STMicroelectronics LD3985M122R

Overview

LD3985M122R from STMicroelectronics is a 1.22 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 low-ESR ceramic capacitors, and targets power-sensitive battery-operated systems such as mobile phone baseband ICs and RF front-end bias rails.

For engineers reviewing the LD3985M122R datasheet, LD3985M122R pinout, LD3985M122R application, or LD3985M122R equivalent, key selection criteria include dropout voltage under light/heavy load, shutdown quiescent current (<1.5 µA), PSRR performance (60 dB @ 1 kHz), thermal shutdown threshold (160 °C), and stability with 1 µF output capacitance.

Technical Context

The LD3985M122R uses a BiCMOS architecture to achieve ultra-low quiescent current and minimal dropout across its 1 mA–150 mA load range. Its internal bypass pin (BYPASS) accepts a 10 nF capacitor to suppress high-frequency noise, while the inhibit input (VINH) provides clean ON/OFF control with defined logic thresholds (≤0.4 V for OFF, ≥1.2 V for ON).

It delivers stable regulation over –40 °C to +125 °C junction temperature, maintains >60 dB supply rejection up to 1 kHz, and sustains low-noise operation even at input voltages as low as 2.5 V - critical for Li-ion and multi-cell alkaline battery discharge profiles.

Key Specifications

Pinout & Package

LD3985M122R is housed in a RoHS-compliant SOT23-5L package (1.45 mm × 1.30 mm × 0.95 mm height), optimized for space-constrained portable designs and compatible with standard reflow processes.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar low-noise, low-I_Q LDO applications.

Compared with XC6206P122MR and MIC5205-1.2YS, LD3985M122R uniquely combines ultra-low dropout (60 mV), sub-100 µA quiescent current, integrated BYPASS noise suppression, and logic-controlled shutdown - making it optimal for battery-critical, noise-sensitive, and power-sequenced systems.

Availability

LD3985M122R is available at Aetrix Electronics and suitable for mobile handset power management, wireless sensor node design, and automotive infotainment subsystems requiring stable component supply, long-lifecycle support, and AEC-Q100-aligned thermal robustness.

Supply support for LD3985M122R 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 silicon solutions for automotive, industrial, and consumer markets since 1987.

The LD3985 series belongs to ST's ultra-low-noise, low-I_Q LDO portfolio, engineered specifically for battery-powered portable electronics demanding extended runtime, clean power for RF/analog circuits, and seamless integration into compact PCB layouts.

FAQ

What is the minimum output capacitor required for stable operation of LD3985M122R?

The LD3985M122R requires a minimum 1 µF ceramic output capacitor (X5R or X7R dielectric) with ESR between 5 mΩ and 5000 mΩ. Stability is maintained across temperature and capacitor tolerance (±30%), and larger values up to 22 µF further improve transient response without compromising phase margin.

Does LD3985M122R require an external pull-down on the VINH pin?

Yes - the VINH pin has no internal pull-down or pull-up. Leaving it floating may cause unpredictable ON/OFF behavior. A 100 kΩ–1 MΩ resistor to GND is recommended when the enable function is unused, ensuring reliable OFF-state operation and preventing false turn-on due to noise coupling.

Can LD3985M122R operate with a 2.0 V input voltage?

No - the absolute minimum operating input voltage is 2.5 V per datasheet specifications. At 2.0 V input, the device cannot regulate 1.22 V output and will enter dropout or cease regulation entirely. For sub-2.5 V inputs, consider alternative LDOs rated for lower V_IN, such as ST's LD39200 series.

How does the BYPASS pin affect output noise performance?

The BYPASS pin connects to a 10 nF capacitor that shunts high-frequency noise from the internal reference amplifier, reducing output noise by up to 50% in the 10 kHz–100 kHz band. Omitting this capacitor increases RMS noise to ~45 µV and degrades PSRR above 10 kHz, directly impacting RF circuit SNR and ADC effective resolution.