L6599 pins: what to measure on an LLC controller

The IC that runs the main supply (12 V / 24 V) in TVs and PC supplies. Mind the high-voltage pins (OUT, HVG, VBOOT): they're measured relative to the midpoint, not ground. Tap a pin.

Where you'll meet it LED TV main supplies, PC power supplies, LED lighting drivers (equivalents: L6599A, NCP1396, FAN7621)

Control / signalClock (crystal)AnalogDrive outputGroundSupply

Pin 1 · Control / signal

CSS

Soft-start. The capacitor here starts the frequency high at power-up and brings it down slowly, avoiding a current surge.

🔧 How to measure

DC against hot-side ground; watch the start-up moment.

Expected on a working boardRises at start-up, steady at ≈2 V while running

On the scope

⚠ If the reading is wrongIf it keeps discharging and recharging, the IC keeps restarting: overcurrent (ISEN) or a protection is tripping.

📋 What order to measure in?

Unplugged: any short between LVG–GND (11–10), HVG–OUT (15–14) or VCC–GND (12–10)?
With the device on, is VCC (12) 12–16 V?
RFmin (4): is ≈2 V present? If not, the IC isn't running.
Is LINE (7) above threshold and DIS (8) ≈0 V? Any protection active?
CF (3): is there a triangle wave?
LVG (11): is there a square wave? Then measure HVG (15) relative to OUT.
Is ISEN (6) below 0.8 V?

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How it works in a circuit and drives the MOSFET

A real circuit using this IC. The line lighting up yellow is the IC's gate output: when the IC drives it HIGH, the MOSFET turns on. Follow the IC's output alongside the circuit's currents and voltages in the slow-motion strip below.

⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)

The IC drives HO and LO HIGH in turn, with a short dead time between so both MOSFETs never conduct together and short the 390 V rail. It lowers the frequency as the load rises and raises it as the load falls.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/2: Q1 on (HO)

The IC turns Q1 on from HO. The midpoint rises to 390 V; current flows through Cr and Lr into the primary. On the secondary, D1 conducts.

L6599 HO / LO outputs: HIGH (≈12 V, switch on) / LOW (0 V, switch off)

1. Midpoint (Q1 source / Q2 drain)

A 50% duty square wave between 0 and 390 V. In the dead times the voltage swings softly (zero-voltage switching).

On the scope

Multimeter≈195 V (average)

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

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Go further

The LLC topology and real circuit How it works: TV power board PC817 pins