Analog Devices Inc. LT1108CN8#PBF

Overview

LT1108CN8#PBF from Analog Devices (formerly Linear Technology) is a micropower, gated-oscillator DC/DC converter IC supporting step-up, step-down, and inverting topologies. It delivers up to 300mA at 5V from 9V input or 150mA at 5V from two AA cells, with 110µA quiescent current and internal 1A power switch. Used in battery-powered logic supplies and portable instrument power rails.

For engineers reviewing the LT1108CN8#PBF datasheet, LT1108CN8#PBF pinout, LT1108CN8#PBF application, or LT1108CN8#PBF equivalent, key selection criteria include its 2V–30V input range, programmable current limit via ILIM pin, on-chip low-battery comparator, and compatibility with fixed-output variants LT1108CN8-5 and LT1108CN8-12.

Technical Context

The LT1108CN8#PBF implements a gated-oscillator architecture where the internal 19kHz oscillator activates only when the feedback voltage falls below the 1.245V reference, enabling ultra-low quiescent current. Its NPN power switch features 70% duty cycle and supports continuous-mode operation down to 2V input.

Pin 6 (AO) provides an open-collector auxiliary gain block usable as a low-battery detector or error amplifier; Pin 7 (SET) accepts external bias for trip-point programming. The ILIM pin allows resistor-programmable switch current limit (e.g., 400mA with 220Ω), critical for stable operation across wide VIN ranges in step-down mode.

Key Specifications

Pinout & Package

LT1108CN8#PBF uses an 8-lead plastic DIP (N8) package with 0.300-inch width, lead pitch 0.100 inch, and JEDEC MS-001 compliant footprint. Thermal resistance θ_JA = 150°C/W.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LT1173CN8#PBF and MAX630CPA+, the LT1108CN8#PBF offers superior current capability at low input voltages, active current limiting for inductor protection, and dual-function auxiliary amplifier-making it optimal for space-constrained, battery-sensitive systems requiring multiple topologies.

Availability

LT1108CN8#PBF is available at Aetrix Electronics and suitable for palmtop computer logic supplies, LCD bias generation, and battery backup systems requiring stable component supply with long-term manufacturability.

Supply support for LT1108CN8#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 series for ultra-low-power portable electronics requiring flexible DC/DC conversion with minimal external components.

The LT1108 product line targets battery-operated instruments, handheld computers, and peripheral cards needing efficient, topology-agile power conversion with integrated protection and monitoring functions.

FAQ

What is the maximum output current achievable with LT1108CN8#PBF in step-down mode?

The LT1108CN8#PBF delivers up to 300mA at 5V from a 9V input in step-down mode, as confirmed in the datasheet's performance curves. Peak switch current is limited to 1.5A internally, but practical output depends on inductor selection, thermal management, and input voltage-e.g., 300mA is achievable at 9V→5V with proper 330µH inductor and 100µF output capacitance. Exceeding this requires external switching transistors per Figure 3 in the datasheet.

Can LT1108CN8#PBF generate negative output voltages?

Yes, the LT1108CN8#PBF supports positive-to-negative conversion using inverting topology (Figure 4). With a 5V input, it generates –5V at 100mA using a 220µH inductor, 1N5818 Schottky diode, and appropriate output capacitors. The SW2 pin must never go below –0.5V, requiring careful layout and Schottky clamping-this capability is explicitly verified in the "Positive-to-Negative Converter" section of the datasheet.

How does the ILIM pin function on LT1108CN8#PBF?

The ILIM pin on LT1108CN8#PBF programs the switch current limit by connecting a resistor between ILIM and VIN. A 220Ω resistor sets ~400mA limit; values down to 100Ω enable up to 1.5A. This feature prevents inductor saturation and switch overstress when input voltage varies-e.g., in 6.5–20V step-down applications. The current-limit circuit adds ~2µs propagation delay, as documented in the "Using the ILIM Pin" section.

What is the purpose of the AO and SET pins on LT1108CN8#PBF?

The AO (Pin 6) and SET (Pin 7) form an auxiliary gain block: SET is the non-inverting input referenced to the internal 1.245V bandgap, and AO is an open-collector output sinking ≤100µA. Together they implement low-battery detection (Figure 9), linear post-regulation, or undervoltage lockout-e.g., a resistor divider from VIN to GND on SET triggers AO pull-down when battery drops below programmed threshold.

Is LT1108CN8#PBF pin-compatible with fixed-output variants like LT1108CN8-5?

Yes, LT1108CN8#PBF is fully pin-compatible with LT1108CN8-5 and LT1108CN8-12. All share identical N8 DIP packaging and pinout. The difference lies internally: LT1108CN8#PBF uses external FB/SENSE resistors for adjustable output, while -5/-12 versions integrate precision voltage-setting resistors-so Pin 8 (FB/SENSE) connects directly to output in fixed versions, eliminating external components.