Microchip Technology LE79555-2FQC

Overview

LE79555-2FQC from Zarlink Semiconductor is a Subscriber Line Interface Circuit (SLIC) designed for high-density POTS linecards, supporting Active, Reverse Polarity, Tip Open, Ringing, Standby, and Open Circuit control states with –40 to –58 V battery operation, programmable constant-current feed up to 30 mA, and on-chip switching regulator for low power dissipation.

For engineers reviewing the LE79555-2FQC datasheet, LE79555-2FQC pinout, LE79555-2FQC application, or LE79555-2FQC equivalent, this device serves as a RoHS-compliant, 32-pin QFN SLIC optimized for space-constrained, low-power telephony systems requiring ground-start detection, loop monitoring, and integrated relay drivers.

Technical Context

The LE79555-2FQC implements a fully integrated two-wire interface with DC feed control, longitudinal signal handling, and AC signal transmission via A(TIP)/B(RING) outputs and VTX transmit audio path. Its internal switching regulator (CHCLK input 200–600 kHz) generates regulated negative supply from VBAT, eliminating need for external –5 V rail.

Control logic accepts TTL-compatible C3–C1 inputs to select six operational states; supervision signals (DET, RD) provide off-hook, ring-trip, and ground-key detection. Three open-collector relay drivers (RINGOUT, RYOUT1, RYOUT2) support ringing and general-purpose switching with built-in snubbers.

Key Specifications

Pinout & Package

LE79555-2FQC uses a 32-pin QFN package (8×8 mm) with exposed thermal pad connected to VBAT potential - not grounded - to maximize heat transfer from the switching regulator and power amplifiers.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LE79555-2BVC, the LE79555-2FQC offers superior thermal performance (25°C/W vs. 52°C/W) and smaller footprint, enabling higher line density; versus LE79202BVC, it eliminates external transformers and reduces system power by >50%, at the cost of increased design complexity for switching regulator layout.

Availability

LE79555-2FQC is available at Aetrix Electronics and suitable for VoIP gateway line cards, central office switches, enterprise IP-PBX systems, and remote terminal units requiring stable component supply and long-term lifecycle support.

Supply support for LE79555-2FQC 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

Zarlink Semiconductor was a Canadian fabless semiconductor company specializing in voice, timing, and RF solutions for telecommunications infrastructure before its acquisition by Microsemi in 2011.

The LE79555-2FQC belongs to the VoiceEdge™ VE580 series, designed specifically for high-density, low-power POTS linecard applications where space, thermal efficiency, and integrated supervision features are critical.

FAQ

What is the maximum loop current supported by the LE79555-2FQC in Active state?

The LE79555-2FQC supports up to 30 mA of constant-current feed in Active, Normal, and Reverse Polarity operation, as specified in the datasheet's Electrical Characteristics table. This rating holds across the full operating temperature range (–40°C to +85°C) and ensures compatibility with international POTS loop standards including long-loop conditions. The LE79555-2FQC achieves this with low overhead voltage and integrated feed control.

Does the LE79555-2FQC require an external –5 V supply?

No, the LE79555-2FQC does not require an external –5 V supply. Its on-chip switching regulator generates the necessary internal negative supply from the main VBAT rail (–40 V to –58 V), eliminating the need for a second voltage source. This feature is explicitly stated in the Features section and confirmed in the Switching Regulator block description of the LE79555-2FQC datasheet.

How is ground-start detection implemented in the LE79555-2FQC?

The LE79555-2FQC implements automatic ground-start detection via its Ground Detector block, which monitors longitudinal current against the IGK threshold (5–13 nA) during Active, Standby, or Tip Open states. When exceeded, the DET pin asserts low - indicating ground key or ring-ground fault. This function is hardware-based and requires no software configuration, as detailed in the Block Descriptions section of the LE79555-2FQC datasheet.

What is the role of the exposed thermal pad on the LE79555-2FQC QFN package?

The exposed thermal pad on the LE79555-2FQC QFN package must be connected directly to the VBAT net (not AGND or BGND) to serve as the primary thermal conduction path. It reduces junction-to-board thermal resistance to 25°C/W, critical for managing power dissipation during high-current Active and Ringing states. The datasheet warns against grounding the pad and specifies soldering to an equally sized copper area with multiple vias.

Can the LE79555-2FQC operate in both normal and reverse polarity modes?

Yes, the LE79555-2FQC supports both Normal and Reverse Polarity operation, with confirmed 63 dB longitudinal rejection in Normal Polarity mode across commercial and industrial temperature ranges. Reverse Polarity mode is enabled via C3–C1 control inputs and maintains ≥54 dB L-T rejection, enabling use in payphone keypad disable and other polarity-sensitive applications as documented in the LE79555-2FQC Performance Grade specifications.