LNK625DG-TL — 6.5W Primary-Side CV Flyback Switcher, No Optocoupler (SO-8C)

What Are the Technical Specifications of LNK625DG-TL?

The number that defines this part: 1.8V at the FB pin, sampled 2.5µs after turn-off. That's the entire regulation mechanism. Bias winding voltage during flyback is proportional to output voltage (through the turns ratio). A resistor divider scales it to 1.8V.

If FB > 1.8V, the IC skips cycles. If FB < 1.8V, it enables them. Simple in concept. But the resistor placement and PCB layout around the FB pin make or break the design.

When Should You Use (and NOT Use) the LNK625DG-TL?

✅ Use LNK625DG-TL when:

• You're building a 5–6.5W isolated flyback and want the simplest possible BOM. Primary-side regulation eliminates the optocoupler, TL431, and their compensation network. That's 4–6 components off the board and out of the cost.
• Cross-primary-secondary isolation creepage is tight. No optocoupler means one less component bridging the isolation barrier. For designs where PCB creepage distance is at a premium, that matters.
• You're replacing a linear transformer with a switch-mode supply. Legacy appliances that used 50/60Hz transformers for a 5V or 12V rail can swap to a LinkSwitch-CV flyback and shed 70% of the weight and 80% of the standby power.
• Load regulation of ±5% is acceptable. Primary-side CV can't match the ±1–2% of an optocoupler-based design. For MCU supplies, relays, LEDs, and most analog circuits, ±5% is fine.

❌ Don't use LNK625DG-TL when:

• You need tight load regulation. If the load swings from 10% to 100% and the rail must stay within ±2%, use TinySwitch-4 (TNY284) with optocoupler feedback.
• You need CC/CV for battery charging. LinkSwitch-CV is constant-voltage only. For lithium chargers that need both CC and CV modes from the primary side, use LinkSwitch-II (LNK60x series).
• You're doing a non-isolated design. If you don't need isolation, a LinkSwitch-TN (LNK306) buck costs less — no transformer, no bias winding.
• No-load consumption must be <30mW. LNK625 draws <200mW no-load (or <70mW with bias). For sub-30mW, TinySwitch-4 is the answer.

LNK623 vs LNK624 vs LNK625 vs LNK626 — How Do They Compare?

The LinkSwitch-CV family scales from 5W to 10.5W. Same pinout, same FB sensing mechanism, same 725V MOSFET — only the current limit and die size change.

Picking the right one: The LNK625 hits the 6–7W sweet spot for 5V/1A and 12V/500mA adapters. The LNK623/624 are pin-compatible cost-downs if your power budget allows. The LNK626 adds headroom without any design change — same feedback, same turns ratio, bigger die.

Pin Functions at a Glance

What Are the Typical Applications of LNK625DG-TL?

USB charger / wall adapter (5V/1.2A). The LNK625 at 6.5W is sized perfectly for a single-port 5V USB charger. Primary-side CV means no optocoupler crossing the isolation barrier — fewer creepage headaches, simpler safety certification.

Set-top box / DVD player standby supply (5V/500mA). Consumer AV gear needs an always-on standby rail that powers the IR receiver and front-panel MCU. The LNK625 delivers isolated 5V without an optocoupler, keeping the standby board compact and cheap.

White goods auxiliary supply (12V/400mA). Washing machines and refrigerators need an isolated low-voltage rail for the control board. The LNK625 handles universal AC input and meets household appliance standby regulations with the external bias winding option.

Industrial sensor / meter supply (24V/200mA). DIN-rail modules and field sensors often need an isolated supply from 230VAC. The LNK625 + an EE16 transformer produces a regulated 24V rail with no secondary-side components beyond the output diode and filter cap.

What Are the Alternatives to LNK625DG-TL?

LNK625 vs TNY284 — the optocoupler question. Both are isolated flyback switchers in SO-8C. LNK625 drops the optocoupler and TL431 — roughly $0.15–0.25 BOM savings. TNY284 offers tighter regulation and <30mW standby.

If your load is predictable and ±5% works, LNK625 wins on cost. If the rail must stay tight across wide load swings, the optocoupler pays for itself.

How Much Does the LNK625DG-TL Cost and Is It in Stock?

Contact ICMASS for a same-day quote. The LNK625 is an active production part. Availability is solid across the full LinkSwitch-CV family. Pricing is roughly on par with the LNK306 (LinkSwitch-TN) — you're paying for the isolation capability, not a premium over the buck parts.

Why Buy LNK625DG-TL from ICMASS?

Full LinkSwitch-CV family, one shipment. We stock LNK623 through LNK626 across SO-8C and DIP packages. If you prototype at 6.5W with LNK625 and production needs to hit a cost target with LNK623, we have both.

Transformer sourcing support. The LNK625 is only as good as its flyback transformer. We can recommend proven transformer vendors with LinkSwitch-CV-tested designs — EE16 or EFD15 cores with the right primary inductance and bias winding ratio. Tell us your output spec, we'll point you to the right magnetics.

Complete BOM, one invoice. The LNK625 needs a transformer, input bulk capacitor, output Schottky, output filter cap, and a handful of resistors for the FB divider. We stock every part in the typical reference design.

Shenzhen warehouse, global shipping. Orders before 15:00 CST ship same day via DHL/FedEx. 5–10 day delivery worldwide.

Frequently Asked Questions About LNK625DG-TL

Q1: How does the LNK625 regulate without an optocoupler?

A: It senses the reflected output voltage on the bias winding through the FB pin. During flyback, the bias winding voltage is proportional to the output voltage (by the turns ratio). The IC samples FB exactly 2.5µs after MOSFET turn-off — this is when the leakage inductance spike has settled but the flyback energy hasn't decayed. If FB > 1.8V, the IC skips cycles. A resistor divider from bias winding to FB sets the output voltage.

Q2: My LNK625 output voltage sags under load. What's wrong?

A: Start with the grounding layout. The most common cause is poor grounding — using a shared return path for power and signal instead of single-point (Kelvin) grounding at the input capacitor negative. Second check: feedback resistor placement. If the divider resistors are more than 10mm from the FB pin, switching noise corrupts the 2.5µs sampling window. Third: verify the bias winding voltage doesn't sag due to transformer leakage inductance.

Q3: My LNK625 transformer makes an audible buzzing noise.

A: Three likely causes, each with a different fix. If noise is at no-load, check the clamp/snubber capacitor — it may be resonating. If noise is at full load, the transformer core isn't properly bonded — replace with a properly varnished transformer where the center leg is glued. If noise happens with pulse grouping (6+ consecutive pulses), add an RC network across the lower FB divider resistor: R × C ≈ 32ms.

Q4: What's the real no-load power I can expect?

A: About 150–200mW without a bias winding; 50–70mW with one. The internal high-voltage current source draws ~200mW to power the IC at no-load. Adding a bias winding to the transformer feeds the BP pin externally and reduces no-load by 60–70%. For sub-30mW standby, switch to TinySwitch-4 — LinkSwitch-CV can't get there.

Q5: Can I use LNK625 as a battery charger?

A: Not by itself — LinkSwitch-CV is constant-voltage only, no CC mode. For lithium battery charging, you need CC/CV: constant current until the battery reaches float voltage, then constant voltage. Add an external CC circuit on the secondary side, or use LinkSwitch-II (LNK606) which does CC/CV from the primary side.

Q6: LNK625 vs LNK626 — what changes if I swap?

A: Nothing except the power headroom. Same pinout, same FB threshold, same frequency. The LNK626 has a larger MOSFET die — that's the only difference. If your LNK625 design runs hot at 6.5W, drop in an LNK626. The transformer, feedback resistors, and output filter stay identical. Just make sure the PCB copper pour can handle the extra heat.

Q7: What clamp circuit should I use?

A: Depends on the transformer leakage inductance. For leakage >125µH, use an RCD clamp with Zener bleed — Zener voltage ~20% above V_OR (reflected output voltage). The Zener clips turn-off spikes without conducting during flyback. For leakage <125µH, a simple RC clamp may work: start with 470kΩ in parallel with 820pF. Use a slow recovery diode (1N4007, not UF4007) — fast diodes cause ~8% voltage error by letting feedback winding ring.

Q8: Can I drop an LNK625 into a board designed for TNY284?

A: No — different pinout, different feedback mechanism. TNY284 uses an EN/UV pin driven by an optocoupler. LNK625 uses an FB pin reading a resistor divider from the bias winding. The feedback networks are completely different. You can't swap them without a board redesign. If you need to switch between primary-side and optocoupler feedback, budget a new PCB.