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FODM121D Optoisolator Equivalent & Substitute Parts
Part Overview
The FODM121D is a 3750Vrms optoisolator with transistor output manufactured by onsemi in a 4-SMD surface mount package. This component provides galvanic isolation for single-channel DC input applications with 80mA output current capability. The FODM121D is classified as obsolete, making equivalent substitute parts necessary for new designs and ongoing production support. Active alternatives from onsemi and other manufacturers maintain identical electrical isolation and switching characteristics while offering current product status and improved availability.
Substiute Parts
Key Parameters
| Parameter | Value |
|---|---|
| Voltage - Isolation | 3750Vrms |
| Number of Channels | 1 |
| Output Type | Transistor |
| Current Transfer Ratio (Min) | 50% @ 5mA |
| Current Transfer Ratio (Max) | 600% @ 5mA |
| Voltage - Output (Max) | 80V |
| Current - Output / Channel | 80mA |
| Vce Saturation (Max) | 400mV |
| Operating Temperature | -40°C ~ 110°C |
| Mounting Type | Surface Mount |
| Input Type | DC |
Substitute Part Grouping Explanation
Substitution for the FODM121D is determined by strict equivalence across the following critical parameters:
- Voltage isolation rating: 3750Vrms (mandatory)
- Output type: Transistor configuration (mandatory)
- Number of channels: Single channel (mandatory)
- Input type: DC (mandatory)
- Current transfer ratio range: 50% to 600% @ 5mA (defines switching behavior)
- Maximum output voltage: 80V (defines application voltage ceiling)
- Output current capability: 50mA to 80mA (defines load capacity)
- Vce saturation: 200mV to 400mV (defines on-state voltage drop)
- Operating temperature range: -40°C to 110°C minimum (defines thermal operating window)
- Surface mount packaging: 4-SMD, 4-SOP, or 4-SOIC configurations (defines board integration)
Substitute parts are grouped into three categories:
Direct onsemi Equivalents: FODM121R2 and FODM121V maintain identical electrical specifications with different packaging formats (Tape & Reel and Box respectively). These are pin-compatible replacements with active product status.
Cross-Manufacturer Equivalents: Broadcom HCPL-181 series and Everlight EL357ND(TA)-G provide equivalent isolation voltage, output type, and switching characteristics. These parts maintain the 3750Vrms isolation and transistor output while offering different current transfer ratio grades and package options.
Package Variants: Differences in package type (4-SMD Gull Wing, 4-SOP, 4-SOIC, 4-Mini-Flat) do not affect electrical substitution but require PCB layout verification for mechanical fit.
Parameter Comparison
| Part Number | Manufacturer | Isolation (Vrms) | CTR Min @ 5mA | CTR Max @ 5mA | Vout Max (V) | Iout (mA) | Vce Sat Max (mV) | Temp Range (°C) | Package | Status |
|---|---|---|---|---|---|---|---|---|---|---|
| FODM121D | onsemi | 3750 | 50% | 600% | 80 | 80 | 400 | -40 to 110 | 4-SMD, Gull Wing | Obsolete |
| FODM121R2 | onsemi | 3750 | 50% | 600% | 80 | 80 | 400 | -40 to 110 | 4-SMD, Gull Wing | Active |
| FODM121V | onsemi | 3750 | 50% | 600% | 80 | 80 | 400 | -40 to 110 | 4-SMD, Gull Wing | Active |
| EL357ND(TA)-G | Everlight Electronics | 3750 | 300% | 600% | 80 | 50 | 200 | -55 to 110 | 4-SOP (2.54mm) | Active |
| HCPL-181-000E | Broadcom Limited | 3750 | 50% | 600% | 80 | 50 | 200 | -55 to 100 | 4-SOIC (0.209", 5.30mm) | Active |
| HCPL-181-00AE | Broadcom Limited | 3750 | 80% | 160% | 80 | 50 | 200 | -55 to 100 | 4-Mini-Flat | Active |
| HCPL-181-00BE | Broadcom Limited | 3750 | 130% | 260% | 80 | 50 | 200 | -55 to 100 | 4-Mini-Flat | Active |
| HCPL-181-00CE | Broadcom Limited | 3750 | 200% | 400% | 80 | 50 | 200 | -55 to 100 | 4-Mini-Flat | Active |
| HCPL-181-00DE | Broadcom Limited | 3750 | 300% | 600% | 80 | 50 | 200 | -55 to 100 | 4-Mini-Flat | Active |
| HCPL-181-06BE | Broadcom Limited | 3750 | 130% | 260% | 80 | 50 | 200 | -55 to 100 | 4-Mini-Flat | Active |
| HCPL-181-06DE | Broadcom Limited | 3750 | 300% | 600% | 80 | 50 | 200 | -55 to 100 | 4-Mini-Flat | Active |
Engineering Selection Recommendations
Primary Recommendation: FODM121R2 or FODM121V from onsemi provide direct electrical and mechanical equivalence to the FODM121D with active product status. Both maintain identical current transfer ratio specifications (50% to 600% @ 5mA), output voltage (80V), and output current (80mA). FODM121R2 is supplied in Tape & Reel packaging for production volumes; FODM121V is supplied in Box packaging for smaller quantities. Both are ROHS3 compliant and REACH unaffected.
Secondary Recommendation: HCPL-181-00DE from Broadcom Limited provides equivalent isolation voltage (3750Vrms), current transfer ratio range (300% to 600% @ 5mA), and output voltage (80V). This part is supplied in 4-Mini-Flat package and requires PCB layout modification. Output current is reduced to 50mA, which is acceptable for applications not requiring the full 80mA capability of the FODM121D. Operating temperature range is -55°C to 100°C, which covers the FODM121D range except for the upper 10°C margin. ROHS3 compliant and REACH unaffected.
Alternative for Extended Temperature: EL357ND(TA)-G from Everlight Electronics extends the operating temperature range to -55°C to 110°C, matching the upper temperature limit of the FODM121D. Isolation voltage and output type are equivalent. Current transfer ratio minimum is 300%, which is higher than the FODM121D minimum of 50%, requiring circuit verification for applications dependent on lower CTR values. Output current is 50mA. Supplied in 4-SOP package. ROHS3 compliant.
Current Transfer Ratio Matching: Applications sensitive to current transfer ratio should select substitute parts with matching CTR grades. HCPL-181-00AE (80% to 160% CTR) is suitable for low-CTR applications. HCPL-181-00BE and HCPL-181-06BE (130% to 260% CTR) are suitable for mid-range CTR applications. HCPL-181-00DE and HCPL-181-06DE (300% to 600% CTR) match the FODM121D upper CTR range.
Frequently Asked Questions (FAQ)
Q: Can FODM121R2 be used as a direct replacement for FODM121D?
A: Yes. FODM121R2 is electrically and mechanically identical to FODM121D. The only difference is packaging format (Tape & Reel versus bulk). Both maintain 3750Vrms isolation, 50% to 600% current transfer ratio, 80V output voltage, and 80mA output current. FODM121R2 has active product status, making it the recommended replacement.
Q: What is the difference between HCPL-181-00BE and HCPL-181-00DE?
A: Both parts are Broadcom HCPL-181 optoisolators with 3750Vrms isolation in 4-Mini-Flat package. The difference is current transfer ratio: HCPL-181-00BE has 130% to 260% CTR, while HCPL-181-00DE has 300% to 600% CTR. Selection depends on the required switching speed and input current of the application. Higher CTR grades (00DE) provide faster switching response.
Q: Can I use HCPL-181-000E instead of FODM121D?
A: HCPL-181-000E is electrically equivalent but requires package change from 4-SMD Gull Wing to 4-SOIC (0.209", 5.30mm Width). Output current is reduced from 80mA to 50mA. Operating temperature range is -55°C to 100°C, which does not cover the FODM121D upper limit of 110°C. Substitution is possible only if the application does not require 80mA output current and operates below 100°C.
Q: Is EL357ND(TA)-G compatible with FODM121D?
A: EL357ND(TA)-G provides equivalent 3750Vrms isolation and transistor output. Current transfer ratio minimum is 300%, which is higher than FODM121D minimum of 50%. Output current is 50mA versus 80mA for FODM121D. Operating temperature range extends to -55°C, providing wider low-temperature coverage. Substitution requires circuit verification for CTR and output current requirements. Package is 4-SOP (2.54mm), requiring PCB layout modification.
Q: What does "Current Transfer Ratio" mean for optoisolator selection?
A: Current Transfer Ratio (CTR) is the ratio of output collector current to input LED current, expressed as a percentage. Higher CTR values indicate more efficient light-to-current conversion. CTR grade selection affects switching speed, input current requirements, and output current linearity. The FODM121D range of 50% to 600% CTR accommodates applications with varying input drive capabilities.
Q: Are all substitute parts ROHS3 compliant?
A: Yes. All active substitute parts listed (FODM121R2, FODM121V, EL357ND(TA)-G, and all HCPL-181 variants) are ROHS3 compliant. The obsolete FODM121D does not have ROHS3 status specified. All parts are REACH unaffected.
Q: What is the difference between 4-SMD Gull Wing and 4-Mini-Flat packages?
A: Both are surface mount packages with four leads. 4-SMD Gull Wing (used by FODM121D, FODM121R2, FODM121V) has leads that bend outward from the package body. 4-Mini-Flat (used by HCPL-181 series) has leads that are flush or slightly bent. These packages have different footprints and require different PCB pad layouts. Mechanical compatibility must be verified before substitution.
Q: Can I mix different HCPL-181 grades in the same circuit?
A: Yes, if the circuit design accommodates the different CTR ranges. However, using identical part numbers throughout a design ensures consistent switching characteristics and simplifies inventory management. If different CTR grades are necessary, circuit analysis must confirm that each grade operates within acceptable input current and output current limits.
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