TDA1302_1
6 Wed Sep 14 13:30:56 1994
Philips Semiconductors
Preliminary specification
Data amplifier and laser supply circuit for
CD player and read only optical systems
FUNCTIONAL DESCRIPTION
The TDA1302T can be divided into two main sections, the
laser control circuit and the photo diode signal filter and
amplification section.
Laser control circuit
The main function of the laser control circuit is to control
the laser diode current in order to achieve a constant light
output power which is based on the current of the monitor
diode which is continuously monitored. The circuit is built
up into three parts.
The first part is the input stage which compares the
monitor diode current with a current which is 10 times the
value of the adjustable current. The adjustable current is
derived from a bandgap reference source, to be
temperature independent, and can be further adjusted by
the external resistor R
ADJ
in order to adapt the circuit to the
parameters of the laser/monitor diode unit to be used. The
difference is fed to the second part.
The second part is the integrator stage which makes use
of an external capacitor CL. This capacitor has two
different functions.
During switch-on of the laser current, it provides a current
slope of typically: dI
LO
/dt
≅
10
−6
/CL (A/s).
After switch-on it ensures that the bandwidth conforms to
the typical formula: f
B
≅
K
×
A
ext
×
90
−9
/(CL
×
I
MON
) (Hz).
where A
ext
represents the AC gain of an extra loop
amplifier, if applied, and K = dI
monitor
/dI
laser
which is
determined by the laser/monitor unit.
I
MON
is the average current (pin 17) at typical light
emission power of the laser diode.
TDA1302T
The third part is the power output stage, its input being the
integrator output signal. This stage has a separate supply
voltage (V
DDL
) thereby offering the possibility of reduced
power consumption by supplying this pin with the minimum
voltage necessary.
It also has a laser diode protection circuit which is enabled
prior to the output drive transistor becoming saturated due
to a large voltage dip on V
DDL
. Saturation will result in a
lower current from the laser diode, which is normally
followed immediately by an increment of the voltage from
the external capacitor CL, which could cause damage to
the laser diode at the end of the voltage dip. The protection
circuit prevents an increment of the capacitor voltage and
thus offers full protection to the laser diode under these
circumstances.
Photo diode signal filter and amplification section
This section has 6 identical current amplifiers. Amplifiers 1
to 4 are designed to amplify the focus photo diode signals.
Each amplifier has two outputs, an LF output and an
internal RF output. Amplifiers 5 and 6 are used for the
radial photo diode currents and have only an LF output. All
6 output signals are low-pass filtered with a corner
frequency at 65 kHz. The internal RF output signals are
summed together and converted into a voltage by means
of a selectable transresistance of 120 kΩ or 240 kΩ. This
signal is available directly at pin 10, however, there is also
an unfiltered signal available at pin 9. The equalization
filter used has 2 different filter curves, one for single-speed
mode and one for double-speed mode.
Table 1
Operational modes.
IF NOT
CONNECTED
DEFAULT
1
1
1
MODE
(1)
GAIN
HIGH
1
X
X
LOW
0
X
X
SPEED
SINGLE
X
1
X
DOUBLE
X
0
X
ON
X
X
1
LASER
OFF
X
X
0
SWITCH
PIN
HG
Control pin
LS
LDON
Note
1. Where X = don’t care.
September 1994
6
PHILIPS [ NXP SEMICONDUCTORS ]
