TEA2028 datasheet(32/47 Pages) STMICROELECTRONICS

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µA

-1

→ I

IDC

→ I

∆V

VREF

(1.26V)

t OUT

Low-pass

Filter

Figure 63

-1.9

µs/mV

VIN

(Pin 9)

µs

1.26V

9.5mV

5.2mV

(Pin 7)

Figure 64 : Conduction Periode (Pin 7) versus

Input Voltage (Pin 9)

µs

t ON

µs (max.)

135V

SECONDARY

WINDING

VOLTAGE

SECONDARY

RECTIFIER DIODE

CURRENT

PRIMARY CURRENT

PRIMARY PULSES

V13

Figure 65

V.7.2 - General operating principles

A fraction of the 135V output voltage to be regu-

lated is compared to the 1.26V reference voltage.

Resulting error signal is amplified and then applied

to phase modulator ”M1”, which will deliver a

square waveform at line frequency whose duty

cycle depends on the value of input voltage ”V9”.

A second phase modulator ”M2” will determine the

conduction period as a function of voltage on

Pin 15. This function is mandatory for system start-

up.

A28

µs window is used to limit the conduction

period of the primary-connected transistor.

Supply output (Pin 7) and line output (Pin 10) will

be disabled if any information indicating abnormal

operation is applied to safety input (Pin 28). Con-

sequently, all power stages are disabled and the

TV set is thus protected.

V.7.3 - Electrical characteristics of the internal

regulation loop

The phase modulator implemented by a simple

transistor ”T1” will compare in current mode, the

image of amplified input (i1) with saw-tooth current

(i2) at line frequency. With ”i2” rising, as soon as the

sum of ”i1 +i2 -IDC” goes positive, the transistor

enters into saturation thus determining the output

conduction period.

A low-pass filter implemented by combination of a

100pF capacitor and the input impedance of tran-

sistor ”T1”, attenuat es all frequency variations

higher than the line frequency.

- Input Amplification : A

di1

dVIN

= 3.3µA/mV

- Modulator conversion gain :

dtOUT

di1

=−0.558µs/µA

- Overall gain of the internal loop :

dtOUT

dVIN

=−1.9µs/mV ×

+ j

(f0 = 15kHz)

SMPS WAVEFORMS

For discontinous mode ”flyback” configuration

The primary-connected transistor is turned-off dur-

ing the line flyback.

All interference signals due to switching and sus-

ceptible to affect the video signal will not therefore

be visible on screen.

Regulation Characteristics

The following characteristics have been measured

on a large screen and yield excellent results :

- 135V voltage regulation as a function of mains

voltage : better than 0.5% for mains voltage vari-

ations of 170VRMS to 270VRMS (P = 60W at 135V)

- 135 V voltage regulation as a function of load :

better than 0.5% for a delivered power of 35W to

120W.

This type of power supply offers the following ad-

vantages :

- Overall efficiency enhancement: better than 80%

- Reduction of interferences by synchronizationon

horizontal frequency

TEA2028 - TEA2029 APPLICATION NOTE

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