OFF PHASE

CONTROL

INPUT

t

ON PHASE

t

dt

=

Figure 5

This is due to the constant base charge in the

turn-ON phase independent from the collector cur-

rent ; hence a high peak current results into a low

storage time of the transistor because the excess

base charge is a minimum and vice versa. In the

active deflection the required function, high peak

current-fast switch-OFF and low peak current-slow

switch-OFF, is obtained by a controlled base dis-

charge current for the power transistor ; the nega-

tive slope of this ramp is proportional to the actual

sensed current.

As a result, the active driving system even im-

proves the sharpness of vertical lines on the screen

compared with the traditional solution due to the

increased stability factor of the loop represented as

the variation of the storage time versus the collector

peak current.

t

t

D

I

Base Bias Current

Off Phase

On Phase

Off Phase

Real Base Current

Ideal Base Current

Figure 4 :

Waveforms of Collector and

Base Current

Figure 6 shows the block diagram of the TDA8143,

the circuit consists of an input transconductance

amplifier composed by Q1, Q2, Q3 and Q4.

The symmetrical output current is fed into the load

resistor R1 and R2 ; the two amplifiers V1 and V2

realize a floating voltage to current converter which

can drive 1.2A sink current and 2A source current

for a wide common output range.

So, the overall transconductance results into :

=

R1

R3

⋅

1

R5

across the resistor R4 which provides an output

bias current of 140mA ; the control input determines

the turn ON/OFF function.

In the ON phase, Q5 shorts the external capacitor

the element realizes the transconductance func-

tion ; lower voltages are clamped by the D1/Q6

configuration.

For input voltages higher than 750mV, Q7 limits the

maximum output current at 1.5A peak.

input voltage, by this way, the output current de-

creases quasi linearly and the system stability is

reached.

During the flyback phase, the IC is enabled via the

sync. detector input ; this function with the limited

sink and source current together with the undervol-

tage turn-OFF and a chip temperature sensor en-

sure a complete protection of the IC.

CIRCUIT DESCRIPTION

TDA8143

6/9