TDA 4862

Semiconductor Group

5

1998-02-16

Functional Description (cont’d)

MuItiplier

A one quadrant multiplier is the crucial circuitry that regulates the gate driver with respect

of the DC output voltage and the AC haversine input voltage of the preregulator. Both

inputs are designed for good linearity over a wide dynamic range, 0 V to 4.0 V for the

MULTIN and 2.5 V to 4.0 V for the

V

Current Sense Comparator and RS Latch

The multiplier output voltage is compared with the current sense voltage which

represents the current through the MOSFET. The current sense comparator in addition

with the logic ensures that only a single pulse appears at the drive output during a

given cycle. The multiplier output and the current sense threshold are internally clamped

at 1.3 V. So the gate drive MOSFET is protected against critical operating, as they occur

during start up. To prevent the input from negative pulses a special protection circuitry

is implemented. Switch-on current peaks are reduced by an internal RC-Filter.

Zero Current Detector

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

ensures that the next on-time is initiated immediately when the inductor current has

reached zero. This diminishes the reverse recovery losses of the boost converter diode.

Output switch conduction is terminated when the voltage drop of the shunt resistor

reaches the threshold 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.

To prevent false tripping the zero current detector is designed as a Schmitt trigger with

a hysteresis of 0.6 V. An internal 5 V clamp protects the input from overvoltage

breakdown, a 0.6 V clamp prevents substrate injection. An external resistor must be

used in series with the auxiliary winding to limit the current through the clamps.

Timer

A restart timer function was added to the IC to eliminate the need for an oscillator when

used in stand-alone applications. The timer starts or restarts the TDA 4862 if the drive

output has been off for more than 15

µs after the inductor current reaches zero.