AO3421 Equivalent & Substitute Parts

Part Overview

The AO3421 is a P-Channel 30V 2.6A surface mount MOSFET in SOT-23-3 packaging manufactured by Alpha & Omega Semiconductor Inc. This device is classified as "Not For New Designs," indicating it has reached end-of-life status. The part is ROHS3 compliant with unlimited moisture sensitivity level (MSL 1) and is suitable for applications requiring P-channel switching and amplification in low-power circuits.

Due to its discontinued design status, equivalent and substitute parts are necessary for ongoing production support, maintenance applications, and design continuity where the AO3421 specifications remain functionally adequate.

Key Parameters

Parameter	Value	Unit
FET Type	P-Channel	—
Drain to Source Voltage (Vdss)	30	V
Continuous Drain Current (Id) @ 25°C	2.6	A
Rds On (Max) @ 2.6A, 10V	110	mOhm
Gate Threshold Voltage (Vgs(th)) @ 250µA	2.4	V
Power Dissipation (Max)	1.4	W
Operating Temperature Range	-55 to 150	°C
Package Type	SOT-23-3	—
Mounting Type	Surface Mount	—

Substitute Part Grouping Explanation

Substitution of the AO3421 is determined by the following critical parameters:

Electrical Compatibility Requirements:

FET Type: P-Channel (mandatory match)
Drain to Source Voltage (Vdss): 30V minimum
Continuous Drain Current (Id): 2.6A or greater
Gate Threshold Voltage (Vgs(th)): Within ±20V maximum gate voltage specification
Operating Temperature Range: -55°C to 150°C (minimum requirement)

Mechanical Compatibility Requirements:

Package Type: SOT-23-3 or equivalent footprint (TO-236-3, SC-59, PG-SOT23)
Mounting Type: Surface Mount

Compliance Requirements:

ROHS3 Compliant
Moisture Sensitivity Level: MSL 1 (Unlimited)

Substitute parts are grouped into two categories:

Category A - Direct Electrical Equivalents: Parts meeting or exceeding all electrical parameters with identical or superior performance characteristics (AO3409, AOSS21319C).

Category B - Functional Substitutes: Parts with reduced continuous drain current (1.5A) but maintaining 30V Vdss rating and SOT-23-3 package compatibility. These substitutes are suitable for applications where the full 2.6A rating is not required (BSS314PEH6327XTSA1, BSS315PH6327XTSA1, FDN358P).

Parameter Comparison

Part Number	Manufacturer	Vdss (V)	Id @ 25°C (A)	Rds On @ Id, 10V (mOhm)	Vgs(th) @ 250µA (V)	Power Dissipation (W)	Package	Product Status
AO3421	Alpha & Omega	30	2.6	110	2.4	1.4	SOT-23-3	Not For New Designs
AO3409	Alpha & Omega	30	2.6	130	3.0	1.4	SOT-23-3	Not For New Designs
AOSS21319C	Alpha & Omega	30	2.8	100	2.2	1.3	SOT-23-3	Active
BSS314PEH6327XTSA1	Infineon	30	1.5	140	2.0	0.5	PG-SOT23	Active
BSS315PH6327XTSA1	Infineon	30	1.5	150	2.0	0.5	PG-SOT23	Active
FDN358P	onsemi	30	1.5	125	3.0	0.5	SOT-23-3	Active

Engineering Selection Recommendations

For Direct Replacement (Equivalent Current Rating):

The AOSS21319C is the primary recommended substitute. It is manufactured by Alpha & Omega Semiconductor Inc. with Active product status, providing long-term availability assurance. The AOSS21319C exceeds the AO3421 specifications with 2.8A continuous drain current, superior Rds On performance (100 mOhm vs. 110 mOhm), and lower gate threshold voltage (2.2V vs. 2.4V). Power dissipation is reduced to 1.3W. ROHS3 compliance and MSL 1 rating are maintained. This part is suitable for direct substitution in all applications where the AO3421 was originally specified.

The AO3409 provides an alternative from the same manufacturer with identical 2.6A rating and 1.4W power dissipation. However, it retains "Not For New Designs" status, limiting its suitability for long-term production applications.

For Current-Reduced Applications:

The BSS314PEH6327XTSA1, BSS315PH6327XTSA1, and FDN358P are suitable substitutes where continuous drain current requirements do not exceed 1.5A. All three maintain 30V Vdss rating and SOT-23-3 compatible packaging. These parts offer Active product status with established long-term availability. Selection among these three depends on specific Rds On and gate threshold voltage requirements for the application.

BSS314PEH6327XTSA1 and BSS315PH6327XTSA1 are Infineon OptiMOS™ series devices with superior gate charge characteristics (2.9 nC and 2.3 nC respectively). FDN358P is an onsemi PowerTrench® device with lower input capacitance (182 pF) and moderate gate charge (5.6 nC).

Compliance Verification:

All recommended substitutes maintain ROHS3 compliance and MSL 1 rating. Operating temperature range (-55°C to 150°C) is consistent across all alternatives.

Frequently Asked Questions (FAQ)

Q: Can the AO3421 be directly replaced with the AOSS21319C?

A: Yes. The AOSS21319C meets all electrical and mechanical requirements for direct substitution. It provides superior performance with higher continuous drain current (2.8A vs. 2.6A), lower on-resistance (100 mOhm vs. 110 mOhm), and Active product status for long-term availability.

Q: What is the primary difference between Category A and Category B substitutes?

A: Category A substitutes (AO3409, AOSS21319C) maintain the 2.6A or higher continuous drain current rating of the AO3421. Category B substitutes (BSS314PEH6327XTSA1, BSS315PH6327XTSA1, FDN358P) reduce continuous drain current to 1.5A. Category B parts are suitable only for applications where peak current requirements do not exceed 1.5A.

Q: Are the Infineon BSS314 and BSS315 parts interchangeable with each other?

A: Both BSS314PEH6327XTSA1 and BSS315PH6327XTSA1 share identical electrical ratings (30V, 1.5A, 500mW). Primary differences are gate charge (2.9 nC vs. 2.3 nC) and gate threshold voltage measurement conditions. Selection depends on specific switching speed and gate drive requirements. Both use PG-SOT23 packaging, which is footprint-compatible with SOT-23-3.

Q: Why does the AO3421 have "Not For New Designs" status?

A: This designation indicates the part has reached end-of-life and is no longer recommended for new circuit designs. Existing inventory remains available for maintenance and legacy production support. The AOSS21319C is the recommended Active alternative for new designs.

Q: Are all substitute parts ROHS3 compliant?

A: Yes. All substitute parts listed maintain ROHS3 compliance and MSL 1 (Unlimited) moisture sensitivity level, matching the AO3421 environmental specifications.

Q: What is the significance of the different packaging designations (SOT-23-3 vs. PG-SOT23)?

A: SOT-23-3 and PG-SOT23 are equivalent package designations for the same physical form factor (TO-236-3, SC-59). All parts in this comparison use the same three-lead surface mount package with identical footprint dimensions and pin assignments. Packaging designation differences reflect manufacturer nomenclature only.

Q: Can the FDN358P be used in place of the AO3421 in a 2.6A application?

A: No. The FDN358P is rated for 1.5A continuous drain current, which is insufficient for applications requiring the full 2.6A capability of the AO3421. Use FDN358P only in applications where continuous current does not exceed 1.5A.

Q: What is the advantage of AOSS21319C over AO3409 for substitution?

A: The AOSS21319C has Active product status, ensuring long-term availability and manufacturing support. The AO3409 retains "Not For New Designs" status, limiting future availability. Additionally, AOSS21319C provides superior electrical performance with higher current rating (2.8A), lower on-resistance (100 mOhm), and reduced power dissipation (1.3W).

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