RAMP

GENERATOR

IC

22nF

22nF

10

Ω

470

Ω

V6

6

V5

5

V3

31

42

7

8

9

10

11

12

13

14

TEA2019

0V

-1V

AS1

10nF

10nF

10k

Ω

8.2k

Ω

4.7

µF

4.7

µF

V12

V14

47nF

1.5nF

3.3nF

22nF

3.9k

Ω

56k

Ω

1%

59k

Ω

V10

100

Ω

TYPICAL CIRCUIT

GENERAL DESCRIPTION

(see application note AN406/0591)

Operating Principles (Figure 1)

On every period, the beginning of the conduction

time of the transistor is triggered by the fall of the

oscillator saw-tooth which acts as clock signal. The

Ω)

The end of the conduction time is determined by a

signal issued from comparing the following signals.

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).

=

Figure 1 : Current Mode Control

C

I

SENSE

OSCILLATOR

ERROR

SIGNAL

ERROR

AMPLIFIER

COMPARATOR

S

R

Q

FLIP-FLOP

V

REF

I

C

V

i

OUTPUT

FILTER

Re

t

t

t

Error

Signal

OSCILLATOR

SAWTOOTH

I

(sample)

FLIP-FLOP

OUTPUT

C

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