Professional IC Distribution & Technical Solutions

The L4981BD is a high-voltage double-ended controller designed for resonant converters, manufactured by STMicroelectronics.

Key Specifications:

• Input Voltage Range: Up to 600V
• Operating Frequency: Adjustable, typically up to several hundred kHz
• Driver Output Current: ±500mA (sink/source)
• Supply Voltage (VCC): 10V to 20V
• Standby Current: Low quiescent current for improved efficiency
• Protection Features:
• Undervoltage lockout (UVLO)
• Overcurrent protection
• Soft-start functionality
• Package: SO-16

Descriptions & Features:

• Designed for resonant half-bridge topologies (LLC, ZVS, etc.).
• Double-ended control for driving two power MOSFETs or IGBTs.
• Adaptive dead-time management to optimize efficiency.
• Integrated high-voltage startup for simplified power supply design.
• Precise frequency control for resonant operation.
• Suitable for AC-DC power supplies, LED drivers, and industrial applications.

For exact electrical characteristics, refer to the official STMicroelectronics datasheet.

# L4981BD: Application Scenarios, Design Pitfalls, and Implementation Considerations

## Practical Application Scenarios

The L4981BD from ST is a high-voltage startup regulator designed for AC-DC and DC-DC converters, particularly in power supply applications requiring efficient pre-biasing of PWM controllers. Its primary use cases include:

1. Offline SMPS (Switched-Mode Power Supplies):

The IC is widely employed in flyback and quasi-resonant converters, providing a stable auxiliary voltage (typically 12–20V) to power control circuits before the main converter starts. Its high-voltage capability (up to 650V) makes it suitable for universal mains input (85–265VAC).

2. LED Driver Systems:

In constant-current LED drivers, the L4981BD ensures reliable startup without excessive inrush current, mitigating stress on LEDs during power-up. Its low operating current (~150µA) enhances efficiency in standby modes.

3. Industrial Power Modules:

The device is used in industrial equipment where robust startup sequencing is critical, such as servo drives and PLCs. Its integrated high-voltage MOSFET simplifies layout and reduces BOM count.

## Common Design Pitfalls and Avoidance Strategies

1. Inadequate Thermal Management:

The L4981BD dissipates heat during startup, especially in high-line voltage conditions. Poor PCB layout (e.g., insufficient copper area or lack of thermal vias) can lead to overheating.

*Mitigation:* Use a minimum 2oz copper layer and place thermal vias beneath the IC’s exposed pad. Ensure adequate airflow in enclosed designs.

2. Improper HV Pin Protection:

The high-voltage pin (HV) is susceptible to transient spikes in noisy environments, risking latch-up or failure.

*Mitigation:* Add a transient voltage suppressor (TVS) diode or RC snubber near the HV pin. Keep HV traces short and away from sensitive low-voltage nodes.

3. Oversizing the Startup Resistor:

A poorly calculated startup resistor (Rstart) can cause excessive power loss or insufficient charging of the VCC capacitor.

*Mitigation:* Follow ST’s datasheet guidelines for Rstart selection, balancing power dissipation and startup time. For 230VAC, a 150–200kΩ resistor is typical.

4. Unstable VCC Decoupling:

Inadequate decoupling on the VCC pin can lead to oscillations or premature shutdown during load transients.

*Mitigation:* Place a low-ESR ceramic capacitor (1–10µF) as close as possible to the VCC pin. Avoid electrolytic capacitors due to higher ESR.

## Key Technical Considerations for Implementation

1. Startup Timing:

The L4981BD features an internal current source to charge the VCC capacitor. Ensure the capacitor value (typically 4.7–22µF) aligns with the desired startup delay and hold-up time.

2. Undervoltage Lockout (UVLO):

The IC’s UVLO thresholds (typ. 8.4V on, 7.6V off) must be considered to avoid erratic behavior. Use an external divider if adjusting UVLO is necessary.

3. Minimizing EMI:

High