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Analog Devices Inc. LT1681ISW#PBF
- Part No.:
- LT1681ISW#PBF
- Manufacturer:
- Analog Devices Inc.
- Category:
- DC DC Switching Controllers
- Package:
- 20-SOIC (0.295", 7.50mm Width)
- Datasheet:
-
LT1681ISW#PBF.pdf
- Description:
- IC SYNCHRONOUS FWRD CTRLR 20SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:293
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Product details
Overview
LT1681ISW#PBF from Analog Devices (formerly Linear Technology) is a high-voltage synchronous forward controller IC for dual-transistor isolated DC/DC converters, supporting input voltages up to 72V, fixed-frequency current-mode control up to 350kHz, and internal 5V reference with ±1% accuracy. It drives external N-channel MOSFETs and enforces ≤50% maximum duty cycle to prevent transformer saturation in telecom and industrial power supplies.
For engineers reviewing the LT1681ISW#PBF datasheet, LT1681ISW#PBF pinout, LT1681ISW#PBF application, or LT1681ISW#PBF equivalent, key selection considerations include its 20-lead SOIC-W package with high-voltage pin spacing, programmable oscillator frequency via FSET, adaptive blanking via BLKSENS, soft-start with 10µA ramp current, and integrated fault protection including OVLO, THERM, IMAX, and UVLO with hysteresis.
Technical Context
The LT1681ISW#PBF implements fixed-frequency current-mode control with an internal oscillator running at twice the switching frequency, synchronized via TTL-compatible SYNC input up to 350kHz. Its toggle flip-flop architecture guarantees ≤50% duty cycle, while phase-lock logic aligns output switch timing to external SYNC edges.
Fault detection includes independent comparators on SHDN (1.25V threshold), OVLO (1.25V), THERM (1.25V), and IMAX (360mV), each triggering a General Fault Condition that discharges SS and resets soft-start. The error amplifier features 1.25V internal reference, 72dB gain, 1MHz GBW, and supports optocoupler feedback via VC output with 25mA source/1mA sink capability.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Operating Input Voltage | 9V to 18V on VCC; supports up to 72V system input via external FETs |
| Max Switching Frequency | 350kHz when synchronized; free-run oscillator up to 700kHz (2×) |
| 5V Reference Accuracy | ±1% over temperature; supplies up to 20mA with short-circuit protection |
| Error Amp Reference | 1.25V ±10mV; used for VFB feedback loop with integrator compensation |
| Current Sense Threshold | 150mV typical at SENSE pin; sets peak primary current limit |
| IMAX Fault Threshold | 360mV rising edge; disables switches in 130ns to prevent primary runaway |
| Soft-Start Ramp Current | –10µA typical; charges external capacitor to gradually increase current limit |
Pinout & Package
The LT1681ISW#PBF is housed in a 20-lead wide SOIC (SW) package with creepage/clearance optimized for high-voltage isolation applications. Pin spacing meets reinforced insulation requirements for systems operating up to 72V input.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| SHDN (1) | Shutdown enable input | Logic-high >1.25V enables operation; 150mV hysteresis prevents chatter during brownout |
| OVLO (2) | Input overvoltage fault sense | Triggers GFC if voltage exceeds 1.25V; resets soft-start for graceful recovery |
| THERM (3) | Thermal shutdown input | Monitors system temperature via resistor divider; same GFC behavior as OVLO |
| SGND (4) | Signal ground reference | Must be isolated from power ground to avoid noise coupling into error amp |
| 5VREF (5) | Local 5V precision reference | Stable ±1% supply for external circuitry; current-limited to 45mA typical |
| FSET (6) | Oscillator timing node | RC network sets frequency; internal 0.8mA discharge and 1.5V/2.5V thresholds define period |
| SYNC (7) | Synchronization input | TTL-compatible; controls both oscillator and switch phase; doubles input frequency internally |
| SS (8) | Soft-start capacitor node | 10µA constant current ramps voltage; 225mV reset threshold ensures zero-current start |
| VFB (9) | Error amplifier inverting input | Connected to output divider; compares against 1.25V reference to regulate voltage |
| VC (10) | Error amplifier output | Drives current-sense comparator threshold; supports optocoupler bias up to 10mA |
| SENSE (11) | Primary current sense input | Measures voltage across ground-referenced shunt; blanked during turn-on via BLKSENS |
| IMAX (12) | Primary runaway protection | Detects excessive primary current independently of blanking window; fast 130ns disable |
| SG (13) | Synchronous rectifier driver | Out-of-phase with BG; leads TG by 150ns to optimize synchronous FET timing |
| VCC (14) | Main IC supply input | 9–18V operation; UVLO at 8.4V falling edge with 0.35V hysteresis |
| PWRGND (15) | Power ground return | Low-impedance path for BG/SG/TG drivers; separate from SGND to reduce noise |
| BG (16) | Bottom-side primary switch driver | Drives low-side FET gate; lags TG during turn-on to concentrate losses on bottom switch |
| BLKSENS (17) | Blanking sense input | Connected to bottom-FET gate; disables SENSE while <5V to suppress turn-on glitches |
| BSTREF (18) | Bootstrap reference node | Connected to source of high-side FET; defines VBST reference plane |
| TG (19) | Top-side primary switch driver | High-side gate driver; disabled until VBST–BSTREF >7.0V for safe startup |
| VBST (20) | Bootstrap supply input | Up to 90V absolute max; charged via external Schottky diode from VCC |
Key Features
| Feature | Design Value |
|---|---|
| Synchronized 350kHz operation | Enables multi-controller phase alignment without external clock distribution networks |
| Adaptive current sense blanking | Uses BLKSENS voltage to dynamically suppress false triggers during MOSFET turn-on |
| Transformer saturation protection | Hardware-enforced ≤50% duty cycle prevents core saturation in forward topologies |
| Integrated 5V ±1% reference | Eliminates need for external reference IC in feedback and bias circuits |
| Multi-fault GFC architecture | Single recovery path (SS discharge to 225mV) unifies response to OVLO, THERM, IMAX, UVLO |
Applications
| Telecom Power Systems | Industrial DC/DC Converters |
|---|---|
Use Scenario: 36V–72V DC input to 5V/7A isolated forward converter in half-brick telecom modules. IC Role / Device Role / Timing Role: Primary-side synchronous forward controller managing dual N-channel FETs with optocoupler feedback. Use Value: Enables high-efficiency, high-power-density conversion with built-in transformer saturation prevention and input overvoltage lockout. | Use Scenario: Lead-acid battery backup system requiring regulated 12V output from variable 24V–60V input. IC Role / Device Role / Timing Role: Dual-transistor forward controller with programmable soft-start and thermal fault shutdown. Use Value: Provides robust start-up sequencing, graceful fault recovery, and stable regulation under wide-input-voltage transients. |
| Automotive Heavy Equipment | Isolated Industrial Supplies |
Use Scenario: 24V vehicle electrical system powering isolated 5V/3A control rail for engine management units. IC Role / Device Role / Timing Role: High-voltage synchronous controller driving external FETs with 5VREF-biased thermistor-based thermal monitoring. Use Value: Delivers reliable operation across –40°C to +85°C ambient with integrated OVLO, THERM, and IMAX fault detection. | Use Scenario: Fully isolated 48V-to-15V forward converter using optocoupler feedback and synchronous rectification. IC Role / Device Role / Timing Role: Error amplifier-based controller with VC output driving optocoupler LED and 1.25V reference for secondary-side sensing. Use Value: Achieves tight output regulation and fast transient response via high-gain voltage-mode error amplifier and integrator compensation. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar synchronous forward controller applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| UCC2891DR | Fixed 500kHz frequency; no programmable FSET; requires external 5V bias | Lacks integrated 5VREF and adaptive blanking; simpler layout but less flexible | Preferred for cost-sensitive designs where 500kHz fixed frequency suffices and external reference is acceptable |
| LM5035MTX/NOPB | Supports up to 1MHz; includes integrated bootstrap diode; no IMAX pin | Higher frequency enables smaller magnetics but lacks dedicated primary runaway detection | Chosen when higher switching frequency and integrated bootstrap are prioritized over discrete IMAX fault protection |
Compared with UCC2891DR and LM5035MTX/NOPB, the LT1681ISW#PBF offers unique adaptive blanking, integrated 5VREF, and dedicated IMAX protection-making it optimal for high-reliability forward converters where transformer saturation and primary current runaway must be actively prevented.
Availability
LT1681ISW#PBF is available at Aetrix Electronics and suitable for telecom power systems, industrial DC/DC converters, and automotive heavy equipment requiring stable component supply with guaranteed long-term availability.
Supply support for LT1681ISW#PBF includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.
Manufacturer
Analog Devices, Inc. (ADI) is a global leader in high-performance analog, mixed-signal, and digital signal processing technologies, serving industrial, automotive, communications, and healthcare markets.
The LT1681ISW#PBF belongs to ADI's legacy Linear Technology power management portfolio, designed specifically for high-voltage, high-reliability synchronous forward DC/DC controllers targeting isolated telecom and industrial power supplies.
FAQ
What is the maximum input voltage supported by the LT1681ISW#PBF controller?
The LT1681ISW#PBF itself operates from a 9V to 18V VCC supply, but it is designed to control external N-channel MOSFETs in systems with up to 72V input voltage. Its high-voltage pinout (e.g., VBST rated to 90V) and fault protection (OVLO, UVLO) ensure safe operation in such high-input applications. The LT1681ISW#PBF does not directly handle 72V on its logic pins-external FETs isolate the high voltage.
How does the LT1681ISW#PBF implement transformer saturation protection?
The LT1681ISW#PBF enforces transformer saturation protection through hardware-limited ≤50% maximum duty cycle, achieved via internal toggle flip-flop logic that gates the primary switch outputs. This prevents excessive volt-second product buildup on the forward transformer regardless of load or line conditions. The LT1681ISW#PBF also includes IMAX and blanking functions to further safeguard against primary current runaway.
Can the LT1681ISW#PBF drive synchronous rectifiers directly without external buffers?
The LT1681ISW#PBF SG, BG, and TG drivers support direct connection to small FETs with total gate capacitance under 5000pF, per datasheet guidance. For higher-efficiency designs or larger FETs, a gate drive buffer is recommended to reduce switching losses and improve timing control. The LT1681ISW#PBF provides 11.5V typical high-side drive and 35ns rise/fall times, enabling robust gate control within these limits.
What is the role of the BLKSENS pin on the LT1681ISW#PBF?
The BLKSENS pin on the LT1681ISW#PBF monitors the gate voltage of the bottom-side primary FET to enable adaptive current sense blanking. When BLKSENS voltage falls below 5V, the SENSE amplifier is disabled-suppressing false overcurrent triggers during MOSFET turn-on transitions. This eliminates the need for fixed-time blanking circuits and ensures accurate current limiting across varying FET sizes and drive conditions. The LT1681ISW#PBF uses this feature to maintain stability in high-dV/dt environments.
Does the LT1681ISW#PBF support optocoupler-based feedback for fully isolated designs?
Yes, the LT1681ISW#PBF supports optocoupler-based feedback via its VC pin, which provides a high-current (25mA source/1mA sink) voltage-output error amplifier. The VC output drives the LED anode of an optocoupler, while the 1.25V internal reference at VFB enables precise secondary-side voltage sensing. This configuration allows full galvanic isolation in forward converters, and the LT1681ISW#PBF's integrator-friendly amplifier simplifies loop compensation for stable regulation.
LT1681ISW#PBF Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Analog Devices Inc.
- Series:
- -
- Package/Case:
- 20-SOIC (0.295", 7.50mm Width)
- Packaging:
- Tube
- Product Status:
- Active
- Output Type:
- -
- Function:
- -
- Output Configuration:
- -
- Topology:
- -
- Number of Outputs:
- -
- Output Phases:
- -
- Voltage - Supply (Vcc/Vdd):
- -
- Frequency - Switching:
- -
- Duty Cycle (Max):
- -
- Synchronous Rectifier:
- -
- Clock Sync:
- -
- Serial Interfaces:
- -
- Control Features:
- -
- Operating Temperature:
- -40°C ~ 85°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 20-SO
LT1681ISW#PBF FAQ
1.How can I place an order for LT1681ISW#PBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LT1681ISW#PBF on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.
2.Are the price and stock information for LT1681ISW#PBF reliable?
The price and inventory of LT1681ISW#PBF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LT1681ISW#PBF is usually 5 days.
3.What payment methods are accepted for LT1681ISW#PBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LT1681ISW#PBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LT1681ISW#PBF?
LT1681ISW#PBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LT1681ISW#PBF order is processed, you will receive an email with the shipment details and tracking number.
Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.
5.How can I obtain technical support or documentation for LT1681ISW#PBF?
For technical support, including LT1681ISW#PBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LT1681ISW#PBF requirements.
6.How does Aetrix verify that LT1681ISW#PBF is sourced from the original manufacturer or authorized distributors?
All LT1681ISW#PBF products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that LT1681ISW#PBF meets industry standards.
7.What is the process for return or replacement of LT1681ISW#PBF?
All LT1681ISW#PBF units undergo pre-shipment inspection (PSI). If there is an issue with LT1681ISW#PBF, returns or replacements are accepted under the following conditions:
1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.
2.The issue is reported within 90 days of delivery.
3.The LT1681ISW#PBF part is unused and in its original packaging.
Return procedure for LT1681ISW#PBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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