Analog Devices Inc. LT3684EDD#PBF

Overview

LT3684EDD#PBF from Analog Devices (formerly Linear Technology) is a monolithic 300 kHz–2.8 MHz current-mode step-down switching regulator with integrated 0.18 Ω bipolar NPN switch and boost Schottky diode, delivering up to 2 A output current across 3.6 V–34 V input range. It features power good flag, soft-start via RUN/SS pin, and 1.265 V feedback reference-designed for automotive battery regulation and industrial distributed supplies.

For engineers reviewing the LT3684EDD#PBF datasheet, LT3684EDD#PBF pinout, LT3684EDD#PBF application, or LT3684EDD#PBF equivalent, key selection considerations include adjustable frequency scaling for inductor size tradeoffs, BIAS pin efficiency optimization, thermal performance of the 3 mm × 3 mm DFN package with exposed pad, and startup behavior under wide VIN transients up to 36 V.

Technical Context

The LT3684EDD#PBF employs constant-frequency current-mode control with an internal oscillator set by an external RT resistor (300 kHz–2.8 MHz), enabling fast transient response and stable loop dynamics without ESR-dependent compensation. Its bipolar NPN switch architecture requires external BOOST capacitor and BD diode to generate gate drive above VIN for full saturation.

Operation includes frequency foldback during low FB voltage (e.g., startup or overload), active VC clamp for current limiting, and dual-supply biasing: internal regulator draws from VIN unless BIAS pin is connected to ≥3 V (e.g., VOUT), improving efficiency at high VIN/VOUT ratios. Power good (PG) asserts when FB reaches 90% of 1.265 V, valid only when VIN > 3.6 V and RUN/SS is high.

Key Specifications

Pinout & Package

LT3684EDD#PBF is housed in a thermally enhanced 10-lead (3 mm × 3 mm) plastic DFN package with exposed pad (Pin 11), which must be soldered to PCB ground for thermal and electrical integrity. The package supports high-power density layouts in space-constrained applications such as automotive ECUs and portable industrial sensors.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar step-down regulator applications.

Compared with LT8609SIV#PBF and TPS54202DDCR, the LT3684EDD#PBF uniquely combines bipolar switch + integrated boost diode for lowest BOM count in 36 V-class applications, BIAS pin for efficiency optimization at high input voltages, and 1.265 V reference for precision output setting - making it optimal for cost-sensitive, thermally constrained industrial and automotive buck stages.

Availability

LT3684EDD#PBF is available at Aetrix Electronics and suitable for automotive battery regulation, industrial distributed supply regulation, and portable product power applications requiring stable component supply across extended temperature and long production lifecycles.

Supply support for LT3684EDD#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 (acquired Linear Technology in 2017) is a global leader in high-performance analog, mixed-signal, and power management ICs, serving aerospace, industrial, automotive, and communications markets with reliability-focused design and manufacturing.

The LT3684EDD#PBF belongs to Linear's legacy high-voltage monolithic buck regulator family, engineered specifically for robust operation in harsh environments - emphasizing wide input range, thermal resilience in compact packages, and integrated features to minimize external components in safety-critical and space-constrained systems.

FAQ

What is the maximum input voltage rating for LT3684EDD#PBF during normal regulation versus transient conditions?

The LT3684EDD#PBF has an absolute maximum VIN rating of 36 V. During normal regulation with no startup or fault events, input transients up to 36 V are acceptable regardless of switching frequency. However, during startup or short-circuit conditions, the maximum safe operating VIN is reduced to 34 V - and further limited by the equation VIN(MAX) = (VOUT + VD)/(1 − fSW·tON(MIN)) − VD + VSW, where tON(MIN) ≈ 150 ns. This ensures the internal switch remains within timing constraints.

How does the BIAS pin improve efficiency in LT3684EDD#PBF, and what voltage must be applied?

The BIAS pin on the LT3684EDD#PBF allows the internal regulator to draw bias current from an external ≥3 V source (typically VOUT) instead of VIN, reducing power dissipation in the control circuitry. When BIAS = VOUT = 5 V and VIN = 24 V, this architecture lowers total quiescent loss by ~0.8 mA, increasing full-load efficiency by approximately 2–3%. The minimum valid BIAS voltage is 2.7 V, with guaranteed operation at 3.0 V.

Can LT3684EDD#PBF operate in discontinuous conduction mode (DCM), and how does it affect output current capability?

Yes, the LT3684EDD#PBF can operate in discontinuous conduction mode (DCM) at light loads or with high-frequency/low-inductance configurations. In DCM, peak inductor current drops below twice the average load current, reducing RMS losses but also lowering maximum deliverable output current due to increased boundary between CCM and DCM. The datasheet graphs (e.g., Figure G04–G05) show IOUT(MAX) degrades by ~15–25% in DCM versus CCM at same VIN/VOUT, especially below 0.5 A load.

What is the purpose of the BD pin on LT3684EDD#PBF, and how is it connected externally?

The BD (Boost Diode) pin on the LT3684EDD#PBF connects to the anode of an external Schottky diode whose cathode ties to the BOOST pin. This forms a charge pump that generates a voltage ~1.6–2.1 V above SW during switch off-time, enabling full saturation of the internal bipolar NPN switch. A typical implementation uses a 1 A, 40 V Schottky (e.g., B130 or MBRM140) with minimal trace length to preserve switching speed and minimize ringing.

Does LT3684EDD#PBF require an external bootstrap capacitor, and where is it placed?

Yes, the LT3684EDD#PBF requires an external bootstrap capacitor connected between the BOOST and SW pins. This capacitor (typically 10–22 nF X7R ceramic) stores charge during switch off-time and supplies the elevated voltage needed to drive the internal NPN switch base above VIN during turn-on. It must be placed as close as possible to BOOST and SW pins with short, low-inductance traces to ensure reliable switch saturation and minimize EMI.