TZA1025T Philips Semiconductors, TZA1025T Datasheet - Page 5

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TZA1025T

Manufacturer Part Number
TZA1025T
Description
Data amplifier and laser supply circuit for CD audio and video optical systems ADALASLC
Manufacturer
Philips Semiconductors
Datasheet

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Philips Semiconductors
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:
where:
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).
1998 Oct 30
G
I
handbook, full pagewidth
o
loop
G
G
G
G
converter is used in the loop; G
DC-to-DC converter is used
250 is a fixed internal resistor value ( )
R
Data amplifier and laser supply circuit for CD
audio and video optical systems (ADALASLC)
=
RGADJ
loop
ALPC
lm
con
I
150 mV
o(LASER)(max)
is the laser-to-monitor transfer (V/A)
=
is the extra gain introduced when a DC-to-DC
is the loop gain
is the ALPC transfer (60 A/V)
G
is the value of the external resistor ( ).
ALPC
V DD(L)
a. N-substrate monitor diode.
G
C CFIL
lm
250
------------------------------------ -
G
+
250
CONVERTER
con
R
DC-TO-DC
RGADJ
------------------------------------ -
250
con
Fig.4 Automatic Laser Power Control (ALPC) loop.
+
250
= 1 when no
R
RGADJ
(2)
(3)
V DD
5
where:
The bandwidth of the loop is determined by the external
filter capacitor C
determine the bandwidth is shown in equation (4).
where:
The TZA1025 has a protection circuit to prevent laser
damage that can occur due to a dip of V
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 C
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.
-3dB
I
I
250 is a fixed internal resistor value ( )
R
C
G
150 mV
o
o(LASER)(max)
RGADJ
CFIL
loop
is output current (mA)
=
is the loop gain.
is the value of the capacitor (F)
C
----------------------------------------- -
is the value of the external resistor ( ).
CFIL
V DD(L)
b. P-substrate monitor diode.
CFIL
G
. After the dip a fully charged capacitor
is the maximum laser output current (mA)
loop
C CFIL
CFIL
16 10
and the loop gain. The formula to
CONVERTER
6
DC-TO-DC
Product specification
DD(L)
TZA1025
V DD
MGR519
. When a dip
(4)

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