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TDA1300T

Photodetector amplifiers and laser supplies

Philips Electronics 

Philips Semiconductors

Photodetector amplifiers and laser

supplies

Preliminary specification

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:

-∆----I∆--o--t(--L--) ≅ 1---C-0---–L---6- (A/s)

– After switch-on it ensures that the bandwidth equals

BP ≅ K------×-----G--C---e--L-x--t-×--×---I--m9---0-o---n×---1----0---–--9- (Hz)

in case of P-sub monitor or

BN ≅ R-----AC---D--L--J---n- K × Gext × 870×10–9 (Hz)

in case of N-sub monitor, where

Gext represents the AC gain of an extra loop amplifier,

if applied, and K = ∆Imon/∆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 kΩ (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 kΩ (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.

1997 Jul 15

5

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