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TZA1024 Datasheet(PDF) 6 Page - NXP Semiconductors

Part No. TZA1024
Description  Data amplifier and laser supply circuit for CD audio and video optical systems ADALAS
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Maker  PHILIPS [NXP Semiconductors]
Homepage  http://www.nxp.com
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TZA1024 Datasheet(HTML) 6 Page - NXP Semiconductors

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1998 Oct 30
6
Philips Semiconductors
Product specification
Data amplifier and laser supply circuit for CD
audio and video optical systems (ADALAS)
TZA1024
Automatic laser power control circuit
The ALPC stabilises the laser output power thereby
reducing the effect of ageing of the laser.
The TZA1024 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:
(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).
(3)
G
loop
G
ALPC
G
lm
×
G
con
×
250
250
R
RGADJ
+
-------------------------------------
×
=
I
o
I
o(LASER)(max)
250
250
R
RGADJ
+
-------------------------------------
×
=
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).
(4)
where:
CCFIL is the value of the capacitor (F)
Gloop is the loop gain.
The TZA1024 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.
τ
-3dB
C
CFIL
16 10
6
×
G
loop
------------------------------------------
=
Fig.4 Automatic Laser Power Control (ALPC) loop.
a. N-substrate monitor diode.
b. P-substrate monitor diode.
handbook, full pagewidth
MGR519
DC-TO-DC
CONVERTER
CCFIL
CCFIL
VDD(L)
150 mV
DC-TO-DC
CONVERTER
VDD(L)
VDD
VDD − 150 mV


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