FDS6685 P-Channel MOSFET Equivalent & Substitute Parts

Part Overview

The FDS6685 is a P-Channel MOSFET manufactured by onsemi in the PowerTrench® series, rated for 30V drain-to-source voltage with 8.8A continuous drain current at 25°C. The device is packaged in 8-SOIC surface mount configuration with a maximum power dissipation of 2.5W and operating temperature range of -55°C to 175°C.

The FDS6685 carries an Obsolete product status. Equivalent and substitute parts are necessary to maintain design continuity, ensure supply chain availability, and support ongoing production requirements for applications utilizing this MOSFET topology.

Key Parameters

Parameter	Value	Unit
Drain to Source Voltage (Vdss)	30	V
Continuous Drain Current (Id) @ 25°C	8.8	A (Ta)
On-Resistance (Rds On Max) @ 8.8A, 10V	20	mOhm
Gate Threshold Voltage (Vgs(th) Max) @ 250µA	3	V
Gate Charge (Qg Max) @ 5V	24	nC
Input Capacitance (Ciss Max) @ 15V	1604	pF
Maximum Gate Voltage (Vgs Max)	±25	V
Power Dissipation (Max)	2.5	W (Ta)
Operating Temperature Range	-55 to 175	°C (TJ)
Package Type	8-SOIC	Surface Mount
FET Type	P-Channel	—
Technology	MOSFET (Metal Oxide)	—

Substitute Part Grouping Explanation

Substitution of the FDS6685 is determined by strict equivalence across the following critical parameters:

Primary Substitution Criteria:

Drain-to-Source Voltage (Vdss): 30V minimum
Continuous Drain Current (Id): 8.8A or greater at 25°C
On-Resistance (Rds On): 20mOhm or lower at specified gate voltage
Gate Threshold Voltage (Vgs(th)): 3V or lower
Package Type: 8-SOIC or compatible 8-pin surface mount
FET Type: P-Channel
Technology: MOSFET (Metal Oxide)

Secondary Compatibility Parameters:

Operating Temperature Range: Minimum -55°C to 150°C or higher
Power Dissipation: 2.5W or greater
Gate Charge (Qg): Electrical performance indicator
Input Capacitance (Ciss): Switching characteristic indicator
Maximum Gate Voltage (Vgs Max): ±20V or greater

Substitute parts are grouped into two categories: Direct Equivalents (matching all primary criteria with Active product status) and Parametric Equivalents (meeting primary electrical specifications with variations in secondary parameters or product status).

Parameter Comparison

Parameter	FDS6685	FDS4435BZ	SI4435DY	DMP3036SSS-13	RS3E075ATTB	RSS075P03TB
Manufacturer	onsemi	onsemi	onsemi	Diodes Inc.	Rohm Semi.	Rohm Semi.
Vdss (V)	30	30	30	30	30	30
Id @ 25°C (A)	8.8 (Ta)	8.8 (Ta)	8.8 (Ta)	19.5 (Tc)	—	7.5 (Ta)
Rds On Max (mOhm)	20 @ 8.8A, 10V	20 @ 8.8A, 10V	20 @ 8.8A, 10V	20 @ 9A, 10V	23.5 @ 7.5A, 10V	21 @ 7.5A, 10V
Vgs(th) Max (V)	3 @ 250µA	3 @ 250µA	3 @ 250µA	3 @ 250µA	2.5 @ 1mA	2.5 @ 1mA
Qg Max (nC)	24 @ 5V	40 @ 10V	24 @ 5V	16.5 @ 10V	25 @ 10V	30 @ 5V
Ciss Max (pF)	1604 @ 15V	1845 @ 15V	1604 @ 15V	1931 @ 15V	1250 @ 15V	2900 @ 10V
Vgs Max (V)	±25	±25	±20	±25	±20	±20
Power Dissipation Max (W)	2.5 (Ta)	2.5 (Ta)	2.5 (Ta)	1.4 (Ta)	2 (Ta)	2 (Ta)
Operating Temp Range (°C)	-55 to 175	-55 to 150	-55 to 175	-55 to 150	to 150	to 150
Package	8-SOIC	8-SOIC	8-SOIC	8-SO	8-SOP-J	8-SOP
Product Status	Obsolete	Active	Active	Active	Not For New Designs	Active
RoHS Status	—	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant

Engineering Selection Recommendations

Recommended Primary Substitute: FDS4435BZ

The FDS4435BZ is the optimal substitute for the obsolete FDS6685. Both devices are manufactured by onsemi within the PowerTrench® series and share identical electrical specifications across all primary parameters: 30V Vdss, 8.8A continuous drain current, 20mOhm Rds On, and 3V gate threshold voltage. The FDS4435BZ carries Active product status and is ROHS3 compliant, ensuring long-term supply availability and regulatory compliance. The device is available in Cut Tape and Digi-Reel® packaging formats with 68,168 units in stock. The only notable difference is a higher gate charge specification (40nC @ 10V versus 24nC @ 5V), which represents a minor variation in switching characteristics that does not affect functional equivalence in most applications.

Secondary Substitute: SI4435DY

The SI4435DY is a parametric equivalent manufactured by onsemi, also within the PowerTrench® series. It matches all primary electrical specifications with the FDS6685 and maintains the same gate charge rating (24nC @ 5V). The SI4435DY carries Active product status and ROHS3 compliance. The maximum gate voltage specification is ±20V compared to ±25V on the FDS6685, and the operating temperature range extends to 175°C, matching the original device. This part is suitable for applications where the lower gate voltage rating does not present a constraint.

Alternative Substitute: DMP3036SSS-13

The DMP3036SSS-13 manufactured by Diodes Incorporated meets the primary voltage and current specifications with enhanced current capability (19.5A continuous drain current). The on-resistance specification is equivalent at 20mOhm. This device offers superior thermal performance with lower power dissipation (1.4W Ta) and is ROHS3 compliant with Active product status. The DMP3036SSS-13 is suitable for applications requiring higher current capacity or improved thermal characteristics. The package designation is 8-SO, which is mechanically compatible with 8-SOIC footprints.

Not Recommended for New Designs: RS3E075ATTB

The RS3E075ATTB carries a "Not For New Designs" product status and should not be selected for new circuit implementations. While it meets the 30V voltage specification, the continuous drain current specification is not provided, and the device is manufactured by Rohm Semiconductor outside the onsemi PowerTrench® series. This part is listed for reference only in legacy system maintenance scenarios.

Supplementary Option: RSS075P03TB

The RSS075P03TB manufactured by Rohm Semiconductor is an Active product with ROHS3 compliance. It meets the 30V voltage specification with 7.5A continuous drain current, which is slightly below the FDS6685 rating. The on-resistance is 21mOhm at 7.5A, and the operating temperature range is limited to 150°C. This device is suitable only for applications where the reduced current rating and temperature range do not present operational constraints.

Frequently Asked Questions (FAQ)

Q: Can the FDS4435BZ be used as a direct replacement for the FDS6685 without circuit modifications?

A: Yes. The FDS4435BZ is electrically equivalent to the FDS6685 across all primary specifications: 30V Vdss, 8.8A continuous drain current, 20mOhm Rds On, and 3V gate threshold voltage. Both devices are packaged in 8-SOIC and share identical pinout. The gate charge difference (40nC @ 10V versus 24nC @ 5V) represents a minor switching characteristic variation that does not require circuit redesign in standard applications.

Q: What is the difference between the FDS6685 and SI4435DY?

A: Both devices are onsemi PowerTrench® MOSFETs with identical primary electrical specifications. The SI4435DY has a maximum gate voltage rating of ±20V compared to ±25V on the FDS6685. The SI4435DY maintains the same gate charge (24nC @ 5V) and operating temperature range (-55°C to 175°C). The SI4435DY is suitable for applications where the ±20V gate voltage specification is acceptable.

Q: Why is the DMP3036SSS-13 listed as a substitute if it has higher current rating?

A: The DMP3036SSS-13 meets all primary substitution criteria: 30V Vdss, 20mOhm Rds On, and 3V gate threshold voltage. The higher continuous drain current (19.5A versus 8.8A) represents an enhancement rather than a limitation. The device is electrically compatible and can be used in applications where the FDS6685 is specified. The enhanced current capability and lower power dissipation (1.4W Ta) provide additional design margin.

Q: Is the RS3E075ATTB suitable for new designs?

A: No. The RS3E075ATTB carries a "Not For New Designs" product status. This designation indicates that the manufacturer does not recommend this part for new circuit implementations. The RS3E075ATTB should be used only for maintenance and repair of existing systems where the FDS6685 is already deployed.

Q: Are all substitute parts ROHS3 compliant?

A: All substitute parts listed with Active or Not For New Designs status are ROHS3 compliant. The FDS6685 original part does not specify RoHS status in the provided data. All recommended substitutes meet current environmental and regulatory compliance requirements.

Q: Can the RSS075P03TB be used in applications requiring 8.8A continuous current?

A: No. The RSS075P03TB is rated for 7.5A continuous drain current at 25°C, which is below the FDS6685 specification of 8.8A. This device should be used only in applications where the reduced current rating is acceptable. For applications requiring the full 8.8A specification, use FDS4435BZ or SI4435DY.

Q: What is the significance of the different gate charge specifications among substitute parts?

A: Gate charge (Qg) affects switching speed and driver circuit requirements. The FDS6685 and SI4435DY specify 24nC @ 5V, while the FDS4435BZ specifies 40nC @ 10V. Higher gate charge requires more driver current to achieve the same switching speed. In most applications, this difference is accommodated by standard gate driver circuits. Applications with stringent switching frequency requirements should verify driver compatibility.

Q: Are the 8-SO and 8-SOP packages mechanically compatible with 8-SOIC?

A: The 8-SO, 8-SOP, and 8-SOIC packages are mechanically compatible for PCB footprint purposes, with all variants measuring 0.154" (3.90mm) in width. Pin pitch and spacing are standardized across these package types. However, verification of specific footprint dimensions with component datasheets is recommended for high-density layouts.

Q: What is the operating temperature difference between the FDS6685 and FDS4435BZ?

A: The FDS6685 operates from -55°C to 175°C (TJ), while the FDS4435BZ operates from -55°C to 150°C (TJ). The FDS4435BZ has a 25°C lower maximum junction temperature. For applications operating near the upper temperature limit, verify that the reduced maximum temperature does not present a constraint.

Q: Can multiple substitute parts be used interchangeably in the same design?

A: FDS4435BZ and SI4435DY are fully interchangeable with identical electrical specifications and can be used interchangeably in the same design. DMP3036SSS-13, RS3E075ATTB, and RSS075P03TB have parameter variations and should not be mixed within a single design without verification that the specific parameter differences do not affect circuit performance.

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