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Analog Devices Inc. LT1681ESW#PBF
- Part No.:
- LT1681ESW#PBF
- Manufacturer:
- Analog Devices Inc.
- Category:
- DC DC Switching Controllers
- Package:
- 20-SOIC (0.295", 7.50mm Width)
- Datasheet:
-
LT1681ESW#PBF.pdf
- Description:
- IC SYNCHRONOUS FWRD CTRLR 20SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:2,446
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Product details
Overview
LT1681ESW#PBF from Analog Devices (formerly Linear Technology) is a high-voltage synchronous forward controller IC for dual-transistor DC/DC converters, supporting input voltages up to 72V, fixed-frequency current-mode control up to 350kHz, and 50% maximum duty cycle enforcement to prevent transformer saturation. It drives external N-channel MOSFETs in isolated or nonisolated forward topologies and integrates soft-start, UVLO, OVLO, thermal shutdown, and primary-side saturation protection.
For engineers reviewing the LT1681ESW#PBF datasheet, LT1681ESW#PBF pinout, LT1681ESW#PBF application, or LT1681ESW#PBF equivalent, key selection considerations include its 20-lead SO package with high-voltage pin spacing, programmable oscillator frequency via FSET, synchronous rectifier driver capability, adaptive blanking via BLKSENS, and integrated 5V reference with ±1% tolerance and 20mA load capacity.
Technical Context
The LT1681ESW#PBF implements fixed-frequency current-mode control with an internal oscillator running at twice the switching frequency (e.g., 700kHz oscillator for 350kHz operation), synchronized via TTL-compatible SYNC input. Its T-flip-flop logic enforces ≤50% duty cycle and coordinates phase relationships between TG, BG, and SG outputs.
Fault management is centralized through General Fault Condition (GFC) logic: events including SHDN low, OVLO/THERM >1.25V, IMAX >360mV, VCC UVLO (<8.4V), or 5VREF overcurrent trigger immediate switch disable and soft-start capacitor discharge to 225mV before recovery.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Operating Input Voltage | 9V–18V on VCC; supports up to 72V system input via external FETs |
| Max Switching Frequency | 350kHz (synchronized); enables compact magnetics and EMI control |
| Duty Cycle Limit | 50% max enforced by internal logic to prevent transformer core saturation |
| 5V Reference Accuracy | ±1% (4.85V–5.10V) at 20mA load; powers optocouplers and sensing circuitry |
| Current Sense Threshold | 150mV typical at SENSE pin; sets peak primary current limit with 130mV hysteresis |
| Soft-Start Current | –10µA typical at SS pin; linearly ramps internal current limit to minimize output overshoot |
| UVLO Threshold | 8.4V (rising), 8.0V (falling) on VCC; prevents operation during insufficient supply |
Pinout & Package
The LT1681ESW#PBF is housed in a 20-lead wide SOIC (SW) package with creepage/clearance optimized for ≥72V system isolation requirements.
| 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 | Resistor-divider monitored; >1.25V disables switching and resets soft-start |
| THERM (3) | Thermal fault sense | Connects to thermistor divider; >1.25V triggers GFC and graceful recovery |
| SGND (4) | Signal ground reference | Must be isolated from power ground to avoid noise coupling into error amp |
| 5VREF (5) | Precision 5V output | ±1% reference; supplies external circuitry up to 20mA; short-circuit protected to 45mA |
| FSET (6) | Oscillator timing node | RC network sets frequency; internal 0.8mA discharge and 2.5V/1.5V thresholds define period |
| SYNC (7) | External clock synchronization input | TTL-compatible; locks internal oscillator phase and controls switch timing in multi-controller systems |
| SS (8) | Soft-start ramp control | 10µA current source charges external capacitor; voltage ramp sets peak current limit |
| VFB (9) | Error amplifier inverting input | 1.25V internal reference; feedback from output divider sets regulation point |
| VC (10) | Error amplifier output | Inverting transconductance stage; drives optocoupler LED or compensation network |
| SENSE (11) | Primary current sense input | Monitors ground-referenced shunt; comparator trips at 150mV to terminate switch on-time |
| IMAX (12) | Primary runaway current protection | Detects >360mV during blanking interval; disables switches in 130ns for fast fault response |
| SG (13) | Synchronous rectifier gate driver | Out-of-phase with BG; leads primary turn-on by 150ns to reduce body diode conduction loss |
| VCC (14) | Main IC supply input | 9–18V operation; UVLO disables all functions below 8.4V to protect startup |
| PWRGND (15) | Power ground return | Low-impedance path for BG/SG/TG driver currents; separate from SGND |
| BG (16) | Bottom-side primary switch driver | Drives N-FET source; lags TG turn-on and leads turn-off to concentrate losses on bottom FET |
| BLKSENS (17) | Current sense blanking control | Connected to bottom-FET gate; disables SENSE while gate voltage <5V to suppress turn-on spikes |
| BSTREF (18) | Bootstrap supply reference | Connected to topside FET source; defines VBST return for high-side gate drive |
| TG (19) | Topside primary switch driver | High-side N-FET driver; gated by VBST-BSTREF UVLO (>7.0V required) |
| VBST (20) | Bootstrapped high-side supply | Charged via Schottky diode from VCC; supports up to 90V referenced to BSTREF |
Key Features
| Feature | Design Value |
|---|---|
| Synchronous rectifier driver (SG) | Provides precise timing lead over BG to minimize synchronous FET body diode conduction loss |
| Adaptive current sense blanking | BLKSENS pin monitors bottom-FET gate voltage to dynamically suppress false triggering during switching transitions |
| Transformer saturation protection | Internal logic enforces ≤50% duty cycle and monitors IMAX to detect primary current runaway before core saturation |
| Optocoupler feedback support | VC pin delivers 25mA source/1mA sink current to directly drive optocoupler LEDs in isolated designs |
| Multi-fault graceful recovery | All faults (OVLO, THERM, IMAX, UVLO, 5VREF overload) trigger GFC, discharge SS capacitor, and require 225mV reset before restart |
Applications
| Telecom Power Systems | Industrial DC Power Supplies |
|---|---|
Use Scenario: 48V telecom rectifier feeding distributed 5V/7A loads in shelf-based equipment. IC Role / Device Role / Timing Role: Dual-transistor synchronous forward controller managing primary-side switching and synchronous rectification timing. Use Value: Enables high-efficiency, low-noise 5V output with transformer isolation and robust fault handling for NEBS-compliant systems. | Use Scenario: High-reliability 24–72V input industrial power module delivering regulated 5V/7A to PLC I/O and fieldbus interfaces. IC Role / Device Role / Timing Role: Primary-side controller enforcing strict 50% duty cycle and coordinating TG/BG/SG timing to prevent transformer saturation under variable load. Use Value: Delivers stable output across wide input range with built-in OVLO, thermal shutdown, and soft-start for cold-start reliability. |
| Lead-Acid Battery Backup | Automotive Heavy-Duty DC/DC |
Use Scenario: 24V/48V battery-backed UPS converting to 5V for critical control electronics during grid failure. IC Role / Device Role / Timing Role: Synchronous forward controller with programmable soft-start and UVLO hysteresis to manage deep-discharge brownout conditions. Use Value: Ensures clean start-up and graceful shutdown during battery voltage sag, extending hold-up time and preventing data corruption. | Use Scenario: 12–72V automotive auxiliary power unit supplying 5V/7A to infotainment and ADAS subsystems. IC Role / Device Role / Timing Role: High-voltage controller with robust ESD-hardened pins, wide-temp operation (–40°C to 85°C), and IMAX-driven primary current limiting. Use Value: Survives load-dump transients and cold-crank conditions while maintaining tight regulation and fast fault response. |
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 oscillator; lacks IMAX pin and adaptive blanking | Designed for lower-power forward converters; requires external blanking circuitry | Choose UCC2891DR only if fixed 500kHz operation and simpler fault architecture suffice |
| MAX5072AESA+ | Integrated 5V LDO instead of 5VREF; no synchronous rectifier driver; max 300kHz sync range | Targeted at cost-sensitive non-isolated forward designs; no optocoupler support | Choose MAX5072AESA+ when local 5V bias is needed without external regulator and isolation is not required |
Compared with UCC2891DR and MAX5072AESA+, the LT1681ESW#PBF provides superior flexibility in frequency programming, dedicated synchronous rectifier timing, adaptive blanking, and comprehensive fault coverage - making it optimal for high-reliability, isolated, high-input-voltage forward converters where transformer protection and graceful recovery are critical.
Availability
LT1681ESW#PBF is available at Aetrix Electronics and suitable for telecom rectifiers, industrial DC power supplies, lead-acid battery backup systems, and automotive heavy-duty DC/DC converters requiring stable component supply, long-term lifecycle support, and guaranteed parametric performance over –40°C to 85°C.
Supply support for LT1681ESW#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. (acquired Linear Technology in 2017) is a global leader in high-performance analog, mixed-signal, and power management semiconductors.
The LT1681ESW#PBF belongs to Linear's high-voltage DC/DC controller product line, designed specifically for robust, isolated synchronous forward converter topologies in telecom, industrial, and transportation applications demanding high efficiency, fault resilience, and wide input voltage range.
FAQ
What is the maximum operating frequency of the LT1681ESW#PBF?
The LT1681ESW#PBF supports a maximum synchronized operating frequency of 350kHz. Its internal oscillator runs at twice that rate (700kHz) to generate precise 50% duty cycle timing. Free-run frequency is programmable via the FSET pin RC network and can exceed 350kHz, but synchronization is limited to 350kHz per datasheet specifications. The LT1681ESW#PBF maintains stable operation across this range with controlled phase relationships between TG, BG, and SG outputs.
Does the LT1681ESW#PBF integrate a 5V reference, and what is its accuracy?
Yes, the LT1681ESW#PBF integrates a precision 5V reference (5VREF pin) with ±1% initial accuracy (4.85V–5.10V) over temperature and load (0–20mA). It features short-circuit protection limiting output current to ~45mA and includes internal current limiting that disables switching if the 5VREF load exceeds safe limits. This reference directly powers optocouplers, sensing resistors, and auxiliary circuitry in isolated forward converter designs using the LT1681ESW#PBF.
How does the LT1681ESW#PBF prevent transformer saturation in forward converters?
The LT1681ESW#PBF prevents transformer saturation through three coordinated mechanisms: (1) hard-coded 50% maximum duty cycle enforced by internal T-flip-flop logic; (2) IMAX pin monitoring primary-side current with 360mV threshold and 130ns disable response; and (3) soft-start ramping that gradually increases peak current limit. These features operate together to ensure the magnetic reset interval is always maintained - a critical requirement for forward topology stability. All three protections are active in every LT1681ESW#PBF implementation.
What is the purpose of the BLKSENS pin on the LT1681ESW#PBF?
The BLKSENS pin on the LT1681ESW#PBF enables adaptive current sense blanking: it monitors the gate voltage of the bottom-side primary switch and disables the SENSE comparator while BLKSENS voltage remains below 5V. This suppresses false triggering caused by switching noise during FET turn-on transitions - a common issue in high-dV/dt forward converters. Unlike fixed-time blanking, this method scales automatically with gate drive strength and external buffer delays, ensuring reliable operation across diverse MOSFET and gate driver combinations used with the LT1681ESW#PBF.
Can the LT1681ESW#PBF be used in non-isolated forward topologies?
Yes, the LT1681ESW#PBF explicitly supports both isolated and non-isolated forward topologies per its datasheet. In non-isolated configurations, the error amplifier (VC/VFB) connects directly to the output divider, eliminating optocoupler latency and simplifying compensation. The IC's 50% duty cycle enforcement, synchronous rectifier timing (SG), and primary-side current protection (IMAX/SENSE) remain fully functional. Designers must still observe layout best practices for power-ground separation and noise immunity, as specified in the LT1681ESW#PBF application notes.
LT1681ESW#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
LT1681ESW#PBF FAQ
1.How can I place an order for LT1681ESW#PBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LT1681ESW#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 LT1681ESW#PBF reliable?
The price and inventory of LT1681ESW#PBF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LT1681ESW#PBF is usually 5 days.
3.What payment methods are accepted for LT1681ESW#PBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LT1681ESW#PBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LT1681ESW#PBF?
LT1681ESW#PBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LT1681ESW#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 LT1681ESW#PBF?
For technical support, including LT1681ESW#PBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LT1681ESW#PBF requirements.
6.How does Aetrix verify that LT1681ESW#PBF is sourced from the original manufacturer or authorized distributors?
All LT1681ESW#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 LT1681ESW#PBF meets industry standards.
7.What is the process for return or replacement of LT1681ESW#PBF?
All LT1681ESW#PBF units undergo pre-shipment inspection (PSI). If there is an issue with LT1681ESW#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 LT1681ESW#PBF part is unused and in its original packaging.
Return procedure for LT1681ESW#PBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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