onsemi FAN2558S18X

Overview

FAN2558S18X from onsemi (formerly Fairchild) is a fixed-output 2.5V, 180mA low-dropout linear regulator with power-good indicator, 1% output accuracy, and ultra-low ground current (100µA). It operates from 2.7V to 5.5V input and delivers stable voltage for processor power-up sequencing in portable electronics.

For engineers reviewing the FAN2558S18X datasheet, pinout, applications, or equivalent options, key selection criteria include dropout voltage (400mV at 180mA), fast enable response (30–300µs), open-drain PG output with 95% threshold, and stability with 0–300mΩ ESR output capacitors.

Technical Context

The FAN2558S18X uses BiCMOS technology with a P-channel MOSFET pass element and trimmed bandgap reference to achieve ±1% output accuracy over temperature and load. Its control loop supports wide ESR output capacitors (0–300mΩ) without external compensation.

It integrates thermal shutdown (150°C trip, 10°C hysteresis), current limiting (260–500mA), and active output discharge via internal 100Ω pull-down in shutdown mode. The open-drain PG pin asserts low when VOUT falls below 95% of nominal 2.5V, requiring an external 47kΩ pull-up.

Key Specifications

Pinout & Package

Package: 5-Lead SOT-23 (JEDEC MO-178AA), footprint-compatible with industry-standard 5-pin SOT-23 layouts.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar LDO regulator applications.

Compared with MCP1700T-2502E/TT and NCP1117ST25T3G, the FAN2558S18X uniquely balances 180mA capability, integrated PG signaling, and 100µA ground current in a 5-pin SOT-23 - making it optimal for sequenced, battery-powered 2.5V rail generation where reliability and footprint are co-prioritized.

Availability

FAN2558S18X is available at Aetrix Electronics and suitable for processor power-up sequencing, PDAs and smartphones, and portable electronic equipment requiring stable component supply across extended production lifecycles.

Supply support for FAN2558S18X 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

onsemi (formerly Fairchild Semiconductor) is a global semiconductor supplier focused on energy-efficient power and sensing solutions for automotive, industrial, cloud, and consumer markets.

The FAN2558 series belongs to onsemi's low-voltage CMOS LDO family, engineered specifically for compact battery-powered systems requiring high PSRR, low noise, and robust sequencing control in minimal PCB area.

FAQ

What is the maximum input voltage rating for the FAN2558S18X?

The FAN2558S18X has an absolute maximum input voltage of 6V, but its recommended operating range is 2.7V to 5.5V. Operation above 5.5V risks permanent damage. For 2.5V output, minimum input is VIN = VOUT + VDROPOUT = 2.5V + 0.4V = 2.9V under full load, though datasheet specifies 2.7V as absolute minimum for all VOUT < 2.0V variants - the FAN2558S18X (2.5V) follows the VOUT ≥ 2.0V condition, so actual minimum VIN is VOUT + VDROPOUT. Always verify with thermal derating at high ambient temperatures.

Does the FAN2558S18X require an external bypass capacitor?

No - the FAN2558S18X does not require an external bypass capacitor. Unlike the FAN2559 (low-noise variant) or adjustable FAN2558ADJ versions, the fixed-output FAN2558S18X lacks a BYP pin and is optimized for standard low-noise operation without additional capacitance. Its noise performance (30µVRMS, 300Hz–50kHz) is achieved internally; adding external capacitance offers no benefit and may impair stability if improperly placed.

Can the FAN2558S18X be used with ceramic output capacitors having zero ESR?

Yes - the FAN2558S18X is explicitly designed to remain stable with output capacitors exhibiting ESR from 0mΩ up to 300mΩ. This includes zero-ESR multilayer ceramic capacitors (MLCCs), which are commonly used for their small size and high-frequency filtering. The internal compensation eliminates need for series resistance, enabling direct use of 1µF X5R/X7R MLCCs without risk of oscillation or overshoot during load transients.

What is the purpose of the PG pin on the FAN2558S18X, and how should it be connected?

On the FAN2558S18X, the PG (Power Good) pin is an open-drain output that goes low when VOUT drops below 95% of 2.5V (i.e., <2.375V), indicating fault conditions like input dropout or thermal shutdown. It must be pulled up externally - a 47kΩ resistor to VIN or system logic rail is recommended per datasheet. Leaving PG unconnected is acceptable if monitoring is unnecessary, but never tie it directly to VOUT or GND, as this disables the diagnostic function and may cause latch-up.

Is the FAN2558S18X still recommended for new designs?

No - the FAN2558S18X is marked "DISCONTINUED" and "NOT RECOMMENDED FOR NEW DESIGN" in the official onsemi documentation. While active inventory remains available through authorized distributors including Aetrix Electronics, engineers designing new products should consult onsemi's product change notification (PCN) process and evaluate qualified replacements such as the NCV8170 or AP2112K series, which offer enhanced features, longer lifecycle assurance, and full technical support.