SI3404-A-GMR from SKYWORKS SILICON LABS, in a QFN-20-EP(4x4). 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	ISNS	input	Average current-sense resistor input for the dc-dc converter. Sets the maximum allowed current via external sense resistor; the overcurrent threshold voltage is V_ISNS_OVC. Note this pin voltage can go below VSS. Vrange: -0.5 to 0 V, referenced to VSS.
2	FBH	input	High-side (VPOS-referenced) dc-dc feedback input, used for buck converter topology. Tie to VPOS when not used. Vrange: 0-5.5 V, referenced to VPOS.
3	EROUT	bidirectional	Error amplifier current output for the dc-dc converter; loop compensation impedance is connected here. I_EROUT = (V_FBH - V_FBREF) x 50 uA or (V_FBL - V_FBREF) x 50 uA. Vrange: 0-5.5 V, referenced to VSS.
4	FBL	input	Low-side (VSS-referenced) dc-dc feedback input, used for flyback converter topology. Tie to VSS when not used. Vrange: 0-5.5 V, referenced to VSS.
5	VDD	power_out	Regulated 5 V output relative to VSS. Powers internal logic; requires external C_REG bypass capacitance. No foldback characteristic; output voltage decreases when reaching VDD_ILIM. Vrange: 0-5.5 V.
6	RDET	bidirectional	Detection signature resistor pin. External R_DET is tied between this pin and VPOS to present the ~25 kOhm PoE detection signature. During detection a high-voltage switch pulls RDET to VNEG; after detection the reference block forces -750 mV vs VPOS to create 30 uA for internal blocks. Vrange: 0-100 V referenced to VPOS.
7	NC	unconnected	No internal connection. Connect to VNEG for better thermal performance.
8	HSO	bidirectional	Hotswap switch output. Internal high-voltage hotswap FET shorts VNEG to HSO when enabled, with inrush current limiting and overcurrent protection. Vrange: 0-100 V referenced to VNEG.
9	RCL	bidirectional	Classification resistor input. External R_EXT sets PoE class current; pin is active only during PoE classification. For Class 0 this pin may be left floating. Vrange: 0-5.5 V referenced to VNEG.
10	RFREQ	bidirectional	Oscillator frequency tuning resistor pin. External resistor from RFREQ to VPOS sets the dc-dc switching frequency (100-500 kHz, inversely proportional to R). Tie directly to VPOS to select the default 250 kHz. Vrange: 0-5.5 V referenced to VPOS.
11	NC	unconnected	No internal connection. Connect to VNEG for better thermal performance.
12	NC	unconnected	No internal connection. Connect to VNEG for better thermal performance.
13	VPOS	power_in	Rectified high-voltage supply positive rail from the PoE bridge; main chip input power. Vrange: 0-100 V. Absolute max 120 V.
14	NC	unconnected	No internal connection. Connect to VNEG for better thermal performance.
15	NC	unconnected	No internal connection. Connect to VNEG for better thermal performance.
16	VT15	input	Optional 15 V auxiliary transformer bias-winding input. If bias-winding voltage falls below V_T15_MIN, the internal 15 V coarse regulator supplies the internal 11 V regulator. Vrange: 0-16.5 V referenced to VSS.
17	NC	unconnected	No internal connection. Connect to VNEG for better thermal performance.
18	SWO	output	Internal dc-dc switching NMOS transistor drain output. Vmax = 120 V. Connects to the switching-converter inductor/transformer primary. Vrange: 0-120 V referenced to VSS.
19	VSS	power_in	dc-dc converter primary ground. Reference for internal analog and dc-dc converter circuitry (distinct from VNEG PoE-input ground).
20	NC	unconnected	No internal connection. Connect to VNEG for better thermal performance.
EP	VNEG	power_in	Exposed thermal pad; rectified high-voltage supply ground (PoE line-side negative return). Also provides the primary thermal relief path and must be soldered to the PCB ground plane.

Key specifications

Parameter	Value
Manufacturer	SKYWORKS SILICON LABS
Package	QFN-20-EP(4x4)
Category	PMIC
Pins	21

Ordering variants

Same part, different packaging or reel option.

SI3404-A-GMR
Si3404-A-GMR

CAD (KiCad official)

Symbol: SI3404-A-GMR
Footprint: Package_DFN_QFN:QFN-20-1EP_4x4mm_P0.5mm_EP2.5x2.5mm
3D model: QFN-20-EP(4x4) (KiCad packages3D, STEP)

Provenance

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

Datasheet: SKYWORKS SILICON LABS datasheet (26 pp, sha256 58b4022c78aa84e6, retrieved 2026-08-17, tier: lcsc)
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