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TDA1300TT Datasheet(PDF) 5 Page - NXP Semiconductors |
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TDA1300TT Datasheet(HTML) 5 Page - NXP Semiconductors |
5 / 20 page 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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