STV2000 datasheet(21/38 Pages) STMICROELECTRONICS

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STV2000

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OPERATING DESCRIPTION (continued)

2.3 PLL2

PLL2 ensures a constant position of the shaped

flyback signal in comparison with the sawtooth of

the VCO, taking into account the saturation time

Ts (see Figure 9).

Figure 9. PLL2 Timing Diagram

The phase comparator of PLL2 (phase type com-

parator) is followed by a charge pump (typical out-

put current: 0.5 mA).

The flyback input consists of an NPN transistor.

This input must be current driven. The maximum

recommended

input

current

5 mA

(see Figure 10).

Figure 10. Flyback Input Electrical Diagram

The duty cycle is fixed at 48%. For a safe start-up

operation, the initial duty cycle (after power-on re-

set) is 85% in order to avoid having too long a con-

duction period of the horizontal scanning transis-

tor.

The maximum storage time (Ts Max.) is :

0.44TH-TFLY/2).

Typically, TFLY/TH corresponds to around 20 %

which means that Ts max represents approxim

tively 34 % of TH.

2.4 Output Section

The H-drive signal is sent to the output through a

shaping stage which also controls the fixed H-

drive duty cycle (see Figure 9). In order to secure

the scanning power part operation, the output is in-

hibited in the following cases :

-when VCC is too low,

-when the ABL protection is activated,

-during the Horizontal flyback,

-when the HDrive I

2C bit control is off.

The output stage consists of a NPN bipolar tran-

sistor. Only the collector is accessible

(see Figure 11).

Figure 11.

This output stage is intended for ”reverse” base

control, where setting the output NPN in off-state

will control the power scanning transistor in off-

state.

The maximum output current is 15 mA, and the

corresponding voltage drop of the output VCEsat is

0.4 V Max.

Obviously, the power scanning transistor cannot

be directly driven by the integrated circuit. An inter-

face either bipolar or MOS type has to be added

between the circuit and the power transistor.

H Osc

Sawtooth

H Drive

1.6V

4.0V

6.4V

7/8T

1/8T

Duty Cycle

Internally

Shaped Flyback

Flyback

400

Ω

HFLY

GND 0V

20k

Ω

VCC

Hout

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