FS431S 1% from FOSAN, in a SOT-23. Pinout and pin descriptions extracted from the manufacturer datasheet by ds2sf; symbol, footprint, and 3D from KiCad-official CAD.

Pinout

Pin	Name	Type	Function
1	REF	input	Reference input. Internally compared to the 2.495 V bandgap; drives the shunt regulator to hold this pin at VREF. Typical reference input current 0.22 µA (max 4 µA). Connect to the R1/R2 divider tap between cathode and anode to set VKA = VREF·(1 + R1/R2) + IREF·R1.
2	A	power_in	Anode. Common/ground return of the shunt regulator. Cathode current flows from K to A when VKA ≥ VREF.
3	K	power_in	Cathode. Sinks the regulated shunt current (IK = -100 to +150 mA absolute max; regulation from IKA(MIN) ≈ 0.05 mA). VKA can range from VREF up to 40 V. Off-state leakage 0.056 µA typ at VKA = 40 V.

Key specifications

Parameter	Value
Manufacturer	FOSAN
Package	SOT-23
Category	PMIC
Pins	3

Ordering variants

Same part, different packaging or reel option.

FS431S 1%S

CAD (KiCad official)

Symbol: FS431S 1%
Footprint: Package_TO_SOT_SMD:SOT-23
3D model: SOT-23 (KiCad packages3D, STEP)

3D model downloads

Two STEP files. Same geometry, different body top:

File	Size	Use it when
`fs431s-1.step`	59 KB	You want the bare part, or you mark your own boards.
`fs431s-1-etched.step`	1.7 MB	You want the part with `FS431S 1%` already on it.

The marking in the etched file is real recessed geometry cut into the solid, not a texture or a decal: 416 faces against 69 in the bare model. It survives export, section views, and rendering in any CAD tool, and it drops straight into KiCad or Fusion with no extra setup.

Provenance

Data extracted from the manufacturer datasheet, not a distributor listing:

Datasheet: FOSAN datasheet (5 pp, sha256 a5eaad65e9b461b4, retrieved 2026-08-17, tier: lcsc)
Datasheet language: Chinese. Pin names, descriptions, and specs on this page were extracted and translated to English by ds2sf; the linked PDF is the manufacturer original.
Extractor: ds2sf (Claude). Every pin and dimension is cited to a datasheet page; it refuses to fabricate.
CAD: KiCad official libraries (CC-BY-SA 4.0).

Datasheet PDF