STMicroelectronics LD3985J33R

Overview

LD3985J33R from STMicroelectronics is a 3.3 V, 150 mA ultra-low-dropout linear regulator in SOT23-5L package, featuring 60 mV typical dropout at full load, 30 µV_RMS output noise (10 Hz–100 kHz), and logic-controlled shutdown. It operates from 2.5 V to 6 V input, supports ceramic capacitors as low as 1 µF, and targets power-sensitive battery-operated systems such as mobile phone baseband ICs and wireless sensor nodes.

For engineers reviewing the LD3985J33R datasheet, LD3985J33R pinout, LD3985J33R application, or LD3985J33R equivalent, key selection criteria include its 85 µA no-load quiescent current, 60 dB PSRR at 1 kHz, guaranteed 150 mA output with thermal/current limiting, and compatibility with low-ESR ceramic bypass (10 nF) and output (1 µF) capacitors.

Technical Context

The LD3985J33R employs BiCMOS process technology to achieve ultra-low dropout and sub-100 µA quiescent current across temperature. Its internal reference and error amplifier enable ±2% output voltage accuracy over -40 °C to +125 °C, with line regulation of ±0.1 %/V and load regulation as tight as 0.0004 %/mA for 3.3 V output.

Shutdown is implemented via a dedicated VINH pin requiring ≥1.2 V for ON and ≤0.4 V for OFF mode, with <1 nA input leakage. The BYPASS pin accepts a 10 nF capacitor to reduce high-frequency noise, while SVR remains >50 dB up to 10 kHz-critical for RF and analog subsystems sharing noisy supply rails.

Key Specifications

Pinout & Package

SOT23-5L (5-pin, 2.95 mm × 1.60 mm × 1.30 mm height), RoHS-compliant, ECOPACK2 rated.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar LDO regulator applications.

Compared with XC6206P332MR and AP2204K-3.3TRG1, LD3985J33R delivers superior noise performance (30 µV_RMS vs. ≥55 µV_RMS) and tighter thermal safety (160 °C shutdown with 20 °C hysteresis), making it optimal for RF-critical and thermally unmanaged deployments.

Availability

LD3985J33R is available at Aetrix Electronics and suitable for mobile baseband power, wireless sensor node core supply, and industrial PLC I/O modules requiring stable component supply with full production lifecycle support.

Supply support for LD3985J33R 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

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, designing and manufacturing microcontrollers, power management ICs, sensors, and analog devices for industrial, automotive, and consumer markets.

The LD3985 belongs to ST's ultra-low-noise, low-quiescent-current LDO family, engineered specifically for battery-powered portable electronics where extended runtime and signal integrity are design-critical.

FAQ

What is the minimum output capacitor required for stability?

The LD3985J33R is stable with a minimum 1 µF ceramic output capacitor (X7R or better), as confirmed in Section 5 of the datasheet. Using capacitors below this value risks oscillation; ESR must remain between 5 mΩ and 5 Ω. A 2.2 µF X7R capacitor is commonly used to improve transient response without compromising stability.

Can the BYPASS pin be left unconnected?

No-the BYPASS pin must be connected to a 10 nF ceramic capacitor to ground to achieve the specified 30 µV_RMS noise performance. Leaving it floating degrades high-frequency PSRR and increases output noise by up to 3×, as shown in Figure 18 of the datasheet. No internal pull-up or pull-down exists on this pin.

Does LD3985J33R support reverse polarity protection?

No-LD3985J33R lacks integrated reverse-voltage protection. Applying reverse polarity to VI or VO may damage the device. External protection (e.g., series Schottky diode on input or back-to-back MOSFETs) is required in systems where reverse connection is possible during service or assembly.

How does thermal shutdown behave during continuous overload?

Thermal shutdown activates at 160 °C junction temperature with ~20 °C hysteresis, causing automatic recovery once die temperature falls below ~140 °C. During sustained overload, the device cycles between regulation and shutdown-this protects against permanent damage but is not intended for continuous operation. Peak output current is limited to 550 mA only for short transients.