GENERAL DESCRIPTION

(see application note AN-086)

Operating Principles (Figure 1)

On every period, the beginning of the conduction

time of the transistor is triggered by the fall of the

oscillator sawtooth which acts as clock signal. The

Ω)

The end of the conduction time is determined by a

signal issued from comparing the following sig-

nals :

a) the sawtooth waveform representing the

collector current of the switching transistor,

sampled across the emitter shunt resistor,

b) the output of the error amplifier.

Base Drive

- Fast turn-on : On each period, a current pulse

ensures fast transistor switch-on.

the beginning of the conduction.

- Proportional base drive : In order to save power,

the positive base current after the starting pulse

becomes an image of the collector current.

The ratio

is programmed as follows Figure 2) :

=

- Efficient and fast switch-off : When the positive

base drive is removed, 1ms (typically) will elapse

before the application of negative current there-

fore allowing a safe and rapid collector current

fall.

Safety Functions

- Overload & short-circuit protection : When the

voltage applied to pin 3 exceeds the current

flip-flop is reset and the transistor is turned off.

obtain the current limitation threshold on pin 3 at

the maximum allowable collector current.

- Demagnetization sensing : This function disables

any new conduction cycle of the transistor as long

as the core is not completely demagnetized.

When not used, pin 7 must be grounded.

absence of current limitation, the maximum con-

duction time is set at about 70 % of the period.

during each period (see Figure 2)

- Supply voltage monitoring : The TEA2018A will

OSCILLATOR

SAW-TOOTH

Error signal

I

(sample)

C

t

FLIP-FLOP

OUTPUT

t

t

TEST CIRCUIT

C

I

SENSE

OS CILLATOR

ERROR

SIGNAL

ERROR

AMPLIFIER

COMPARATO R

S

R

Q

FLIP-FLOP

I

C

OUTPUT

FILTER

LO AD

Re

TEA2018A

4/7