OB3330XCPA

The OB3330XCPA is a high power factor (PF), primary-side regulation (PSR) LED lighting driver controller from On-Bright Electronics, housed in an 8-pin SOP package. It is designed for isolated flyback LED driver applications requiring high efficiency, high power factor correction, and precise constant-current control without an optocoupler or secondary-side feedback components.

The device integrates an analog multiplier for power factor correction (PFC), achieving PF greater than 0.9 in single-stage flyback topology. The primary-side control eliminates the need for an optocoupler and TL431, significantly reducing BOM count and system cost while improving reliability.

The OB3330X operates in transition mode (TM / boundary conduction mode), which ensures zero-voltage switching (ZVS) or near-ZVS turn-on of the external MOSFET, minimizing switching losses and maximizing efficiency. The zero current detector (ZCD) pin monitors the auxiliary winding to detect transformer demagnetization, ensuring the next switching cycle begins only after the transformer is fully reset.

Key specifications include: VCC operating range with UVLO on at 8-10V and UVLO off at 13.6-15.6V, VCC over-voltage protection at 30V, VCC clamp at 32V, ultra-low start-up current of 5-15 uA, static operating current of 1.5-2.5 mA (no switching), and internal over-temperature protection at 120-160C (typical 140C).

The current sense (CS) comparator features built-in leading-edge blanking (300 ns) to filter out turn-on spikes, and the OCP propagation delay from comparator to gate drive is approximately 100 ns for fast current limiting response. The OCP threshold is selectable via the OCP_TH pin: 2.1V when OCP_TH is below 0.3V, 1.8V for 0.3V-1.5V, and 1.5V when above 1.5V.

The gate drive output is a totem-pole configuration capable of driving external N-channel MOSFETs with rise time of 80-150 ns and fall time of 30-70 ns (CL=1nF). The output is clamped at 11-12.5V (VCC=28V) for MOSFET gate protection.

Comprehensive protection features include: cycle-by-cycle over-current protection (OCP) with selectable threshold, VCC under-voltage lockout (UVLO), VCC over-voltage protection (VCC OVP), output over-voltage protection (OVP via ZCD), over-temperature protection (OTP), LED open circuit protection, and LED short circuit protection. The soft-start function gradually increases the peak current limit during startup to prevent output overshoot and transformer saturation.

The XCPA suffix denotes: X = extended features variant, C = SOP-8 package, P = Pb-free, A = tape and reel packaging (4000 pcs/reel). The device is RoHS compliant and operates over a junction temperature range of -40C to +150C.

The OB3330XCPA operates as a primary-side regulated flyback LED controller with integrated power factor correction, using transition mode (boundary conduction mode) switching for high efficiency.

Primary-Side Regulation (PSR): Unlike conventional secondary-side regulated converters that require an optocoupler to feed back output voltage/current information across the isolation boundary, the OB3330X derives output current information from the primary side. During the MOSFET off-time, the transformer secondary current flows through the output diode to the LED load. This secondary current is reflected to the primary side as a voltage across the current sense resistor (RCS) at the CS pin. The peak primary current and the demagnetization time (measured by the ZCD pin) together determine the average output current. The internal error amplifier (gm amplifier) with reference voltage (400 mV typical) maintains constant output current regulation by adjusting the MOSFET peak current set point.

Power Factor Correction (PFC): The analog multiplier takes the rectified AC line voltage (sampled at the MULT pin from a voltage divider off the DC bus) and multiplies it with the error amplifier output (COMP pin). The multiplier output sets the peak current threshold for each switching cycle, forcing the input current envelope to follow the sinusoidal input voltage waveform. This achieves high power factor (PF > 0.9) without an additional PFC stage. The multiplier gain is 0.9 1/V (typical) with the MULT input linear range of 0 to 3V.

Transition Mode Operation: The ZCD (Zero Current Detection) pin monitors the auxiliary winding voltage. When the transformer is fully demagnetized (secondary current reaches zero), the auxiliary winding voltage drops, and the ZCD comparator detects this transition. After a minimum off-time of 3.5 us, the controller initiates the next switching cycle. This boundary conduction mode ensures the transformer resets completely before the next cycle, preventing transformer saturation and enabling near-zero-voltage switching (the MOSFET drain voltage rings to a valley before turn-on). The maximum off-time is 55 us; if no ZCD signal is detected within this period, the controller restarts a switching cycle.

Current Limiting: The CS pin senses the MOSFET source current through an external sense resistor. When the CS voltage exceeds the threshold set by the OCP_TH pin (selectable at 2.1V, 1.8V, or 1.5V), the gate drive is immediately turned off with a propagation delay of approximately 100 ns. The built-in 300 ns leading-edge blanking prevents false tripping from the turn-on current spike. The maximum on-time is limited to 50 us as an additional safety measure.

Output Over-Voltage Protection: The ZCD pin also monitors the auxiliary winding voltage during the MOSFET off-time. If the output voltage rises abnormally (e.g., LED open circuit), the reflected auxiliary voltage increases. When the ZCD voltage exceeds the OVP threshold of 3.75-4.25V (typical 4V), the controller shuts down and enters a restart cycle.

VCC Supply: The VCC pin is powered from the auxiliary winding through a rectifier diode and filter capacitor. During startup, the VCC is charged by the internal high-voltage start-up circuit (or an external start-up resistor) until it exceeds the UVLO off threshold (14.6V typical), at which point switching begins. If VCC drops below the UVLO on threshold (9V typical), switching stops and the controller re-enters the start-up mode. VCC over-voltage protection activates at 30V.