PLL frequency synthesizer with bypass path
The National Semiconductor LMX2331LTMX is a single-circuit frequency synthesizer that integrates a phase-locked loop with a bypass mode — the PLL can be engaged for frequency synthesis or bypassed to pass the reference clock through unmodified. It accepts CMOS or TTL reference inputs and delivers a CMOS-level output, making it a straightforward clock-conditioning block for mixed-logic boards. The 3:1 input-to-output ratio (three selectable inputs to one output) gives the designer a way to multiplex clock sources without an external mux.
Package and footprint
Housed in a 20-lead TSSOP (LSSOP) package with a 4.40 mm body width, the LMX2331LTMX is a surface-mount part that shares the common 20-TSSOP land pattern. The supplier device package is listed as 20-TSSOP.
Sourcing and compliance note
The LMX2331LTMX is listed as Active in production — no last-time-buy risk for current designs. However, it is marked RoHS non-compliant, which means it contains lead (Pb) above the RoHS threshold. This is not a problem for military, aerospace, or industrial equipment with a RoHS exemption, but it blocks use in EU consumer goods and any RoHS-mandated assembly line. If your BOM requires lead-free solder, this part will not pass incoming inspection without a waiver.