Identify an unknown SMD part: marking code “21B”

Packages rarely print the full part number. The marking below is shared by several part numbers — use the package, pin count and the manufacturer family together to narrow it down, then confirm against the datasheet.

8 candidates source-traceable

Candidate part numbers

Part number	Manufacturer	Package	Mapping source
1PMT5921B	—	SOD-323	Public marking directory
AAT60021B-S15	—	SOT-143	Public marking directory
AAT60021B-S16	—	SOT-89	Public marking directory
AAT60021B-S2	—	SOT-23 16vdb VD7 3d Aat	Public marking directory
APR3021-17A	—	SOT-23 16vda VD7 3b Anp	Public marking directory
APR3021-17B	—	SOT-23-5	Public marking directory
APR3021-17D	—	SOT-89	Public marking directory
MMBZ5221BPT	—	SOT-23 16dc - 3a Chm	Public marking directory

How we narrow this down

1. Package first — SOT-23 vs SOD-123 vs SC-59 eliminates most candidates immediately.
2. Pin count and body size — a 3-pin SOT-23 LDO cannot be a 6-pin dual transistor.
3. Marking style — laser-marked parts are usually newer production; ink marking often indicates older or second-source vendors.
4. Circuits around it — a part sitting on a rail with an inductor is a regulator, not a diode.
5. Datasheet check — every credible candidate lists its marking scheme in the “marking information” section.

FAQ

• Same code, several parts — which one do I actually have? Compare package + pin count + surrounding circuit; if it still fits two parts, request a sample verification before production.
• Can I substitute between the candidates? No — the same marking may sit on electrically different parts. Always match the electrical spec sheet, not just the marking.
• Where do these mappings come from? Public SMD marking directories and historical stock tables; each row keeps its source so you can trace it.

Other codes starting with “2”

How these mappings are compiled: public SMD marking directories plus our own stock records. Each row keeps its original reference internally, so any mapping can be traced on request. Markings are indicative only — confirm with the physical batch marking and the original datasheet before ordering.