TEA2028 datasheet(33/47 Pages) STMICROELECTRONICS

33 / 47 page

- Full protection of the primary-connected transis-

tor in case of short-circuit or open-load on secon-

dary terminals

- Can provide 1W to 7W, for TV standby mode

operation (refer to TEA2164 application note).

V.7.4 - Power supply soft-start

When the TV set is initially turned on, control pulses

are not yet available and consequently the control-

ler block on primary side will impose a low-power

transfer to the secondary winding. This power is

produced by an intermittent switching mode called

”Burst Mode”.

As soon as the VCC supply to TEA2028B exceeds

6V level, line and SMPS outputs are enabled. Since

the filtering capactitors on secondary side cannot

charge up instantaneously, the voltage to be regu-

lated would not yet be at its nominal value. Without

conduction period limitation upon start-up, the de-

vice will set a maximum cycle of 28

µs which will

result in a high current flow through the primary

winding and thus through the switching transistor

which will in turn activate the protection function

implemented on primary side.

Consequently, the primary controller block will be

inhibited and the set will not turn-on.

A start-up system has been implemented within

TEA2028B to overcome this problem.

This soft start system, will upon initial start-up, use

the image of the falling voltage on Pin 15 to in-

crease progressively the conduction cycle. The

phase modulator ”M2” compares this voltage with

line saw-tooth voltage and delivers the correspond-

ing limitation cycle.

During supply voltage rising cycle [VCC (Pin 8)

< 6V], the capacitor Pin 15 will charge up rapidly

while the voltage across it follows VCC.

At VCC

≥ 6V, the capacitor is discharged via an

internal current generator and the voltage across it

decays linearly.

At V15

≤ 3.5V (line saw-tooth peak-to-peak volt-

age), phase comparator ”M2” delivers a low con-

duction period which will gradually increase.

The conduction period (Pin 7) will rise until the

secondary voltage reaches the value set by poten-

tiometer ”P”. When this occurs, the loop is acti-

vated.

The Pin 15 discharge current value is 100

µAfor a

duration of 2

µs line frequency.

Therefore ID(AV)

= 100 ×

= 3.1µA

Conduction period limitation voltage (Pin 15)

TON(LIM) =56

µs- 16 x V15 (in µs)

5.5V

12V

3.5

VCC

Soft-start area

Regulated Mode

Active area

PIN 8 SUPPLY

VOLTAGE (V)

PIN 15

VOLTAGE (V)

SMPS CONTROL

OUTPUT VOLTAGE

(PIN 7)

LINE OUTPUT

VOLTAGE (PIN 10)*

* Lin e output (Pin 10) an d Thyristo rco ntrol output (Pin 4) for TEA2029C

Figure 66

TEA2028 - TEA2029 APPLICATION NOTE

33/46

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