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Analog Devices Inc. LT3781EG#TRPBF
- Part No.:
- LT3781EG#TRPBF
- Manufacturer:
- Analog Devices Inc.
- Category:
- DC DC Switching Controllers
- Package:
- 20-SSOP (0.209", 5.30mm Width)
- Datasheet:
-
LT3781EG#TRPBF.pdf
- Description:
- IC REG CTRLR FWRD CONV 20SSOP
- Quantity:
- Payment:
- Shipping:
Inventory:3,130
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Product details
Overview
LT3781EG#TRPBF from Analog Devices (formerly Linear Technology) is a high-voltage synchronous forward controller IC designed for isolated and nonisolated DC/DC converters using dual N-channel MOSFETs. It operates up to 72V input, delivers fixed-frequency current-mode control up to 350kHz, integrates a ±1% 1.25V error amplifier reference, supports programmable soft-start, and features undervoltage lockout with 6V hysteresis for self-biased power. It is used in telecom quarter-brick power supplies delivering 5V/7A.
For engineers reviewing the LT3781EG#TRPBF datasheet, LT3781EG#TRPBF pinout, LT3781EG#TRPBF application, or LT3781EG#TRPBF equivalent, key selection considerations include its 20-lead SSOP package, bootstrap-driven high-side gate driver (TG), synchronous switch output (SG), VCC UVLO threshold (8.4V/14.5V), 150mV current sense threshold, and thermal/fault protection via THERM/OVLO pins.
Technical Context
The LT3781EG#TRPBF employs fixed-frequency current-mode control with internal logic enforcing ≤50% maximum duty cycle to prevent transformer saturation. Its oscillator runs at twice the switching frequency and supports synchronization up to 350kHz via TTL-compatible SYNC pin, enabling phase-aligned multi-controller systems.
It integrates a voltage-output error amplifier referenced to 1.25V, adaptive blanking on SENSE pin tied to BG gate voltage, and independent UVLO monitoring for both VCC (8.4V/14.5V) and VBST–BSTREF (6.4V/7.0V). Fault detection triggers graceful soft-start reset on SHDN, OVLO, or THERM pin overvoltage events.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Input Voltage Range | 36V to 72V - supports wide-range industrial and telecom input rails |
| Max Switching Frequency | 350kHz - enables compact magnetics and high power density designs |
| Error Amp Reference | 1.25V ±1% - provides precision output regulation without external reference |
| Current Sense Threshold | 150mV - sets peak current limit with low-loss, ground-referenced sensing |
| VCC UVLO Threshold | 8.4V (falling), 14.5V (rising) - ensures reliable start-up and brownout recovery |
| Operating Temp Range | –40°C to 85°C - qualified for industrial ambient environments |
| Package | 20-lead SSOP - surface-mount footprint compatible with automated assembly |
Pinout & Package
LT3781EG#TRPBF is housed in a 20-lead plastic SSOP (Shrink Small Outline Package) with exposed pad for thermal enhancement. Pin 1 is located at the top-left corner of the top view, with pins numbered counterclockwise.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| SHDN (1) | Shutdown Control Input | Logic-enable input with 1.25V rising threshold and 150mV hysteresis; disables all switching when pulled low |
| OVLO (2) | Input Overvoltage Sense | Monitors input rail via resistor divider; disables switching if >1.25V to protect downstream circuitry |
| THERM (3) | Thermal Fault Input | Accepts thermistor-based voltage; triggers shutdown if >1.25V for system-level thermal protection |
| SGND (4) | Signal Ground Reference | Low-noise analog ground for feedback and error amp; must be isolated from high-current PWRGND |
| 5VREF (5) | Internal 5V Reference Output | Stable 5V supply capable of sourcing up to 20mA; short-circuit protected to ~45mA |
| FSET (6) | Oscillator Timing Input | RC-programmed node setting free-run frequency; charges/discharges between 1.5V and 2.5V |
| SYNC (7) | External Clock Input | TTL-compatible sync input; locks internal oscillator phase and controls max duty cycle |
| SS (8) | Soft-Start Capacitor Node | Sinks 10µA to ramp VC output; resets to 225mV during faults for controlled recovery |
| VFB (9) | Error Amp Inverting Input | Feedback node referenced to internal 1.25V; open-circuit bias current ~50nA prevents floating |
| VC (10) | Error Amp Output | Inverting gain stage driving current-sense comparator; sources/sinks up to 25mA/1mA |
| SENSE (11) | Current Sense Input | Non-inverting input for ground-referenced shunt; 150mV threshold with adaptive blanking |
| SG (12) | Synchronous Switch Driver | Gate driver for secondary-side synchronous rectifier; leads BG by ~150ns |
| VCC (13) | Main Supply Input | 9V–18V operating range; includes 18V shunt clamp and 8.4V/14.5V UVLO |
| PWRGND (14) | Power Ground Return | High-current return path for BG/SG/TG drivers; connects to VIN decoupling capacitor |
| BG (15) | Bottom-Side Primary Driver | Drives low-side primary FET; lags TG turn-on and leads TG turn-off to concentrate losses |
| BSTREF (18) | Bootstrap Reference | Connects to source of high-side FET; defines VBST reference point for bootstrapped drive |
| TG (19) | Top-Side Primary Driver | High-side gate driver; disabled until VBST–BSTREF > 7.0V for safe start-up |
| VBST (20) | Bootstrap Supply Input | Bootstrapped rail referenced to BSTREF; supports up to 90V absolute max |
Key Features
| Feature | Design Value |
|---|---|
| Synchronous Forward Control | Enables high-efficiency isolated DC/DC conversion using dual N-MOSFETs with precise timing coordination |
| Programmable Soft-Start | 10µA constant-current ramp on SS pin ensures controlled current rise and eliminates output overshoot |
| Multi-Fault Protection | Dedicated SHDN, OVLO, and THERM inputs with independent 1.25V thresholds and soft-start reset recovery |
| Bootstrap-Driven High-Side Gate | TG driver supports up to 90V VBST–BSTREF with UVLO (7.0V) to prevent linear-region operation during start-up |
| Adaptive Current Sense Blanking | Blanking triggered by BG gate voltage prevents false current-limit trips during FET turn-on transitions |
Applications
| Telecom Quarter-Brick Power Supply | Lead-Acid Battery Backup System |
|---|---|
Use Scenario: 36V–72V input telecom rectifier supplying isolated 5V/7A to line cards in central office equipment. IC Role / Device Role / Timing Role: Primary-side synchronous forward controller managing dual N-MOSFET switching, current-mode regulation, and fault-safe sequencing. Use Value: Enables compact quarter-brick form factor with high efficiency (>90%) and robust input transient handling via OVLO and soft-start recovery. | Use Scenario: Off-grid UPS system converting 48V battery input to regulated 5V for critical control electronics during grid failure. IC Role / Device Role / Timing Role: High-voltage DC/DC controller providing isolated step-down with thermal derating and battery undervoltage protection. Use Value: Delivers stable 5V output across wide battery discharge range while preventing deep discharge via VCC UVLO hysteresis. |
| Automotive Heavy Equipment DC/DC | Distributed Power Step-Down Converter |
Use Scenario: 24V/48V vehicle electrical system powering 5V microcontrollers and sensors in construction machinery. IC Role / Device Role / Timing Role: Nonisolated synchronous forward controller supporting wide input transients (load dump, cold crank). Use Value: Survives 100V load-dump spikes via 90V VBST rating and maintains regulation through 6V VCC hysteresis during cranking. | Use Scenario: Industrial rack-mounted power shelf distributing 5V to multiple blade servers from a common 48V backplane. IC Role / Device Role / Timing Role: Synchronized multi-phase forward controller enabling parallel operation with phase interleaving. Use Value: Reduces input/output ripple and EMI via SYNC pin phase alignment across multiple LT3781EG#TRPBF units. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar synchronous forward controller applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| LT3782EG#TRPBF | Includes integrated 5V bias supply for optocoupler; same 20-SSOP package and pinout | Better suited for fully isolated secondary-side feedback architectures requiring local 5V bias | Select LT3782EG#TRPBF when optocoupler bias is needed; LT3781EG#TRPBF saves cost where 5VREF suffices |
| UCC28950PWPR | Phase-shifted full-bridge controller; 16-pin HTSSOP; no integrated 5VREF or VCC shunt regulator | Targets higher-power (>500W) isolated PSFB topologies; requires external bias and protection circuits | Choose UCC28950PWPR only for phase-shifted full-bridge designs; LT3781EG#TRPBF is optimized for forward topology simplicity |
Compared with LT3782EG#TRPBF, LT3781EG#TRPBF lacks optocoupler bias but offers lower BOM cost and identical forward-control functionality; versus UCC28950PWPR, it provides simpler forward-specific integration with built-in references and protection, avoiding external bias design effort.
Availability
LT3781EG#TRPBF is available at Aetrix Electronics and suitable for telecom power supplies, battery backup systems, and automotive DC/DC converters requiring stable component supply, long-term lifecycle support, and traceable sourcing.
Supply support for LT3781EG#TRPBF 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 power management semiconductors, formed through the acquisition of Linear Technology in 2017.
The LT3781EG#TRPBF belongs to ADI's high-voltage power controller product line, designed specifically for rugged, high-efficiency isolated and nonisolated forward converter applications in telecom, industrial, and transportation systems.
FAQ
What is the maximum input voltage supported by the LT3781EG#TRPBF?
The LT3781EG#TRPBF supports an input voltage range of 36V to 72V for normal operation. Its VCC pin has an absolute maximum rating of 20V, and the VBST pin supports up to 90V referenced to BSTREF. The IC includes dedicated UVLO circuitry for both VCC (8.4V/14.5V thresholds) and VBST–BSTREF (6.4V/7.0V thresholds) to ensure safe start-up and operation under varying input conditions. This makes LT3781EG#TRPBF suitable for high-voltage industrial and telecom power systems.
Does the LT3781EG#TRPBF require external gate drivers for MOSFETs?
The LT3781EG#TRPBF integrates high-current gate drivers for TG, BG, and SG outputs-each capable of sourcing/sinking >11V with <35ns rise/fall times-making external drivers optional for small-to-medium FETs (CGATE < 5000pF). However, for high-power designs with large gate capacitance or stringent efficiency targets, a dedicated gate driver like the LTC7001 is recommended to minimize switching losses. The LT3781EG#TRPBF's drivers are optimized for direct connection in quarter-brick telecom forward converters.
How does the soft-start function work on the LT3781EG#TRPBF?
The LT3781EG#TRPBF implements soft-start by sinking a constant 10µA current from the SS pin to charge an external capacitor. As the SS pin voltage ramps, the internal current limit threshold increases proportionally, gradually raising the peak switch current and limiting inrush. The SS pin resets to 225mV during shutdown, UVLO, OVLO, or THERM faults-ensuring zero-output-current recovery. This behavior is confirmed in the datasheet's Electrical Characteristics table (ISS = –10µA typ) and Applications Information section, and applies directly to LT3781EG#TRPBF.
Can the LT3781EG#TRPBF be synchronized with other controllers?
Yes, the LT3781EG#TRPBF supports synchronization via its TTL-compatible SYNC pin (Pin 7), which accepts external clock signals up to 350kHz. The internal oscillator doubles the SYNC frequency and phase-locks the toggle flip-flop to align switching edges. During synchronized operation, the LT3781EG#TRPBF also controls output switch phase-enabling interleaved or anti-phase operation in multi-controller systems. This capability is explicitly documented in the "Synchronizable Operating Frequency and Output Switch Phase" feature list and verified in the SYNC timing specifications.
What protection features are integrated into the LT3781EG#TRPBF?
The LT3781EG#TRPBF integrates five independent protection mechanisms: (1) VCC undervoltage lockout (8.4V/14.5V), (2) VBST–BSTREF UVLO (6.4V/7.0V), (3) input overvoltage shutdown (OVLO pin, 1.25V threshold), (4) thermal shutdown (THERM pin, 1.25V threshold), and (5) 5VREF overcurrent protection (~45mA limit). All fault conditions trigger a General Fault Condition (GFC) that disables switching and discharges the SS capacitor for graceful restart. These protections are detailed in the Features list, Electrical Characteristics table, and Applications Information section of the LT3781EG#TRPBF datasheet.
LT3781EG#TRPBF Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Analog Devices Inc.
- Series:
- -
- Package/Case:
- 20-SSOP (0.209", 5.30mm Width)
- Packaging:
- Tape & Reel (TR)
- 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#TRPBF FAQ
1.How can I place an order for LT3781EG#TRPBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LT3781EG#TRPBF 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#TRPBF reliable?
The price and inventory of LT3781EG#TRPBF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LT3781EG#TRPBF is usually 5 days.
3.What payment methods are accepted for LT3781EG#TRPBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LT3781EG#TRPBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LT3781EG#TRPBF?
LT3781EG#TRPBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LT3781EG#TRPBF 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#TRPBF?
For technical support, including LT3781EG#TRPBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LT3781EG#TRPBF requirements.
6.How does Aetrix verify that LT3781EG#TRPBF is sourced from the original manufacturer or authorized distributors?
All LT3781EG#TRPBF 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#TRPBF meets industry standards.
7.What is the process for return or replacement of LT3781EG#TRPBF?
All LT3781EG#TRPBF units undergo pre-shipment inspection (PSI). If there is an issue with LT3781EG#TRPBF, 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#TRPBF part is unused and in its original packaging.
Return procedure for LT3781EG#TRPBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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