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TDA1300T 데이터시트(Datasheet) 5 Page - NXP Semiconductors

부품명 TDA1300T
상세내용  Photodetector amplifiers and laser supplies
PDF  20 Pages
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제조사  PHILIPS [NXP Semiconductors]
홈페이지  http://www.nxp.com
Logo 

 
 5 page
background image
1997 Jul 15
5
Philips Semiconductors
Preliminary specification
Photodetector amplifiers and laser
supplies
TDA1300T; TDA1300TT
FUNCTIONAL DESCRIPTION
The TDA1300T; TDA1300TT can be divided into two main
sections:
• Laser control circuit section
• Photo diode signal filter and amplification section.
Laser control circuit section
The main function of the laser control circuit is to control
the laser diode current in order to achieve a constant light
output power. This is done by monitoring the monitor
diode. There is a fixed relation between light output power
of the laser and the current of the monitor diode. The circuit
can handle P-sub or N-sub monitor diodes.
N-sub MONITOR
In this event pin 14 (ADJ) must be connected to the
positive supply voltage VDD to select the N-sub mode. With
an adjustable resistor (RADJn) across the diode the monitor
current can be adjusted (and so the laser light output
power) if one knows that the control circuit keeps the
monitor voltage Vmon at a constant level of
approximately 150 mV.
P-sub MONITOR
In this event pin 14 (ADJ) is connected via resistor RADJp
to ground. The P-sub mode is selected and pin 14 (ADJ)
acts as reference band gap voltage, providing together
with RADJp an adjustable current lADJ. Now the control
circuit keeps the monitor current at a level which is 10lADJ.
The circuit is built up in three parts:
• The first part is the input stage which is able to switch
between both modes (N-sub or P-sub).
• The second part is the integrator part 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:
(A/s)
– After switch-on it ensures that the bandwidth equals
(Hz)
in case of P-sub monitor or
∆I
o(L)
t
--------------
10
6
C
L
-----------
B
P
KG
ext
×
90
9
×10
×
C
L
I
mon
×
--------------------------------------------------
(Hz)
in case of N-sub monitor, where
Gext represents the AC gain of an extra loop amplifier,
if applied, and K =
∆I
mon/∆IL which is determined by
the laser/monitor unit. Imon is the average current
(pin 17) at typical light emission power of the laser
diode.
• The third part is the power output stage, its input being
the integrator output signal. This stage has a separate
supply voltage (VDDL), 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 comes into
action just before the driving output transistor will get
saturated due to a large voltage dip on VDDL. Saturation
will result in a lower current of the laser diode, which
normally is followed immediately by an increment of the
voltage of the external capacitor CL. This could cause
damage to the laser diode at the end of the 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 only have
an LF output. All 6 output signals are low-pass filtered with
a corner frequency at 69 kHz. The internal RF output
signals are summed together and converted to a voltage
afterwards by means of a selectable transresistance.
This transresistance RRF can be changed between 140 kΩ
(3.3 V application) or 240 k
Ω (5 V application) in
combination with the P-sub monitor. In the event of the
N-sub monitor selection, RRF can be changed between
70 k
Ω (3.3 V application) and 120 kΩ (5 V application).
The RF signal is available directly at pin 10 but there is
also an unfiltered signal available at pin 9.
The used equalization filter has 2 different filter curves:
• One for single-speed mode
• One for double-speed mode.
B
N
R
ADJn
C
L
---------------- KG
ext
×
870
9
×10
×




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