LT8643SIV#PBF is a high-efficiency, 42 V input, synchronous step-down (buck) DC/DC regulator designed and manufactured by Analog Devices Inc. (following its acquisition of Linear Technology). It belongs to the ultra-low-noise, high-performance LT864x family, engineered for demanding applications requiring exceptional EMI performance, wide input range, high reliability, and compact solution size — especially in automotive, industrial, medical, and communications systems where clean power and small footprint are critical.
The “SIV” suffix denotes the 24-lead 4 mm × 4 mm LQFN package with exposed thermal pad, optimized for superior thermal dissipation and minimal EMI radiation; the “#PBF” indicates Pb-free, RoHS-compliant, and tape-and-reel packaging (typically 2,500 units per reel), suitable for automated SMT assembly.
Introduction
The LT8643SIV#PBF integrates a 42 V, 3.5 A peak-switching-current power MOSFET pair, precision reference, current-mode PWM controller, internal compensation, and integrated bootstrap diode into a single, thermally enhanced LQFN package. It operates from an ultra-wide input voltage range of 3.4 V to 42 V, making it ideal for:
- Automotive systems (cold-crank down to 3.4 V, load-dump up to 42 V);
- Industrial 24 V/36 V rails;
- Telecom intermediate buses;
- Battery-powered equipment (e.g., 12 V LiFePOβ, 3S–10S Li-ion packs).
Its standout feature is Silent Switcher® 2 architecture, which combines:
πΉ Integrated input capacitors (within the package) to minimize high-frequency loop area;
πΉ Symmetrical, balanced internal power stages to cancel magnetic fields;
πΉ Enhanced pinout and internal layout for optimal EMI suppression — enabling CISPR-22 Class B compliance without a metal shield or ferrite beads, even at full load.
The device delivers up to 95% peak efficiency, supports adjustable output voltages from 0.97 V to 40 V, and features ultra-low quiescent current of just 2.5 µA (in Burst Mode®) — enabling always-on operation in energy-sensitive applications (e.g., telematics, remote sensors, battery backup systems). It also includes robust protection: thermal shutdown, current limiting, and undervoltage lockout (UVLO) with precise, adjustable thresholds.
The SIV package (24-lead 4 mm × 4 mm LQFN) features a large exposed thermal pad (EP) that — when soldered to a multilayer PCB ground plane — achieves θJA ≈ 25°C/W, supporting continuous 3 A output at ambient temperatures up to +85°C without external heatsinking.
Key Features
β
Ultra-Wide Input Range & High Reliability:
• VIN = 3.4 V to 42 V — survives automotive cold-crank (down to 3.4 V) and load-dump transients (up to 42 V);
• Qualified per AEC-Q100 Grade 1 (–40°C to +125°C junction) — suitable for under-hood automotive use.
β
Silent Switcher® 2 EMI Performance:
• Meets CISPR-22/25 Class B limits with > 20 dB margin — no shielding, snubbers, or complex layout tricks required;
• Integrated input capacitors + symmetrical layout reduce peak EMI by up to 30 dBµV vs. conventional buck regulators.
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High Efficiency & Ultra-Low IQ:
• Peak efficiency > 95% (e.g., 12 V → 3.3 V @ 2 A);
• Quiescent current = 2.5 µA (Burst Mode®) — enables >1 year battery life in 10 mA standby applications (e.g., GPS trackers on coin cells);
• Supports forced continuous conduction mode (FCCM) for low-noise, constant-frequency operation.
β
Precision Regulation & Flexibility:
• Output voltage adjustable from 0.97 V to 40 V, or fixed options (e.g., 3.3 V, 5 V) via feedback resistor divider;
• ±1% output voltage accuracy over line, load, and temperature (–40°C to +125°C);
• Programmable switching frequency: 200 kHz to 2.2 MHz, synchronized to external clock.
β
Robust Protection & Integration:
• Internal boot diode — eliminates external Schottky;
• Power-good (PG) open-drain output with accurate threshold (±1.5%);
• Adjustable soft-start (via external capacitor) and precision enable (EN) input (2.5 V ±150 mV);
• Overcurrent protection (OCP) with hiccup-mode recovery.
β
Thermally Optimized LQFN-24 (SIV):
• 4 mm × 4 mm footprint with 2.5 mm² exposed copper pad;
• Supports full 3 A continuous output at +85°C ambient — verified with ≥ 200 mm² EP copper area and ≥ 16 thermal vias.
Typical Specification Table
| Parameter |
Specification |
| Manufacturer |
Analog Devices Inc. (formerly Linear Technology) |
| Product Series |
LT864x Family (Silent Switcher® 2 Buck Regulators) |
| Model |
LT8643SIV#PBF |
| Function |
Monolithic Synchronous Step-Down (Buck) Regulator |
| Input Voltage Range |
3.4 V to 42 V |
| Output Voltage Range |
0.97 V to 40 V (adjustable) |
| Max Continuous Output Current |
3 A (with proper thermal design) |
| Peak Switch Current |
3.5 A |
| Switching Frequency |
200 kHz to 2.2 MHz (externally programmable/sync) |
| Efficiency (typ.) |
Up to 95% (e.g., 12 V → 3.3 V @ 2 A) |
| Quiescent Current (Burst Mode®) |
2.5 µA (typ.) |
| Reference Accuracy |
±1% (over line, load, temp: –40°C to +125°C) |
| Power-Good (PG) |
Open-drain, ±1.5% threshold accuracy |
| Enable Threshold (EN) |
2.5 V ±150 mV |
| Operating Junction Temp. |
–40°C to +125°C (AEC-Q100 Grade 1) |
| Package |
24-Lead LQFN (4 mm × 4 mm) with Exposed Pad (SIV) |
| RoHS / Green |
Yes (Pb-free, Halogen-free) |
| Packaging |
Tape-and-Reel (PBF), typically 2,500 units/reel |
Typical Applications
πΉ Automotive Electronics: ADAS cameras, infotainment head units, body control modules, and telematics gateways — leveraging 42 V rating, Silent Switcher®2 EMI immunity, and AEC-Q100 qualification for harsh environments.
πΉ Industrial Sensors & PLC I/O: Powering 3.3 V/5 V microcontrollers, ADCs, and RS-485 transceivers from noisy 24 V factory buses — enabled by wide VIN, high PSRR, and robust UVLO.
πΉ Medical Devices: Portable ultrasound, patient monitors, and infusion pumps — benefiting from low EMI (no interference with sensitive analog front-ends), high reliability, and quiet operation (no audible coil whine).
πΉ Battery-Powered Test Equipment: Handheld multimeters, data loggers, and field calibrators — where 2.5 µA IQ extends battery life and wide VIN supports multiple chemistries (Li-ion, LiFePOβ, NiMH).
πΉ Distributed Power Architectures: Intermediate bus conversion (e.g., 24 V → 5 V/3.3 V) in telecom shelves, base stations, and edge servers — simplified by integrated switch, bootstrap diode, and EMI robustness.
πΉ LED Drivers & Bias Supplies: Driving logic-level MOSFET gates or powering op-amp rails in precision analog front-ends — using adjustable VOUT and tight regulation.
Development & Design Notes
π§ Thermal Management & Layout:
- Solder the exposed pad (EP) to a ≥ 200 mm² internal ground plane using ≥ 16 thermal vias (0.3 mm diameter) — essential for maintaining TJ < 125°C at full 3 A load and +85°C ambient.
- Keep power loops (VIN → SW → L1 → OUT → GND) extremely short and wide; use solid copper pours for VIN, SW, and PGND — minimizes EMI and switching losses.
- Place input capacitors directly at the IC’s VIN and GND pins, using X5R/X7R ceramics (e.g., 10 µF × 2, 0805) — critical for Silent Switcher®2 performance.
π§ Inductor & Capacitor Selection:
- Choose shielded power inductors rated for ≥ 4 A saturation current (e.g., Coilcraft XAL7030, Würth WE-PD). Avoid unshielded types — they radiate strongly despite Silent Switcher®2.
- Use low-ESR ceramic output capacitors (e.g., 22 µF × 2 in parallel, X5R/X7R, 6.3 V rating). Place them < 3 mm from SW and GND pins — critical for stability and transient response.
π§ EMI Optimization (Silent Switcher®2 Best Practices):
- Do not add external input LC filters unless absolutely necessary — the integrated caps already optimize high-frequency noise. If used, keep filter inductor close to VIN and add damping resistor (e.g., 1 Ω) in series.
- Use a 4-layer PCB with dedicated ground and power planes — avoid splitting the ground plane under the LT8643.
- For maximum EMI margin: set fSW = 600 kHz (reduces harmonics) and enable spread-spectrum modulation (SSFM) via SYNC pin.
π§ Enable & Sequencing Design:
- Use the precise 2.5 V EN threshold to sequence with other rails (e.g., tie EN to a 2.5 V supervisor or another regulator’s PG pin). Add a 10 nF capacitor from EN to GND for noise immunity.
- Configure PG to drive a microcontroller GPIO or reset generator — enables safe power-up/down sequencing and brown-out detection.
π§ Reliability & Safety:
- For functional safety (IEC 61508), monitor PG and thermal warning (via junction temp estimation using RθJA and power dissipation) — ADI provides FIT rate data (12) and reliability reports.
- Add a 27 V TVS diode (e.g., SMAJ27A) on VIN for automotive load-dump protection — though LT8643 withstands 42 V continuously, it’s not rated for 60 V/100 ms pulses.
