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Q: What happens if I leave the BP/M pin floating on TNY284?

The IC won't start. The BP/M pin must have a capacitor to SOURCE. Without it, the internal 6V regulator has no bypass, the control circuit can't power up, and the MOSFET never switches.

Q: Can I use a 10µF capacitor on BP/M if I only need standard current limit?

No. It's not about what you need - it's about what the IC detects. TNY284 reads the capacitor value during startup. 10µF signals increased current limit mode, which the TNY284's MOSFET physically cannot support. Use 100nF for standard mode.

Q: Why does TNY284 SO-8C only have 7 leads instead of 8?

Pin 6 is removed for HV creepage compliance. The gap between DRAIN (pin 5) and adjacent SOURCE pins must meet safety isolation requirements. Removing pin 6 increases creepage distance between the high-voltage DRAIN node and low-voltage side.

Q: Does TNY284 need an external startup circuit?

No. It's fully self-starting. An internal high-voltage current source draws from the DRAIN pin to charge the BP/M capacitor to ~6V. Once BP/M reaches the UVLO threshold, the IC starts switching. No external startup resistor or auxiliary supply is needed.

Q: Can I drive the EN/UV pin directly from a microcontroller GPIO?

Yes, with a series resistor. Configure the GPIO as open-drain output. Add a 1-10kΩ series resistor to limit current. When GPIO is LOW, EN/UV current >240µA → switching disabled. When GPIO is HIGH-Z, EN/UV current <240µA → switching enabled.

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TNY284DG-TL Pinout Reference — SO-8C Functions, Specs, and Common Design Mistakes

2026/8/11 11:39:30

TNY284DG-TL Pinout Reference — SO-8C Functions, Specs, and Common Design Mistakes

The TNY284DG-TL packs a 725V flyback switcher into a 7-lead SO-8C. Five functional pins — but the BP/M pin alone has three valid configurations and one trap that catches new designers every time.

This page covers the pinout, what each pin actually does, and the five most common mistakes that cause a TNY284 board to fail on first power-up.

Quick Specs at a Glance

Parameter	Value
Family	TinySwitch-4
Package	SO-8C (7 leads, 4 SOURCE pads)
MOSFET	725V, integrated
f_SW	132kHz (124–140kHz, with jittering)
Duty Cycle	67% max
Power (85–265VAC)	5W adapter / 8.5W open frame
No-Load	<30mW (with bias winding)
Feedback	EN/UV pin, optocoupler-driven ON/OFF control
BP/M Modes	100nF = Standard ILIMIT / 1µF = Reduced / 10µF = NOT SUPPORTED

Pinout Diagram

Pin Functions

Pin	Name	Type	What It Does
1, 2, 7, 8	SOURCE (S)	Power Ground	MOSFET source return. All four internally connected. Solder to copper pour for heatsink. This is the quietest node on the board — use it as the single-point ground for the bias winding return and BP/M capacitor.
3	BP/M	Bypass / Multi-function	Internal 6V regulator bypass. The capacitor value selects the current limit mode. 100nF = standard. 1µF = reduced (next-smaller device). Must be placed as close as possible to BP/M and SOURCE pins. Rated ≥10V.
4	EN/UV	Enable / Undervoltage	Pulling current from this pin (>240µA) disables switching. Normally driven by an optocoupler transistor. Also sets line undervoltage threshold via a resistor to the DC bus — ±5% accurate threshold. When EN/UV current <240µA, switching resumes.
5	DRAIN (D)	High-Voltage Input	725V MOSFET drain. Connect to flyback transformer primary. Self-biases the IC at startup through an internal HV current source. Keep drain trace away from EN/UV trace — capacitive coupling injects switching noise into the enable signal.

BP/M Capacitor Selection: The Pin That Bites Back

The BP/M pin is the most misunderstood pin on a TinySwitch-4. It does two things at once — bypass the internal 6V regulator AND select the MOSFET current limit.

The capacitor value determines the mode. Get it wrong and the IC either runs at reduced power or enters an undefined state.

BP/M Capacitor	Mode	Supported on TNY284?	Effect
100nF	Standard I_LIMIT	✓ Yes	Full rated current limit. Default for new designs.
1µF	Reduced I_LIMIT	✓ Yes	Current limit drops to next-smaller device level. Use when the transformer core would saturate at standard limit or you're trading efficiency for lower peak flux.
10µF	Increased I_LIMIT	NO	Only TNY285 and above. On TNY284, 10µF causes low output voltage, erratic switching at 6–60kHz, and potential MOSFET overstress. PI Expert sometimes defaults to 10µF incorrectly.

TNY284 Current Limit Modes (valid BP/M capacitor values):

100nF

Standard ILIMIT - full power

1µF

Reduced ILIMIT - derated

10µF

WARNING: NOT VALID - erratic operation

Five Common Design Mistakes (and How to Avoid Them)

Mistake 1: BP/M Capacitor = 10µF on a TNY284

Symptom: Output voltage stuck at 60–70% of target. Switching frequency drops to 6–60kHz. IC runs hot. The design looks like it should work but doesn't.

Root cause: PI Expert sometimes defaults BP/M to 10µF for all TinySwitch-4 devices. But TNY284 only supports 100nF (standard) and 1µF (reduced). The 10µF mode requires a larger MOSFET die that the TNY284 doesn't have.

Fix: Replace BP/M capacitor with 100nF. Verify in PI Expert that "Increased Current Limit" is NOT selected for TNY284.

Mistake 2: EN/UV Pin Floating or Poorly Routed

Symptom: IC starts, runs for a few cycles, then enters auto-restart. Or output voltage is correct at no-load but collapses under any load.

Root cause: The EN/UV pin is high-impedance and capacitive coupling from the DRAIN trace injects switching noise. If the pin picks up enough noise to cross the 240µA threshold, it falsely disables switching cycles.

Fix: Route EN/UV trace on the opposite side of the board from the DRAIN trace, or place a ground guard trace between them. Keep the optocoupler as close to the EN/UV pin as physically possible. Add a 100pF capacitor from EN/UV to SOURCE if noise persists.

Mistake 3: Forgetting the Line UV Resistor

Symptom: Supply starts at 230VAC but won't start at 100VAC. Or it starts at low line but the output power sags because the IC thinks it's in undervoltage.

Root cause: The EN/UV pin requires a resistor to the DC bus to set the undervoltage threshold. Without it, the IC defaults to an internal threshold that may not match your input range.

Brown-out detection is only reliable for brown-IN, not brown-OUT — the IC turns on at the correct voltage but may not turn off until the bus collapses far below the expected threshold.

Fix: Calculate R_UV = (V_DC(min) − 2V) / 25µA. For 85VAC input, V_DC(min) ≈ 120V, giving R_UV ≈ 4.7MΩ. For reliable brown-out, add an external circuit that pulls BP/M below 4.9V when mains drops.

Mistake 4: Skipping the Bias Winding

Symptom: No-load consumption measures 150–200mW instead of <30mW. The supply fails standby compliance testing.

Root cause: Without a bias winding, the IC powers itself from the DRAIN pin through an internal current source, wasting ~150mW even at no-load. The bias winding supplies the BP/M pin externally during normal operation.

Fix: Add a bias winding to the flyback transformer (typically 10–15 turns, phased to deliver ~8–12V). Connect through a diode and small filter cap to BP/M. This drops no-load consumption from 150mW to <30mW. For always-plugged-in products, this is not optional.

Mistake 5: Grounding the SOURCE Pins Separately

Symptom: Erratic switching, poor regulation, unexpected EMI, or the IC randomly entering auto-restart.

Root cause: The four SOURCE pins are the return path for both the power MOSFET current and the control circuitry. If they connect to different ground pours or through thin traces, the control circuit ground bounces with the power current.

Fix: Connect all four SOURCE pins to a single, solid copper pour. Use Kelvin (single-point) grounding: bring the input filter capacitor negative, bias winding return, and BP/M capacitor return ALL to the same point at the SOURCE pins. No exceptions.

Component Placement Priority (closer to IC = more critical):

BP/M cap

<5mm from pins - HIGHEST priority

EN/UV trace

Keep separate from DRAIN - HIGH

Optocoupler

Within 10mm of EN/UV - MEDIUM

SOURCE pour

Solid copper, all pins - baseline

Reference Circuit

Reference Circuit Quick Check

Component	Value / Type	Connected To	Critical Placement Rule
BP/M capacitor	100nF, X7R, ≥10V	BP/M (pin 3) to SOURCE	Must be <5mm from pins. Traces <2mm.
EN/UV resistor (R_UV)	~4.7MΩ (for 85VAC)	EN/UV (pin 4) to DC bus (+)	Use 2× 1206 in series for voltage rating.
Optocoupler transistor	PC817 or similar, CTR 100–300%	Collector to EN/UV, emitter to SOURCE	Place within 10mm of EN/UV pin.
TL431 reference	Standard, 2.5V, 1% tolerance	Secondary-side feedback divider	Keep cathode close to optocoupler LED.
Clamp diode	UF4007 or similar fast recovery	DRAIN to clamp network	Keep loop area small: DRAIN → diode → clamp cap → SOURCE.
Transformer primary	EE13 or EFD15, 1–2mH typical	DRAIN (pin 5) to DC bus (+)	Minimize DRAIN node copper area to reduce EMI.

Frequently Asked Questions

Q1: What happens if I leave the BP/M pin floating?

A: The IC won't start. The BP/M pin must have a capacitor to SOURCE. Without it, the internal 6V regulator has no bypass, the control circuit can't power up, and the MOSFET never switches. The IC draws zero current because it never leaves UVLO.

Q2: Can I use a 10µF capacitor on BP/M if I only need standard current limit?

A: No. It's not about what you need — it's about what the IC detects. The TNY284 reads the capacitor value during startup. 10µF signals "increased current limit" to the control circuit, which the TNY284's MOSFET physically cannot support. This causes the control loop to enter an undefined state. Use 100nF for standard mode regardless of what current you actually draw.

Q3: The datasheet shows 8 pins. Why does the SO-8C only have 7 leads?

A: Pin 6 is removed for HV creepage compliance. The gap between DRAIN (pin 5) and the adjacent SOURCE pins must meet safety isolation requirements. Removing pin 6 increases the creepage distance between the high-voltage DRAIN node and the low-voltage side. This is standard across all PI SO-8C packages.

Q4: Does TNY284 need an external startup circuit?

A: No. It's fully self-starting. An internal high-voltage current source draws from the DRAIN pin to charge the BP/M capacitor to ~6V. Once BP/M reaches the UVLO threshold, the IC starts switching. After startup, the bias winding (if present) takes over supplying BP/M. No external startup resistor or auxiliary supply is needed.

Q5: Can I drive the EN/UV pin directly from a microcontroller GPIO?

A: Yes, with a series resistor. The EN/UV pin expects current to be pulled from it to ground. A microcontroller GPIO configured as open-drain output can drive it. Add a 1–10kΩ series resistor to limit current. When GPIO is LOW, EN/UV current >240µA → switching disabled. When GPIO is HIGH-Z (input mode), EN/UV current <240µA → switching enabled. This is how you implement MCU-controlled output on/off.