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TZA1025

Data amplifier and laser supply circuit for CD audio and video optical systems (ADALASLC)

Philips Electronics 

Philips Semiconductors

Data amplifier and laser supply circuit for CD

audio and video optical systems (ADALASLC)

Product specification

TZA1025

Automatic laser power control circuit

The ALPC stabilises the laser output power thereby

reducing the effect of ageing of the laser.

The TZA1025 automatically detects when an

N- or P-substrate monitor diode is used and selects the

correct reference voltage. A simplified diagram for the use

of an N- or P-substrate monitor diode is given in Fig.4.

The gain of the loop can be controlled (reduced) by adding

an external resistor between pins RGADJ and GND.

The loop gain then becomes:

Gloop = GALPC × Glm × Gcon × 2----5---0-----+--2---R-5---0-R---G----A---D----J-

(2)

where:

Gloop is the loop gain

GALPC is the ALPC transfer (60 A/V)

Glm is the laser-to-monitor transfer (V/A)

Gcon is the extra gain introduced when a DC-to-DC

converter is used in the loop; Gcon = 1 when no

DC-to-DC converter is used

250 is a fixed internal resistor value (Ω)

RRGADJ is the value of the external resistor (Ω).

The minimum available output current is also reduced

when an external resistor is used. The formula to

determine the minimum available output current is shown

in equation (3).

Io = Io(LASER)(max) × 2----5---0-----+--2---R-5---0-R---G----A---D----J-

(3)

where:

Io is output current (mA)

Io(LASER)(max) is the maximum laser output current (mA)

250 is a fixed internal resistor value (Ω)

RRGADJ is the value of the external resistor (Ω).

The bandwidth of the loop is determined by the external

filter capacitor CCFIL and the loop gain. The formula to

determine the bandwidth is shown in equation (4).

τ-3dB = C-----C----F---I-L-G---×--l--o-1--o--6p----⋅---1----0---6-

(4)

where:

CCFIL is the value of the capacitor (F)

Gloop is the loop gain.

The TZA1025 has a protection circuit to prevent laser

damage that can occur due to a dip of VDD(L). When a dip

occurs the output transistor (see Fig.4) will go into

saturation making it unable to supply the required laser

current. Without the protection circuit the ALPC would still

try to supply the required laser current by charging the filter

capacitor CCFIL. After the dip a fully charged capacitor

would create a large output current during the few

milliseconds it needs to discharge the capacitor to a

normal level. The protection circuit monitors the output

transistor and switches off the ALPC when saturation

occurs by discharging the capacitor. The ALPC will

automatically restart within a few milliseconds after the dip

has passed.

handbook, full pagewidVtDh D(L)

150 mV

DC-TO-DC

CONVERTER

CCFIL

VDD(L)

VDD − 150 mV

DC-TO-DC

CONVERTER

CCFIL

VDD

a. N-substrate monitor diode.

MGR519

b. P-substrate monitor diode.

Fig.4 Automatic Laser Power Control (ALPC) loop.

1998 Oct 30

5

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