Microchip Technology HT18LG-G

Overview

HT18LG-G from Supertex Inc. is an electronic telephone line switch IC that replaces mechanical hookswitch contacts in telecom interface circuits. It features 350V breakdown voltage, ≤18Ω on-state resistance, current-limiting via external resistor, 2.3V minimum logic-compatible control, and operates with TPG-referenced inputs for tip-side switching in analog telephone lines.

For engineers reviewing the HT18LG-G datasheet, HT18LG-G pinout, HT18LG-G application, or HT18LG-G equivalent, this device supports pulse-dialing compatibility, dual-control (HKS/LS) activation, DP-driven turn-off, and requires Zener clamping on TP to meet 18V output protection - critical for handset, modem, and remote meter reading designs.

Technical Context

The HT18LG-G integrates a high-voltage charge pump and logic-controlled MOSFET switch to isolate and route the positive (tip) side of POTS lines. Its three independent control inputs - HKS (hookswitch sense), LS (logic enable), and DP (dial pulse) - implement combinational logic per truth table, enabling ON/OFF states without external level shifters.

Current limiting is externally set via R_EXT between CS and TP, delivering 188–330mA ILIM range. Switching times are ≤1.0ms both ways at 4.5V TPG, and the device withstands 350V transient on TPG while maintaining <2.0µA leakage up to 100V.

Key Specifications

Pinout & Package

HT18LG-G is housed in an 8-Lead SOIC (Narrow Body) package: 4.90×3.90mm body, 1.27mm pitch, 1.75mm max height, RoHS-compliant lead-free finish.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar telephone line switch applications.

Compared with SL18001DZ and MAX1395ESE+, the HT18LG-G uniquely balances 350V ruggedness, native pulse-dial compatibility, and 2.3V logic drive - making it optimal for cost-sensitive, standards-compliant telephone interface products where mechanical switch replacement is the primary goal.

Availability

HT18LG-G is available at Aetrix Electronics and suitable for telephone handsets, modems, and remote meter reading terminals requiring stable component supply across extended product lifecycles.

Supply support for HT18LG-G 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

Supertex Inc. was a U.S.-based analog semiconductor company specializing in high-voltage interface, LED driver, and telecom ICs before its acquisition by Microchip Technology in 2014.

The HT18LG-G belongs to Supertex's telephone line interface product line, engineered specifically to replace electromechanical hookswitches in analog voice equipment while meeting POTS line voltage and surge requirements.

FAQ

What is the maximum allowable voltage on the TPG pin of the HT18LG-G?

The HT18LG-G supports up to +350V on the TPG pin under absolute maximum ratings. This rating allows direct connection to standard POTS telephone lines including ring voltage transients. Operation above this voltage risks permanent damage. The device exhibits <2.0µA leakage at 100V and remains functional up to 290V per test conditions in the datasheet. Always verify line conditioning and transient protection upstream of TPG in final designs using the HT18LG-G.

How does the HT18LG-G implement current limiting, and what resistor value sets 250mA?

The HT18LG-G implements current limiting via an external resistor (R_EXT) connected between CS and TP pins. A 200Ω ±1% resistor sets the nominal current limit to 250mA, as specified in the Electrical Characteristics table. The relationship is linear: lower resistance increases current limit, higher resistance reduces it. The HT18LG-G does not include internal current-sense amplification - R_EXT directly defines the trip point, making calibration straightforward and predictable in production.

Can the HT18LG-G be used with 3.3V microcontroller GPIOs without level shifting?

Yes, the HT18LG-G accepts 3.3V logic signals directly on LS and DP pins. Its VIH(LS)/VIH(DP) specification is 1.5V to 10V, and VIL is 0–0.2V, fully compatible with standard 3.3V CMOS outputs. No level shifter is required. However, HKS input expects connection to TPG (up to 350V), so it must be driven via mechanical switch or optocoupler - not directly from MCU. The HT18LG-G's 2.3V minimum operating voltage further ensures robustness with margin against brownout.

Why is a Zener diode required on the TP pin of the HT18LG-G?

A Zener diode (18V, cathode to TP, anode to GND) is mandatory on the TP pin because the HT18LG-G's absolute maximum rating for V_TP is +18V. Without clamping, inductive kickback or line transients could exceed this, causing irreversible damage. The Zener ensures TP never rises above 18V while sinking fault current safely to GND. This requirement is explicitly stated in the Pin Description and General Description sections of the HT18LG-G datasheet and is non-negotiable in layout.

Does the HT18LG-G support both tone and pulse dialing, and how is pulse dialing implemented?

The HT18LG-G natively supports pulse dialing via its DP (Dial Pulse) pin, which forces immediate switch turn-off when driven high - interrupting loop current to generate dial pulses. Tone dialing (DTMF) does not require DP involvement; the switch remains ON during tone transmission. The HT18LG-G's truth table confirms DP has priority over HKS and LS. This behavior matches legacy telephone system requirements, and the ≤1.0ms turn-off time ensures accurate pulse generation. No firmware or external timing circuitry is needed to implement pulse dialing with the HT18LG-G.