Fixed 5V rail with 200 mA headroom and a PSRR profile worth reading
The STMicroelectronics LDK320ADU50R is a single-output, positive fixed 5 V linear regulator rated for 200 mA continuous output. The 0.35 V maximum dropout at full load means the input rail needs to stay above 5.35 V across the load range — a 5.0 V rail is too tight; a 5.5 V or 6.0 V rail gives the headroom for regulation to hold through transients. The PSRR curve starts at 88 dB at 120 Hz and rolls off to 48 dB by 10 kHz. That 88 dB at line frequency means the regulator attenuates 120 Hz ripple from a full-wave rectifier by a factor of roughly 25,000 — useful for cleaning up a lightly loaded analog supply. By 10 kHz the attenuation drops to 48 dB, still meaningful but worth checking if the upstream switcher runs above that corner. For a 5 V rail that stays alive in standby or on a battery-backed domain, the regulator's own draw is under half a milliwatt — it does not dominate the sleep budget.
SOT-89 package: common footprint, inspectable joints, no thermal pad tricks
The LDK320ADU50R ships in a SOT-89 package (also designated TO-243AA). It is a three-lead surface-mount package with a large collector/ground tab on the back — the tab is the main thermal path. The package is available in Tape & Reel (TR) and Cut Tape (CT) formats. The TR variant is the production-reel quantity; CT suits prototype or low-volume builds. Both are surface-mount compatible with standard reflow profiles for the SOT-89 body.
Temperature grade and protection suit industrial and automotive domains
The operating junction temperature range is -40°C to 125°C. The 125°C TJ ceiling means the part can sit next to a hot MCU or a power stage without derating the output current, as long as the board-level thermal resistance keeps TJ below that limit. On-chip protection includes over-current and over-temperature shutdown. These are the standard safeguards for a linear regulator — the current limit folds back the output during a short, and the thermal shutdown trips when the die exceeds the safe junction temperature. No UVLO, no soft-start, no power-good flag; the protection set is minimal but adequate for a fixed-output LDO in a non-critical rail.
Active lifecycle and ROHS3 compliance — no urgency on the BOM
ROHS3 compliant (2011/65/EU + 2015/863) — no exemption declarations needed. The part is free of the ten restricted substances plus the four phthalates added in the 2015 amendment.
How it sits against the closest SOT-89 peer: LD39020DTPU33R
The LD39020DTPU33R is the same regulator family in the same SOT-89 package, but it is a fixed 3.3 V output with a 5.5 V maximum input voltage. The LDK320ADU50R accepts up to 18 V input and outputs 5 V. Both deliver 200 mA and share the same -40 to 125°C temperature grade and protection set (over-current, over-temperature). The LD39020 adds a soft-start feature and an enable pin; the LDK320 has neither. The PSRR profiles differ: the LD39020 quotes 80 dB to 55 dB (1 kHz to 100 kHz), while the LDK320 starts higher at 88 dB (120 Hz) but rolls off to 48 dB by 10 kHz. They are not pin-compatible substitutes — the output voltage and input range are different — but they share the same package footprint and thermal characteristics.