only when FB/SENSE falls below threshold, minimizing idle current. Its dual-switch topology (SW1 collector, SW2 emitter) supports reconfigurable operation - SW2 grounded for boost, SW1 tied to VIN for buck, and SW2 floating for inverting modes.

It integrates an auxiliary gain block (A0/SET pins) usable as a low-battery detector or error amplifier, and features user-programmable current limiting via external resistor from ILIM to VIN - enabling stable operation across wide input ranges without inductor saturation or switch overstress. Thermal shutdown is not included; max junction temperature is 90°C.

Key Specifications

Pinout & Package

LT1108CS8#PBF is housed in an 8-lead plastic SOIC (S8) package, 3.9mm × 4.9mm body, 1.27mm lead pitch, RoHS-compliant and Pb-free.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LT1173CN8#PBF and TPS61040DRVR, the LT1108CS8#PBF uniquely combines triple-mode operation, programmable current limiting, and on-chip analog functions in a legacy-proven SOIC-8 package - making it optimal for cost-sensitive, low-power, multi-rail portable designs where design reuse and long-term availability matter.

Availability

LT1108CS8#PBF is available at Aetrix Electronics and suitable for palmtop computers, portable instruments, LCD bias generators, and battery backup supplies requiring stable component supply, long-lifecycle support, and proven field reliability.

Supply support for LT1108CS8#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; Linear originally designed the LT1108 family for ultra-low-power, multi-topology DC/DC conversion in portable and battery-critical systems.

The LT1108 product line targets space- and power-constrained applications including handheld computing, medical instrumentation, and remote sensors - emphasizing micropower operation, minimal external components, and flexible voltage conversion without external MOSFETs.

FAQ

What is the maximum output current achievable with the LT1108CS8#PBF in step-up mode?

The LT1108CS8#PBF delivers up to 150mA at 5V from a 2 AA cell input (2V–3V). Output current depends on input voltage, inductor value, and diode selection; at higher inputs (e.g., 5V), peak switch current reaches 1.5A, but practical continuous output is limited by thermal dissipation and efficiency. The internal 1A switch and 1.5A peak rating define absolute limits - sustained >150mA at 5V requires careful thermal design and low-DCR magnetics.

Can the LT1108CS8#PBF be used in step-down (buck) configuration, and what are the key wiring differences versus boost mode?

Yes, the LT1108CS8#PBF supports step-down operation: SW1 connects to VIN, SW2 drives the inductor, and the diode anode ties to SW2 (cathode to output). Unlike boost mode (where SW2 is grounded), buck mode places the switch in high-side position, requiring attention to SW2's –0.5V minimum voltage limit. Efficiency remains high (e.g., 300mA at 5V from 9V), and ILIM programming maintains consistent peak current as VIN varies.

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

The ILIM pin senses a fraction of switch current via an internal current mirror; connecting a resistor (RLIM) between ILIM and VIN sets the trip point. Per the datasheet's "Maximum Switch Current vs RLIM" curve, a 220Ω resistor yields ~400mA limit in step-up mode. This prevents inductor saturation and switch overstress when input voltage rises - critical for battery-powered systems where VIN can vary from 2V (fresh) to 3.2V (fully charged).

Is the LT1108CS8#PBF pin-compatible with the LT1173, and what advantages does it offer over that part?

Yes, the LT1108CS8#PBF is pin-for-pin compatible with the LT1173CN8#PBF. Key improvements include a higher 70% duty cycle (vs. 60%), lower 110µA quiescent current (vs. 250µA), added ILIM current-limit programming, and an integrated auxiliary gain block (A0/SET) for low-battery detection or error amplification - all in the same SOIC-8 package, enabling drop-in upgrades with enhanced functionality and efficiency.

What is the role of the A0 and SET pins on the LT1108CS8#PBF, and how can they be configured as a low-battery detector?

The A0 (open-collector output) and SET (non-inverting input) pins form an auxiliary gain block referenced to the internal 1.245V bandgap. To implement low-battery detection: connect a resistor divider from VIN to GND, feeding the midpoint to SET; A0 pulls low when VIN drops below the trip point (e.g., 3.0V = 1.245V × (1 + R1/R2)). A 47kΩ pull-up on A0 provides a clean logic-level flag to a microcontroller - all without external op amps or comparators.