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Analog Devices Inc. LT3781EG#PBF
- Part No.:
- LT3781EG#PBF
- Manufacturer:
- Analog Devices Inc.
- Category:
- DC DC Switching Controllers
- Package:
- 20-SSOP (0.209", 5.30mm Width)
- Datasheet:
-
LT3781EG#PBF.pdf
- Description:
- IC REG CTRLR FWRD CONV 20SSOP
- Quantity:
- Payment:
- Shipping:
Inventory:4,591
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Product details
Overview
LT3781EG#PBF from Analog Devices (formerly Linear Technology) is a high-voltage synchronous forward controller IC for isolated DC/DC conversion, supporting input voltages up to 72V and operating frequencies up to 350kHz. It drives external N-channel MOSFETs in dual-transistor forward topologies, features ±1% internal 1.25V error amplifier reference, programmable soft-start, and integrated undervoltage lockout with 6V hysteresis for self-biased power. Used in telecom quarter-brick converters delivering 5V/7A.
For engineers reviewing the LT3781EG#PBF datasheet, LT3781EG#PBF pinout, LT3781EG#PBF application, or LT3781EG#PBF equivalent, key selection criteria include bootstrap gate drive capability (VBST up to 90V), synchronous switch phase control for multi-controller synchronization, current-mode regulation with 150mV sense threshold, and fault protection via SHDN/OVLO/THERM pins.
Technical Context
The LT3781EG#PBF employs fixed-frequency current-mode control with internal T-flip-flop logic enforcing ≤50% maximum duty cycle to prevent transformer saturation. Its oscillator runs at twice the switching frequency (e.g., 300kHz FSET → 150kHz system frequency), and SYNC pin accepts TTL-level inputs up to 350kHz to align phase across multiple controllers.
Fault management is centralized through General Fault Condition (GFC) logic: events including VCC UVLO (<8.4V falling), 5VREF overcurrent (>45mA), OVLO/THERM pin voltage >1.25V, or SHDN <0.3V trigger immediate shutdown and soft-start capacitor discharge to 225mV before recovery.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VIN Range | Up to 72V max input - enables direct integration into 48V telecom or 60V industrial bus systems without pre-regulation. |
| fSW Max | 350kHz synchronized operation - supports compact magnetics and high-power density quarter-brick designs. |
| VFB Reference | 1.25V ±1% over –40°C to 85°C - ensures precise output voltage regulation across temperature in isolated feedback loops. |
| ISENSE Threshold | 150mV typical - sets peak current limit with low-loss, ground-referenced sensing using standard shunt resistors. |
| VBST Rating | 90V max (VBST–BSTREF) - supports high-side N-MOSFET gate drive in boost-referenced topologies with robust start-up margin. |
| UVLO Hysteresis | 6V (14.5V rise / 8.4V fall) - prevents oscillation during brown-out and enables reliable trickle-charge startup from auxiliary supplies. |
| 5VREF Output | 5.0V ±2%, 20mA load capacity - powers optocouplers, TL431 references, or thermistor bias networks without external LDO. |
Pinout & Package
LT3781EG#PBF is housed in a 20-lead plastic SSOP package (0.154" wide, 0.025" lead pitch), thermally optimized for high-current gate driver operation with exposed paddle (GND-connected).
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| SHDN (1) | Shutdown enable input | Logic-high >1.25V enables operation; 150mV hysteresis prevents chatter; clamps VCC to 18V during shutdown. |
| OVLO (2) | Input overvoltage fault sense | Resistor-divider monitored; >1.25V disables switching and resets soft-start for graceful recovery from transients. |
| THERM (3) | Thermal fault sense | Accepts thermistor divider; >1.25V triggers GFC shutdown - enables system-level thermal protection without external comparator. |
| SGND (4) | Signal ground reference | Separate from PWRGND; must be routed away from high-di/dt paths to preserve error amp accuracy and stability. |
| 5VREF (5) | Internal precision reference output | Stable 5V supply for optocoupler LEDs or sensor bias; short-circuit protected to ~45mA; loading reduces VC sourcing capacity. |
| FSET (6) | Oscillator timing node | RC network (RFSET + CFSET) programs frequency; internal 2.5V/1.5V hysteresis defines charge/discharge thresholds. |
| SYNC (7) | External clock synchronization input | TTL-compatible; rising/falling edges reset internal oscillator; forces phase alignment and limits max duty cycle to SYNC duty. |
| SS (8) | Soft-start control | 10µA current source charges external capacitor; ramps internal current limit from zero to reduce output overshoot. |
| VFB (9) | Error amplifier inverting input | Compares output voltage (via resistor divider) against 1.25V reference; open-circuit bias current ~50nA pulls high. |
| VC (10) | Error amplifier output | Inverting gain stage with 10mA source/1mA sink capability; interfaces directly with optocoupler anode or transimpedance networks. |
| SENSE (11) | Current sense amplifier input | Monitors ground-referenced shunt; adaptive blanking disables sensing while BG <5V to reject turn-on glitches. |
| SG (12) | Synchronous switch gate driver | Drives bottom-side sync FET; leads BG by 150ns to ensure proper dead-time sequencing and ZVS conditions. |
| VCC (13) | Main IC supply input | 9–18V operational range; 20V absolute max; internal 18V shunt clamp active in shutdown to protect trickle-start circuits. |
| PWRGND (14) | Power ground return | Low-impedance path for BG/SG/TG drivers; must connect directly to VIN decoupling capacitor ground plane. |
| BG (15) | Bottom-side primary switch driver | Drives main low-side FET; lags TG on turn-on and leads on turn-off to concentrate transitional losses on BG FET. |
| BSTREF (18) | Bootstrap supply reference | Connected to source of high-side FET; defines return for VBST; UVLO activates if VBST–BSTREF <7.0V at startup. |
| TG (19) | Top-side primary switch driver | Drives high-side N-MOSFET; requires bootstrap capacitor (0.1–1µF) and Schottky diode from VCC to VBST. |
| VBST (20) | Boosted gate drive supply | 90V max rating referenced to BSTREF; supplies TG driver; charged each cycle via bootstrap diode/capacitor network. |
Key Features
| Feature | Design Value |
|---|---|
| Synchronous dual-transistor forward control | Enables high-efficiency isolated DC/DC conversion with external N-MOSFETs - eliminates need for gate drive transformers or discrete level shifters. |
| Programmable soft-start with fault-triggered reset | 10µA SS current source ramps current limit linearly; automatic discharge to 225mV on GFC events ensures zero-output-current restart after OVLO/THERM/SHDN faults. |
| Integrated 5V ±2% reference with 20mA drive | Powers optocoupler LEDs, TL431 references, or thermistor bias networks directly - eliminates external LDO and reduces BOM count in isolated feedback paths. |
| Adaptive current-sense blanking | Disables SENSE comparator while BG <5V - rejects turn-on spikes regardless of FET size or gate charge, enabling robust operation with diverse MOSFETs. |
| Multi-controller synchronization with phase control | SYNC pin locks oscillator phase and enforces switch timing alignment across parallel converters - essential for interleaved high-current telecom supplies. |
Applications
| Telecom Quarter-Brick Converters | Lead-Acid Battery Backup Systems |
|---|---|
Use Scenario: 36–72V input telecom rectifier feeding 5V/7A distributed power rail in NEBS-compliant equipment. IC Role / Device Role / Timing Role: Primary controller for synchronous forward topology; regulates output via optocoupler-isolated feedback; generates 150kHz switching with precise 50% max duty cycle. Use Value: Enables quarter-brick footprint compliance with high efficiency (>92%) and low EMI due to fixed-frequency current-mode control and synchronized phase alignment. | Use Scenario: Off-grid solar + battery hybrid system where 48V nominal battery bank powers 5V digital control subsystems during grid outage. IC Role / Device Role / Timing Role: High-input-voltage DC/DC controller managing battery-to-load conversion; uses OVLO pin to disconnect during overcharge and THERM for thermal cutoff. Use Value: Built-in 6V UVLO hysteresis prevents brown-out oscillation during deep discharge; 72V absolute max input tolerates battery surge transients up to 80V. |
| Automotive Heavy Equipment Power | Distributed Industrial Step-Down |
Use Scenario: 24V/48V vehicle electrical architecture powering 5V infotainment and ADAS ECUs from alternator or traction battery. IC Role / Device Role / Timing Role: Forward controller driving SiC or trench MOSFETs; SHDN pin tied to MCU-controlled GPIO for sequenced power-up; SS pin manages inrush into large downstream capacitance. Use Value: Robust 125°C junction rating and AEC-Q100-unqualified but automotive-grade design allow deployment in engine bay-adjacent enclosures with minimal derating. | Use Scenario: Factory automation PLC backplane supplying 5V to I/O modules from 48V DC distribution bus. IC Role / Device Role / Timing Role: Centralized forward controller in modular power architecture; SYNC pin daisy-chained to coordinate switching across multiple rails and suppress beat frequencies. Use Value: Phase-aligned 350kHz synchronization reduces conducted EMI peaks and simplifies EMI filter design across multi-rail systems. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar synchronous forward controller applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| UCC28950RTJT | 4-phase interleaved controller with adaptive dead-time control; no integrated 5VREF; requires external VREF for optocoupler bias. | Targeted at higher-power (>1kW) phase-shifted full-bridge designs; lacks single-chip 5V bias for isolated feedback. | Choose UCC28950RTJT when scaling beyond 7A output or requiring multi-phase interleaving; retain LT3781EG#PBF for simpler 5VREF-integrated quarter-brick solutions. |
| LTC3766EFE#PBF | Similar forward controller but with integrated MOSFET drivers only (no external FET support); max VIN = 38V; no OVLO/THERM pins. | Designed for non-isolated forward or active-clamp forward; unsuitable for >48V input or system-level fault monitoring. | Select LTC3766EFE#PBF for cost-sensitive, lower-voltage non-isolated designs; use LT3781EG#PBF where 72V input, isolation, and comprehensive fault detection are required. |
Compared with UCC28950RTJT and LTC3766EFE#PBF, the LT3781EG#PBF uniquely integrates a 5V reference, dedicated OVLO/THERM fault pins, and bootstrap-supply UVLO - making it the only option among the three that supports fully self-contained, isolated, high-input-voltage forward conversion with minimal external components.
Availability
LT3781EG#PBF is available at Aetrix Electronics and suitable for telecom power systems, battery backup architectures, and automotive heavy-equipment power supplies requiring stable component supply, long-term lifecycle support, and guaranteed parametric performance across –40°C to 85°C.
Supply support for LT3781EG#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 semiconductor leader specializing in high-performance analog, mixed-signal, and power management ICs, acquired Linear Technology in 2017 to strengthen its power conversion portfolio.
The LT3781EG#PBF belongs to ADI's high-voltage DC/DC controller product line, engineered specifically for isolated synchronous forward converters in telecom, industrial, and transportation applications demanding reliability under wide input ranges and harsh thermal conditions.
FAQ
What is the maximum input voltage supported by the LT3781EG#PBF?
The LT3781EG#PBF supports input voltages up to 72V on its VIN-related circuitry, with absolute maximum ratings specifying VCC up to 20V and VBST–BSTREF up to 90V. Its undervoltage lockout activates at 8.4V (falling) and 14.5V (rising), ensuring robust operation across 36–72V telecom and industrial bus systems. This makes the LT3781EG#PBF suitable for applications like 48V telecom rectifiers and 60V battery backup systems where transient tolerance is critical.
Does the LT3781EG#PBF include an internal voltage reference?
Yes, the LT3781EG#PBF integrates two precision references: a 1.25V ±1% error amplifier reference (VFB pin) used for output voltage regulation, and a 5.0V ±2% buffered output (5VREF pin) capable of sourcing up to 20mA. The 5VREF pin can power optocoupler LEDs, TL431 references, or thermistor bias networks directly - eliminating the need for an external LDO in isolated feedback designs. Both references are specified over the full –40°C to 85°C operating range.
How does the LT3781EG#PBF handle fault conditions such as overtemperature or overvoltage?
The LT3781EG#PBF implements centralized fault management via its General Fault Condition (GFC) logic. Events including THERM pin voltage >1.25V (thermal fault), OVLO pin voltage >1.25V (input overvoltage), SHDN pin <0.3V (shutdown), or 5VREF overcurrent trigger immediate switching disable and soft-start capacitor discharge to 225mV. Recovery only begins once the fault clears and the SS pin recharges - ensuring zero-output-current restart. This behavior is documented in the LT3781EG#PBF datasheet Section "System Fault Detection".
Can the LT3781EG#PBF synchronize multiple controllers, and what is the maximum SYNC frequency?
Yes, the LT3781EG#PBF supports synchronization of multiple controllers via its TTL-compatible SYNC pin, which accepts input frequencies up to 350kHz. The internal oscillator runs at twice the SYNC frequency, and the SYNC decoder forces phase alignment of both oscillator and switch outputs. This allows precise interleaving or phase-matching across parallel converters - critical for reducing EMI and managing thermal distribution in high-current telecom power systems using the LT3781EG#PBF.
What package type and thermal characteristics does the LT3781EG#PBF have?
The LT3781EG#PBF is packaged in a 20-lead plastic SSOP (0.154" wide) with exposed thermal pad connected to PWRGND. It has a maximum junction temperature of 125°C and thermal resistance θJA = 90°C/W. The exposed paddle improves heat dissipation from high-current gate drivers (TG/BG/SG), supporting continuous operation in ambient temperatures up to 85°C when properly mounted on a 2-oz copper PCB with thermal vias. This package is specified in the LT3781EG#PBF ordering information table.
LT3781EG#PBF Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Analog Devices Inc.
- Series:
- -
- Package/Case:
- 20-SSOP (0.209", 5.30mm Width)
- Packaging:
- Tube
- Product Status:
- Active
- Output Type:
- Transistor Driver
- Function:
- Step-Up/Step-Down
- Output Configuration:
- Positive, Isolation Capable
- Topology:
- Forward Converter
- Number of Outputs:
- 1
- Output Phases:
- 1
- Voltage - Supply (Vcc/Vdd):
- 9V ~ 18V
- Frequency - Switching:
- 700kHz
- Duty Cycle (Max):
- -
- Synchronous Rectifier:
- No
- Clock Sync:
- Yes
- Serial Interfaces:
- -
- Control Features:
- Enable, Soft Start
- Operating Temperature:
- -40°C ~ 85°C (TA)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 20-SSOP
LT3781EG#PBF FAQ
1.How can I place an order for LT3781EG#PBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LT3781EG#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 LT3781EG#PBF reliable?
The price and inventory of LT3781EG#PBF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LT3781EG#PBF is usually 5 days.
3.What payment methods are accepted for LT3781EG#PBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LT3781EG#PBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LT3781EG#PBF?
LT3781EG#PBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LT3781EG#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 LT3781EG#PBF?
For technical support, including LT3781EG#PBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LT3781EG#PBF requirements.
6.How does Aetrix verify that LT3781EG#PBF is sourced from the original manufacturer or authorized distributors?
All LT3781EG#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 LT3781EG#PBF meets industry standards.
7.What is the process for return or replacement of LT3781EG#PBF?
All LT3781EG#PBF units undergo pre-shipment inspection (PSI). If there is an issue with LT3781EG#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 LT3781EG#PBF part is unused and in its original packaging.
Return procedure for LT3781EG#PBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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