FSA1256AL8X Equivalent & Substitute Parts

Part Overview

The FSA1256AL8X is a 2-circuit IC switch manufactured by onsemi, featuring SPST-NO (Single Pole Single Throw - Normally Open) configuration with 1.15Ohm on-state resistance. This component is classified as obsolete, making identification of functionally equivalent alternatives essential for ongoing design support and production continuity. The part operates across a 2.7V to 5.5V supply range and is housed in an 8-MicroPak™ surface mount package.

Key Parameters

Parameter	FSA1256AL8X
Manufacturer	onsemi
Switch Circuit Type	SPST - NO
Number of Circuits	2
Multiplexer/Demultiplexer Circuit	1:1
On-State Resistance (Max)	1.15Ohm
Channel-to-Channel Matching (ΔRon)	60mOhm
Supply Voltage Range (V+)	2.7V ~ 5.5V
Switch Time (Ton/Toff Max)	35ns / 15ns
-3dB Bandwidth	300MHz
Charge Injection	20pC
Channel Capacitance (CS(off), CD(off))	11.5pF
Leakage Current (IS(off) Max)	2nA (Typ)
Crosstalk	-100dB @ 1MHz
Operating Temperature Range	-40°C ~ 85°C
Package / Case	8-UFQFN
RoHS Status	ROHS3 Compliant
Moisture Sensitivity Level (MSL)	1 (Unlimited)

Substitute Part Grouping Explanation

Substitution of the FSA1256AL8X is determined by strict alignment of the following electrical and mechanical parameters:

Switch circuit topology: SPST - NO configuration with 1:1 multiplexer/demultiplexer ratio
Number of circuits: 2-circuit requirement
On-state resistance: Maximum 1.15Ohm specification
Supply voltage compatibility: Overlap with 2.7V ~ 5.5V range
Switching performance: Ton/Toff specifications within acceptable operational limits
Bandwidth and signal integrity: -3dB bandwidth and crosstalk performance
Package form factor: Surface mount 8-pin configuration
Environmental compliance: RoHS3 compliance and MSL rating

The NX3L2T66GM,125 from NXP Semiconductors meets the core functional requirements of a 2-circuit SPST-NO switch with 1:1 configuration. However, substitution feasibility depends on tolerance of higher on-state resistance (750mOhm vs. 1.15Ohm) and reduced bandwidth (60MHz vs. 300MHz) in the target application.

Parameter Comparison

Parameter	FSA1256AL8X (onsemi)	NX3L2T66GM,125 (NXP)	Compatibility Notes
Switch Circuit Type	SPST - NO	SPST - NO	Identical
Number of Circuits	2	2	Identical
Multiplexer/Demultiplexer Circuit	1:1	1:1	Identical
On-State Resistance (Max)	1.15Ohm	750mOhm	NXP part has lower resistance
Channel-to-Channel Matching (ΔRon)	60mOhm	20mOhm	NXP part has tighter matching
Supply Voltage Range (V+)	2.7V ~ 5.5V	1.4V ~ 4.3V	Partial overlap: 2.7V ~ 4.3V
Switch Time (Ton/Toff Max)	35ns / 15ns	28ns / 20ns	NXP Ton faster; Toff slower
-3dB Bandwidth	300MHz	60MHz	FSA1256AL8X has 5x higher bandwidth
Charge Injection	20pC	6pC	NXP part has lower charge injection
Channel Capacitance (CS(off), CD(off))	11.5pF	35pF	NXP part has higher capacitance
Leakage Current (IS(off) Max)	2nA (Typ)	10nA	NXP part has higher leakage
Crosstalk	-100dB @ 1MHz	-90dB @ 100kHz	FSA1256AL8X has better crosstalk isolation
Operating Temperature Range	-40°C ~ 85°C	-40°C ~ 125°C	NXP part supports extended temperature
Package / Case	8-UFQFN	8-XFQFN Exposed Pad	Different package variants; both surface mount 8-pin
RoHS Status	ROHS3 Compliant	ROHS3 Compliant	Identical
Moisture Sensitivity Level (MSL)	1 (Unlimited)	1 (Unlimited)	Identical

Engineering Selection Recommendations

The FSA1256AL8X is classified as obsolete. The NX3L2T66GM,125 provides functional equivalence as a 2-circuit SPST-NO switch with matching circuit topology and pin count.

Selection between these parts must account for the following trade-offs:

The NX3L2T66GM,125 exhibits higher on-state resistance (750mOhm) compared to the FSA1256AL8X (1.15Ohm). Applications requiring minimal insertion loss or operating at lower signal levels may experience performance degradation.
The FSA1256AL8X provides significantly higher bandwidth (300MHz vs. 60MHz). High-frequency signal switching applications may not be compatible with the NX3L2T66GM,125.
The NX3L2T66GM,125 operates across a narrower supply voltage range (1.4V ~ 4.3V vs. 2.7V ~ 5.5V). Designs requiring operation at 5.5V are not compatible with the NXP substitute.
The NX3L2T66GM,125 supports extended operating temperature to 125°C, compared to 85°C for the FSA1256AL8X.
Both parts maintain RoHS3 compliance and MSL Level 1 ratings, ensuring environmental and moisture handling compatibility.
Package form factors differ (8-UFQFN vs. 8-XFQFN Exposed Pad). PCB layout and thermal management considerations may require evaluation.

Frequently Asked Questions (FAQ)

Q: Can the NX3L2T66GM,125 directly replace the FSA1256AL8X in all applications?

A: No. While both are 2-circuit SPST-NO switches with 1:1 configuration, the NX3L2T66GM,125 has reduced bandwidth (60MHz vs. 300MHz), higher on-state resistance (750mOhm vs. 1.15Ohm), and a narrower supply voltage range (1.4V ~ 4.3V vs. 2.7V ~ 5.5V). Substitution is application-dependent and requires verification of these parameter tolerances.

Q: What is the primary limitation of using the NX3L2T66GM,125 as a substitute?

A: The most significant limitation is the 5-fold reduction in -3dB bandwidth (60MHz vs. 300MHz). Applications requiring high-frequency signal switching or broadband signal routing will not be compatible with the NXP part.

Q: Are there supply voltage compatibility issues?

A: Yes. The NX3L2T66GM,125 operates from 1.4V to 4.3V, while the FSA1256AL8X operates from 2.7V to 5.5V. Designs operating at 5.5V supply voltage cannot use the NXP substitute. The overlap range is 2.7V to 4.3V.

Q: How do the package differences affect PCB design?

A: The FSA1256AL8X uses 8-UFQFN packaging, while the NX3L2T66GM,125 uses 8-XFQFN with exposed pad. Both are surface mount 8-pin packages, but pin layouts and thermal characteristics may differ. PCB footprint verification is required before substitution.

Q: Do both parts meet the same environmental compliance standards?

A: Yes. Both the FSA1256AL8X and NX3L2T66GM,125 are RoHS3 compliant with MSL Level 1 (Unlimited) moisture sensitivity ratings, ensuring equivalent environmental and handling requirements.

Q: What is the impact of higher on-state resistance in the NXP part?

A: The NX3L2T66GM,125 has 750mOhm on-state resistance compared to 1.15Ohm for the FSA1256AL8X. This lower resistance reduces insertion loss and power dissipation, which may be beneficial in some applications. However, applications designed around the FSA1256AL8X specifications should verify compatibility with the different resistance value.

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