TDA4863

Functional Description

Data Sheet

9

V1.0, 2003-05

2

Functional Description

2.1

Introduction

Conventional electronic ballasts and switch mode power supplies are designed with a

bridge rectifier and a bulk capacitor. Their disadvantage is that the circuit draws power

from the line when the instantaneous AC voltage exceeds the capacitors voltage. This

occurs near the line voltage peak and causes a high charge current spike with following

characteristics: The apparent power is higher than the real power that means low power

factor condition, the current spikes are non sinusoidal with a high content of harmonics

causing line noise, the rectified voltage depends on load condition and requires a large

bulk capacitor, special efforts in noise suppression are necessary.

With the TDA4863 preconverter a sinusoidal current is achieved which varies in direct

instantaneous proportional to the input voltage half sine wave and so provides a power

factor near 1. This is due to the appearance of almost any complex load like a resistive

one at the AC line. The harmonic distortions are reduced and comply with the IEC555

standard requirements.

2.2

IC Description

The TDA4863 contains a wide bandwidth voltage amplifier used in a feedback loop, an

overvoltage regulator, an one quadrant multiplier with a wide linear operating range, a

current sense comparator, a zero current detector, a PWM and logic circuitry, a totem-

pole MOSFET driver, an internal trimmed voltage reference, a restart timer and an

undervoltage lockout circuitry.

2.3

Voltage Amplifier

With an external capacitor between the pins VSENSE and VAOUT the voltage amplifier

acts like an integrator. The integrator monitors the average output voltage over several

line cycles. Typically the integrator´s bandwidth is set below 20 Hz in order to suppress

the 100 Hz ripple of the rectified line voltage. The voltage amplifier is internally

compensated and has a gain bandwidth of 5 MHz (typ.) and a phase margin of 80

degrees. The non-inverting input is biased internally to 2.5 V. The output is directly

connected to the multiplier input.

The gate drive is disabled when VSENSE voltage is less than 0.2 V or VAOUT voltage

is less than 2.2 V.

If the MOSFET is placed nearby the controller switching interferences have to be taken

into account. The output of the voltage amplifier is designed in a way to minimize these

inteferences.