Oemstron What Is L4981A IC?

What Is L4981A?
The L4981A is a Power Factor Correction (PFC) pre-regulator IC designed to achieve a high power factor of up to 0.99. Built using BCD 60II technology, it enables the design of efficient power supplies with sinusoidal line current for input voltages from 85V to 265V without a line switch.
Operating at a fixed frequency, it features an average current mode PWM controller, MOSFET gate driver, precise voltage reference, error amplifier, undervoltage lockout, current sensing, and soft start. Integrated overvoltage and overcurrent protection reduce external component needs, with programmable current limits using a simple resistor.
Pin Connection of L4981A
P-GND: Powerground.
IPK: Peak current limit using resistors between Pin 2, sense resistor, and Pin 11 (VREF) for higher precision.
OVP: Overvoltage protection via a resistive divider and 5.1V reference.
IAC: AC input current generates the current reference for the current amplifier.
CA-OUT: Current amplifier output; loop gain set by external RC network.
LFF: Load feedforward input (1.5V-5.3V) to modify the multiplier output. Connect to VREF if unused.
VRMS: RMS line voltage input for line compensation (1.5V-5.5V). Connect to VREF if unused.
MULT-OUT: Multiplier output and current amplifier input. Must stay above -0.5V.
ISENSE: Inverting input of current amplifier. Must stay above -0.5V.
S-GND: Signal ground.
VREF: 5.1V reference voltage, ±2% accuracy, supplies up to 10mA.
SS: Soft-start input; capacitor defines start-up time. Discharges in OVP or UVLO conditions.
VA-OUT: Voltage error amplifier output; loop gain set by external RC network.
VFEED: Inverting input of voltage error amplifier connected to the boost output.
P-UVLO: Programmable under-voltage lockout threshold input, set by a voltage divider.
SYNC: Synchronization input/output for external clock pulse sync.
ROSC: Resistor sets charging current of COSC.
COSC: Capacitor sets switching frequency.
VCC: Power supply input.
GDRV: Gate driver output for MOSFET/IGBT with peak current >1A.
Circuit of L4981A
The circuit flow of the L4981A can be divided into three main sections: input rectification, PFC control, and output rectification. First, the AC power supply (85V-265V) is converted into DC voltage through a rectification circuit, and high-frequency interference signals are filtered out using a filter. The rectified voltage is then fed into the PFC control circuit, with the L4981A chip as its core component.
Inside the L4981A, the voltage detection circuit and current detection circuit monitor the input voltage and current, respectively. The multiplier multiplies the voltage and current signals to generate a current reference signal that is in phase with the input voltage. The PWM controller receives the difference between the current reference signal and the actual current, generating a control signal to drive the switching state of the MOSFET.
This process adjusts the waveform of the input current to be in phase with the input voltage and close to a sinusoidal shape, thereby achieving a high power factor. The output voltage is fed back to the error amplifier via a feedback circuit to stabilize the output voltage.
Specification of L4981A
Supply Voltage Range: 14.5V ~ 19.5V
Control Method: Average current mode PWM control
Power Factor: Up to 0.99
Gate Driver Output Current: Over 1A
Voltage Reference: 5.1V
Startup Current: 100 µA
Switching Frequency: 100kHz
Operating Temperature Range: -40°C ~ 125°C
Package: 20-DIP
Technology: BCD 60II technology
Features of L4981A
Control Boost PWM Up to 0.99P.F.
Limit Line Current Distortion to < 5%
Universal Input Mains
Feed Forward Line And Load Regulation
Average Current Mode PWM For Minimum Noise Sensitivity
High Current Bipolar And DMOS Totem Pole Output
Low Start-Up Current (0.3mA TYP.)
Under Voltage Lockout With Hysteresis And Programmable Turn On Threshold
Overvoltage, Overcurrent Protection
Precise 2% On Chip Reference Externally Available
Soft Start
Where to Use L4981A?
The L4981A is widely used in the PFC circuits of electronic devices. It is commonly found in household appliances, industrial power supplies, and LED lighting power supplies. By controlling the input current waveform to synchronize with the input voltage, the L4981A achieves a power factor of up to 0.99, reducing harmonic interference in the power grid and improving energy efficiency. Additionally, it is frequently used in uninterruptible power supplies (UPS), server power supplies, and electric vehicle charging equipment.
With support for a wide input voltage range, fixed frequency control, and multiple protection features, the L4981A is well-suited for use in global switch-mode power supplies, industrial control devices, and communication base station power systems.
How to Connect L4981A?
First, connect the AC power supply to the input of the rectifier bridge circuit. The rectified DC voltage passes through a filter capacitor to eliminate high-frequency interference signals and then flows through the boost inductor to the input pin of the L4981A. The IAC and VRMS pins receive the input current and voltage signals, respectively, which are used by the internal multiplier to calculate the current control signal.
In the control circuit, the ISENSE pin is connected to a current-sensing resistor to detect the actual current signal and compare it with the reference current signal output by the multiplier. The VFEED pin is connected to the output voltage via a voltage divider. The CA-OUT pin adjusts the current loop gain through an RC filter network. Finally, the GDRV pin of the L4981A connects to the gate of the MOSFET, controlling its switching state to regulate the waveform of the input current.
Package of L4981A
The L4981A comes in a 20-DIP package, a common through-hole package designed for easy installation and soldering of electronic components. This package features 20 pins arranged in two parallel rows, making it easy to insert into printed circuit boards (PCBs) for through-hole mounting. The typical dimensions of the 20-DIP package are 24.38mm (length) x 7.62mm (width), with appropriate pin spacing for electrical connections and heat dissipation.
FAQs
What is the main function of the L4981A?
The primary function of the L4981A is to control the input current waveform to be in phase with the input voltage, achieving high power factor correction (PFC). It is used to reduce harmonic distortion and improve energy efficiency in AC-to-DC converters.
What is the difference between L4981A and L4981B?
The key difference is in control of the switching frequency:
L4981A operates at a fixed switching frequency, providing stable operation.
L4981B supports variable frequency modulation, optimizing the size of the input filter.
How to achieve overcurrent protection with L4981A?
The ISENSE pin is connected to a shunt resistor. By adjusting the value of this resistor, you can set the overcurrent protection (OCP) level. If the sensed current exceeds a threshold, the L4981A will limit or shut down the MOSFET gate drive to protect the circuit.