Texas Instruments LMX2531LQ1778E/NOPB

Overview

LMX2531LQ1778E/NOPB from Texas Instruments is a high-performance fractional-N delta-sigma PLL frequency synthesizer with integrated VCO, operating in the 1726–1840 MHz high-band and 863–920 MHz low-band. It delivers ultra-low phase noise (–92 dBc/Hz @ 10 kHz offset), supports 1.8-V MICROWIRE serial interface, and features FastLock with timeout for rapid frequency settling in RF local oscillator applications.

For engineers reviewing the LMX2531LQ1778E/NOPB datasheet, LMX2531LQ1778E/NOPB pinout, LMX2531LQ1778E/NOPB application, or LMX2531LQ1778E/NOPB equivalent, key selection considerations include its dual-band VCO tuning range, integrated loop filter support, 36-pin WQFN package, and programmable charge pump current up to 1440 µA for optimized phase noise vs. lock time trade-offs.

Technical Context

The LMX2531LQ1778E/NOPB implements a fourth-order delta-sigma modulator for fractional-N synthesis, enabling fine frequency resolution without spurs. Its fully integrated VCO includes on-die tank inductor and adjustable third/fourth poles, eliminating external resonators while maintaining <±600 kHz frequency pulling under VSWR = 2:1.

It uses three independent LDOs-VregPLL1/VregPLL2 for charge pump and digital PLL circuitry, VregVCO for VCO core, and VregBUF for output buffer-to isolate supply noise and ensure consistent phase noise performance across temperature (–40°C to +85°C) and supply variation (2.8–3.2 V).

Key Specifications

Pinout & Package

The LMX2531LQ1778E/NOPB is housed in a 36-pin 6 mm × 6 mm × 0.8 mm WQFN package (NJG0036A variant), with exposed thermal pad for enhanced heat dissipation and ground connection.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LMX2531LQ1700E/NOPB and LMX2531LQ1742/NOPB, the LMX2531LQ1778E/NOPB provides optimal overlap for 863–920 MHz low-band and 1726–1840 MHz high-band-matching EU DECT, LTE Band 20, and 868 MHz ISM allocations with no frequency gap or excess range.

Availability

LMX2531LQ1778E/NOPB is available at Aetrix Electronics and suitable for cellular base stations, satellite communications equipment, and automotive radar systems requiring stable component supply, full traceability, and long-term lifecycle support.

Supply support for LMX2531LQ1778E/NOPB 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

Texas Instruments is a global semiconductor company specializing in analog and embedded processing technologies, with leadership in RF, power management, and precision analog ICs.

The LMX2531 product line delivers highly integrated, low-noise frequency synthesizers targeting wireless infrastructure, test equipment, and automotive radar-designed to replace discrete PLL+VCO solutions with single-chip reliability and reduced layout risk.

FAQ

What is the maximum phase detector frequency supported by the LMX2531LQ1778E/NOPB?

The LMX2531LQ1778E/NOPB supports a maximum phase detector frequency of 32 MHz, as specified in its electrical characteristics table. This limit applies regardless of reference input frequency or division settings. Operating above 32 MHz may cause incorrect lock detection or increased phase noise due to charge pump nonlinearity. The LMX2531LQ1778E/NOPB's internal prescaler and R-divider allow flexible reference scaling to maintain optimal fPD within this bound.

Does the LMX2531LQ1778E/NOPB require external loop filter components?

Yes, the LMX2531LQ1778E/NOPB requires an external passive loop filter between CPout (Pin 24) and Vtune (Pin 23). While the device integrates third and fourth poles that are digitally adjustable, the primary RC network-including series resistor, shunt capacitor, and optional series capacitor-must be placed externally to set loop bandwidth, phase margin, and reference spur suppression. The LMX2531LQ1778E/NOPB datasheet provides design equations and example values for common loop configurations.

Can the LMX2531LQ1778E/NOPB operate with a 1.8-V reference clock?

No, the LMX2531LQ1778E/NOPB OSCin input accepts only 5–80 MHz single-ended or differential crystal/reference signals with 0.5–2.0 Vpp amplitude-not logic-level clocks. A 1.8-V square wave violates the VIH/VIL thresholds for OSCin (which expects AC-coupled sine wave or clipped sine). To use a 1.8-V digital clock source, an external AC-coupling network and amplitude-limiting circuit must condition the signal to meet the OSCin specification before applying it to Pin 31 or 32.

What is the purpose of the FLout pin on the LMX2531LQ1778E/NOPB?

The FLout pin (Pin 25) on the LMX2531LQ1778E/NOPB is an open-drain NMOS output used primarily for FastLock status indication. When configured for FastLock mode, FLout asserts low upon successful lock acquisition. It can also be repurposed as a general-purpose digital output controlled by register bits, enabling synchronization with other system events. Unlike Ftest/LD, FLout does not reflect PLL lock state by default and requires explicit register configuration to activate FastLock functionality.

How does temperature drift affect continuous lock performance of the LMX2531LQ1778E/NOPB?

The LMX2531LQ1778E/NOPB specifies a maximum allowable temperature drift of ±125°C for continuous lock-meaning the ambient temperature may vary up to 125°C above or below the value present when the R0 register was last programmed, while maintaining lock. If drift exceeds this limit, the VCO calibration must be re-executed by rewriting R0. This behavior is inherent to its integrated VCO architecture and applies across its full –40°C to +85°C operating range.