TDA4863-2

Functional Description

Version 2.1

10

22 Feb 2005

2.4

Overvoltage Regulator

Because of the integrator´s low bandwidth fast changes of the output voltage can’t be

regulated within an adequate time. Fast output changes occur during initial start-up,

sudden load removal, or output arcing. While the integrator´s differential input voltage

remains zero during this fast changes a peak current is flowing through the external

capacitor into pin VAOUT. If this current exceeds an internal defined margin the

overvoltage regulator circuitry reduces the multiplier output voltage. As a result the on

time of the MOSFET is reduced.

2.5

Multiplier

The one quadrant multiplier regulates the gate driver with respect of the DC output

voltage and the AC half wave rectified input voltage. Both inputs are designed to achieve

good linearity over a wide dynamic range to represent an AC line free from distortion.

Special efforts are made to assure universal line applications with respect to a 90 to

270 V AC range.

The multiplier output is internally clamped at 1.3 V. So the MOSFET is protected against

critical operating during start up.

2.6

Current Sense Comparator, LEB and RS Flip-Flop

The source current of the MOS transistor is transferred into a sense voltage via the

external sense resistor. The multiplier output voltage is compared with this sense

voltage. Switch on time of the MOS transistor is determined by the comparison result.

To protect the current comparator input from negative pulses a current source is inserted

which sends current out of the ISENSE pin every time when V

below ground potential. An internal RC-filter is connected to the ISENSE pin which

smoothes the switch-on current spike. The remaining switch-on current spike is blanked

out via a leading edge blanking circuit with a blanking time of typ. 200 ns.

The RS Flip-Flop ensures that only one single switch-on and switch-off pulse appears at

the gate drive output during a given cycle (double pulse suppression).

2.7

Zero Current Detector

The zero current detector senses the inductor current via an auxiliary winding and

ensures that the next on-time of the MOSFET is initiated immediately when the inductor

current has reached zero. This reduces the reverse recovery losses of the boost

converter diode to a miniumum. The MOSFET is switched off when the voltage drop of

the shunt resistor reaches the voltage level of the multiplier output. So the boost current

waveform has a triangular shape and there are no deadtime gaps between the cycles.

This leads to a continuous AC line current limiting the peak current to twice of the

average current.