LT3753IFE - 100V Active Clamp Forward Controller | Analog Devices
MPN: LT3753IFE ✓ Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $12.97 | $12.97 |
| 10 | $11.65 | $116.50 |
| 100 | $9.95 | $995.00 |
| 500 | $8.65 | $4,325.00 |
| 1,000 | $4.84 | $4,840.00 |
LT3753IFE Overview
An active clamp forward controller is a current-mode PWM IC purpose-built for isolated forward converter topologies used in industrial, telecom, and distributed-bus power designs. Active clamp resets the power transformer using an auxiliary MOSFET and capacitor instead of a dissipative snubber or simple reset winding, which boosts efficiency, enables duty cycles above 50%, and lowers primary switch voltage stress. Forward controllers sit within the broader hierarchy: PWM controller -> DC-DC switching controller -> power management IC -> semiconductor.
Key features include programmable volt-second clamp, current-mode control with slope compensation, programmable soft-start, an internal 5V LDO for housekeeping, and an internal 100V start-up bias regulator that eliminates external bootstrap components. The synchronous rectifier (SR) timing output simplifies secondary-side synchronous MOSFET control. The 38-TSSOP-EP package exposes a thermal die pad (theta_JA around 30 C/W with proper PCB copper), enabling high-power isolated designs without external heatsinking.
Architecture depth: the LT3753 uses peak current-mode control with programmable slope compensation to support both voltage- and current-mode transformers without subharmonic instability. The volt-second clamp circuitry monitors the OUT/transformer reset waveform and terminates the primary switch ON-time when the programmed volt-second product is reached, eliminating duty-cycle dependence on input voltage variations. The internal 5V LDO powers the IC's logic and SR gate drive from the high-voltage INTVCC rail.
Typical applications include 24V to 48V isolated telecom bus converters, industrial 24V/48V distributed bus intermediate bus converters (IBCs), isolated bias supplies for gate drivers and analog front-ends, and high-density 48V-input server/ATE point-of-load front ends. Synchronous rectification enables efficiency peaks of 92-95% in well-tuned designs.
Design consideration: the active clamp capacitor value is set by the desired transformer reset time and volt-second clamp setting; too-small capacitance causes incomplete reset and transformer saturation, while too-large capacitance slows regulation response. Use the manufacturer datasheet to size the clamp capacitor, primary inductance, and slope compensation resistor for the chosen transformer turns ratio.
Drop-in alternatives for LT3753IFE — same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
LT3753IFE#PBF
✅ Drop-In📋 Reference alternative (not in catalog)
LT3753IFE#TRPBF
✅ Drop-In📋 Reference alternative (not in catalog)
LT3753MPFE#PBF
✅ Drop-In📋 Reference alternative (not in catalog)
LT3753EFE#PBF
✅ Drop-In ⚠️ Specs Unverified📋 Reference alternative (not in catalog)
LT3753AIFE#PBF
✅ Drop-In ⚠️ Specs Unverified📋 Reference alternative (not in catalog)
LT3753AEFE#PBF
✅ Drop-In ⚠️ Specs Unverified📋 Reference alternative (not in catalog)
LT3753IFE Maximum Ratings & Electrical Characteristics
| Topology | Active Clamp Forward Converter Controller |
| Input Voltage Range | 8.5 V to 100 V |
| Switching Controller Type | Current-Mode PWM |
| Maximum Duty Cycle | >50% (programmable via volt-second clamp) |
| Synchronous Rectifier Output | Yes (secondary-side SR timing) |
| Internal LDO | 5 V (INTVCC) |
| Internal Start-up Regulator | Yes (100 V rated) |
| Soft-Start | Programmable |
| Slope Compensation | Programmable |
| Output Type | Transistor Driver (single output, isolation-capable) |
| Isolation | Capable (forward converter topology) |
| Operating Temperature Range | -40C to +125C |
| Package | 38-TSSOP-EP (Thermally Enhanced) |
| Mounting Type | Surface Mount |
| MSL Level | 1 (unlimited at 30C/85% RH) |
| Lead-Free / RoHS | Yes (PBF suffix indicates lead-free, RoHS compliant) |
LT3753IFE Pin Configuration
| Pin 1 | SENSE — Current sense input (peak current-mode feedback) |
| Pin 2 | SLOPE — Slope compensation programming |
| Pin 3 | COMP — Error amplifier compensation output |
| Pin 4 | VREF — Reference voltage output |
| Pin 5 | FB — Feedback input from optocoupler/isolated error amplifier |
| Pin 6 | SS — Soft-start programming |
| Pin 7 | RT — Timing resistor for oscillator frequency |
| Pin 8 | SYNC — Synchronization input |
| Pin 9 | GND — Ground (signal) |
| Pin 10 | PGND — Power ground (gate driver return) |
| Pin 11 | INTVCC — Internal 5V LDO output |
| Pin 12 | DRVCC — Gate driver supply |
| Pin 13 | BG — Bottom gate (primary switch gate drive) |
| Pin 14 | BST — Boost (bootstrap for top gate drive) |
| Pin 15 | TG — Top gate (active clamp MOSFET gate drive) |
| Pin 16 | SW — Switch node (primary MOSFET drain sense) |
| Pin 17 | VIN — Input voltage supply (8.5V to 100V) |
| Pin 18 | RUN/UVLO — Run control / undervoltage lockout |
| Pin 19 | VSC — Volt-second clamp programming |
| Pin 20 | SR — Synchronous rectifier timing output |
| Pin 21 | SRT — Synchronous rectifier timing programming |
| Pin 22 | NC — Not connected (per datasheet) |
| Pin 23 | NC — Not connected (per datasheet) |
| Pin 24 | NC — Not connected (per datasheet) |
| Pin 25 | NC — Not connected (per datasheet) |
| Pin 26 | NC — Not connected (per datasheet) |
| Pin 27 | NC — Not connected (per datasheet) |
| Pin 28 | NC — Not connected (per datasheet) |
| Pin 29 | NC — Not connected (per datasheet) |
| Pin 30 | NC — Not connected (per datasheet) |
| Pin 31 | NC — Not connected (per datasheet) |
| Pin 32 | NC — Not connected (per datasheet) |
| Pin 33 | NC — Not connected (per datasheet) |
| Pin 34 | NC — Not connected (per datasheet) |
| Pin 35 | NC — Not connected (per datasheet) |
| Pin 36 | NC — Not connected (per datasheet) |
| Pin 37 | NC — Not connected (per datasheet) |
| Pin 38 | NC — Not connected (per datasheet) |
Typical Applications
LT3753IFE is suitable for 6 applications: 48V Telecom Isolated Bus Converter, Industrial 24V Distributed Bus Power Supply, Isolated Gate Driver Bias Supply, ATE and Test Equipment Point-of-Load, Battery Energy Storage DC-DC Conversion, LED Display and Sign Backlight Driver.
48V Telecom Isolated Bus Converter
The LT3753IFE is designed into 48V telecom rectifier front-ends and intermediate bus converters (IBCs) where isolated step-down from 36V-72V to 5V-12V rails is required. The 8.5V-100V input range directly accepts 48V telecom battery rails, while the >50% programmable duty cycle (via volt-second clamp) maximizes transformer utilization, shrinking the magnetic footprint and reducing primary switch conduction losses. Its synchronous rectifier timing output drives secondary-side MOSFETs for efficiency peaks of 92-95%. The internal 100V start-up regulator eliminates the auxiliary bootstrap winding at power-up, simplifying PCB layout and reducing BOM count. Designers typically pair it with a planar power transformer and an isolated gate driver IC on the secondary side, achieving 100W-300W isolated outputs in standard quarter-brick or sixteenth-brick footprints.
Recommended
Industrial 24V Distributed Bus Power Supply
In industrial automation systems running on 24V nominal bus (19V-32V actual), the LT3753IFE provides isolated step-down to 5V, 12V, or 24V secondary rails for PLC backplanes, motor drive control boards, and sensor isolation barriers. Its 8.5V-100V VIN tolerates 24V industrial transients and load-dump events that exceed nominal 24V by 30-50%. The current-mode control with programmable slope compensation supports both voltage-mode and current-mode transformer designs without subharmonic instability. Active clamp reset recovers transformer leakage energy, boosting light-load efficiency versus dissipative RCD snubbers. The programmable soft-start manages inrush into large bulk capacitors on downstream rails, and the 5V INTVCC LDO powers housekeeping logic from the high-voltage rail, reducing auxiliary supply complexity. Real-world reference designs deliver 50W-150W isolated rails at 90%+ efficiency.
Recommended
Isolated Gate Driver Bias Supply
High-voltage IGBT and SiC MOSFET gate drivers in motor drives, solar inverters, and traction inverters require isolated bias supplies at 15V-25V referenced to the switching node. The LT3753IFE generates these isolated rails from a 24V or 48V DC bus with reinforced isolation provided by the power transformer. Its active clamp topology keeps peak reverse voltage on the primary MOSFET well below the 100V rating, allowing the use of standard 150V MOSFETs with generous derating for long-term reliability. The programmable volt-second clamp prevents transformer saturation across the full input voltage range. The synchronous rectifier timing output directly drives secondary-side synchronous MOSFETs, eliminating Schottky diode losses on the 15V-25V secondary rail. Engineers typically pair the LT3753 with gate driver ICs such as the ADuM4135 or ADuM3223 for complete high-side/low-side drive solutions.
Recommended
ATE and Test Equipment Point-of-Load
Automated test equipment (ATE) and bench instruments require clean, isolated rails for analog signal conditioning and digital control sections. The LT3753IFE delivers isolated 5V/12V/24V rails from a 48V or 24V bench supply bus with low output ripple and high line-noise rejection. Current-mode control provides tight load regulation, while active clamp reset eliminates the ringing and voltage spikes associated with conventional forward converters, reducing conducted and radiated EMI. The synchronous rectifier output enables >90% efficiency at moderate loads, reducing thermal stress in dense instrument enclosures. Programmable slope compensation allows optimization for the specific transformer turns ratio used in the design, supporting both small-signal and power-grade isolation transformers. The 38-TSSOP-EP package with exposed pad enables compact PCB layouts that fit standard instrument card cages.
Recommended
Battery Energy Storage DC-DC Conversion
Battery energy storage systems (BESS) and industrial battery chargers use the LT3753IFE to convert 24V, 36V, or 48V battery stacks to isolated 12V or 24V auxiliary rails for BMS controllers, contactors, and cooling fans. The wide 8.5V-100V input accommodates depleted-to-charged battery voltage swings, while the active clamp topology handles the dynamic load steps from contactor switching and fan inrush. The internal 100V start-up regulator operates directly from the high-voltage battery rail, simplifying cold-crank and surge scenarios common in mobile and stationary battery systems. Programmable soft-start manages inrush into large downstream bulk capacitors, preventing battery overcurrent trips at system power-up. Efficiency peaks of 92-95% via synchronous rectification reduce thermal dissipation in sealed battery enclosures.
Recommended
LED Display and Sign Backlight Driver
Large-format LED displays, traffic signs, and architectural lighting strips require isolated constant-current drivers fed from 24V or 48V power distribution. The LT3753IFE can be configured with secondary-side constant-current control to drive high-brightness LED strings with galvanic isolation, satisfying safety regulations for outdoor and commercial LED installations. Active clamp topology minimizes transformer stress from the high di/dt switching required for PWM dimming at 1kHz-10kHz rates. The synchronous rectifier timing output on the secondary side enables high-efficiency LED current regulation without lossy linear regulators. The 8.5V-100V input range accepts both 24V architectural bus and 48V telecom-grade power. Programmable soft-start and slope compensation allow tuning for the specific transformer and LED string characteristics in each design.
Recommended
Recommended Products Summary
Engineering reference data for LT3753IFE — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | LT3753IFE#PBF | LT3753IFE#TRPBF | LT3753MPFE#PBF | LT3753AIFE#PBF |
|---|---|---|---|---|---|
| Package | 38-TSSOP-EP | 38-TSSOP-EP - same | 38-TSSOP-EP - same | 38-TSSOP-EP - same | 38-TSSOP-EP - same |
| Brand | Analog Devices | Analog Devices - same | Analog Devices - same | Analog Devices - same | Analog Devices - same |
| Input Voltage Range | 8.5 V to 100 V | 8.5 V to 100 V | 8.5 V to 100 V | 8.5 V to 100 V | 8.5 V to 100 V |
| Maximum Duty Cycle | >50% (V-sec clamp) | >50% (V-sec clamp) | >50% (V-sec clamp) | >50% (V-sec clamp) | |
| Synchronous Rectifier Output | Yes | Yes | Yes | Yes | |
| Operating Temperature Range | -40C to +125C (industrial) | -40C to +125C | -55C to +125C (military) | -40C to +125C | |
| Lead-Free / RoHS | No (standard, needs #PBF suffix for RoHS) | Yes (RoHS compliant) | Yes (RoHS compliant) | Yes (RoHS compliant) | |
| Light-Load Efficiency | Standard | Standard | Standard | Improved (next-gen) | |
| 1ku List Price (USD) | 4.84 | 4.84 (Analog Devices 1ku list) | Higher (military grade) | Comparable (next-gen) |
Key Differentiators
- Wide 8.5V-100V input range with integrated 100V start-up regulator (vs LT3748 (lower-power 100V forward controller))
- Programmable volt-second clamp enables >50% duty cycle (vs Conventional forward controllers (LT3748, basic UC3842 family))
- Synchronous rectifier timing output on the secondary side (vs LT3748 (no SR timing output))
- Pin-compatible upgrade path to LT3753A family (vs LT3753A second-generation variants)
Design Notes
The 38-TSSOP-EP package requires soldering the exposed thermal pad to a copper pour on the top PCB layer, with multiple thermal vias connecting the top pour to inner and bottom copper planes. The thermal pad pad must be at least 1 square inch of copper (preferably 2 sq-in on a 4-layer board) to keep theta_JA below 30 C/W. Place the input bypass capacitor (1uF-10uF X7R 100V-rated) directly between the VIN pin and PGND, within 5mm of the IC. Keep the high-di/dt switching node traces (SW, BST, TG) short and away from sensitive analog feedback traces (FB, COMP, SENSE) to minimize noise coupling.
Estimated: the LT3753 itself dissipates approximately 0.3W-0.6W at typical 100kHz-500kHz switching frequencies (gate charge + quiescent current). With the 38-TSSOP-EP theta_JA of ~30 C/W on 2 sq-in of 4-layer copper, junction temperature rise above ambient is roughly 9-18 C. At maximum 125C ambient industrial operation, the IC's junction temperature remains within the -40-125C operating range with proper PCB layout. Active clamp MOSFET and synchronous rectifier MOSFETs dissipate the bulk of the power; size heatsinks or copper pours accordingly.
Critical pitfalls: (1) Sizing the active clamp capacitor too small causes incomplete transformer reset and saturation - use the datasheet's volt-second clamp formula to calculate C_clamp = I_primary * t_reset / V_clamp. (2) Forgetting slope compensation on duty cycles above 50% causes subharmonic oscillation - use the SLOPE pin resistor per the datasheet guidance. (3) The synchronous rectifier timing output is a control signal, not a gate driver - it must drive the secondary-side SR controller IC, not the synchronous MOSFET gate directly. (4) The INTVCC 5V LDO cannot supply heavy external loads - it is sized for IC housekeeping only.
Separate analog ground (GND) and power ground (PGND) planes, joining them only at the IC's GND/PGND pins. Keep the SENSE resistor Kelvin-connected to the SENSE pin with minimal trace length to avoid false current-mode trips. The transformer primary should be designed with low leakage (Leakage < 1% of primary inductance) for efficient active clamp reset. Use a gate-drive RC snubber (1-10 ohm + 100pF-1nF) on each primary MOSFET gate to dampen parasitic ringing.
Compliance Information
Standard LT3753IFE requires the #PBF suffix for lead-free RoHS compliance (e.g., LT3753IFE#PBF). The base LT3753IFE datasheet does not explicitly state RoHS, REACH, halogen-free, or conflict minerals status - these must be verified per suffix variant on the Analog Devices compliance documentation. AEC-Q100 is not applicable - this is not an automotive-qualified part.
