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TZA1026T Datasheet(PDF) 6 Page - NXP Semiconductors |
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TZA1026T Datasheet(HTML) 6 Page - NXP Semiconductors |
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6 / 20 page  1998 Oct 30 6 Philips Semiconductors Product specification Data amplifier and laser supply circuit for CD audio and video optical systems (ADALASAuto) TZA1026 Automatic laser power control circuit The ALPC stabilises the laser output power thereby reducing the effect of ageing of the laser. The TZA1026 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: (2) where: Gloop is the loop gain GALPC is the ALPC transfer (60 A/V) Glm is the laser-to-monitor transfer (V/A) Gcon is the extra gain introduced when a DC-to-DC converter is used in the loop; Gcon = 1 when no DC-to-DC converter is used 250 is a fixed internal resistor value ( Ω) RRGADJ is the value of the external resistor (Ω). 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). (3) G loop G ALPC G lm × G con × 250 250 R RGADJ + ------------------------------------- × = I o I o(LASER)(max) 250 250 R RGADJ + ------------------------------------- × = where: Io is output current (mA) Io(LASER)(max) is the maximum laser output current (mA) 250 is a fixed internal resistor value ( Ω) RRGADJ is the value of the external resistor (Ω). The bandwidth of the loop is determined by the external filter capacitor CCFIL and the loop gain. The formula to determine the bandwidth is shown in equation (4). (4) where: CCFIL is the value of the capacitor (F) Gloop is the loop gain. The TZA1026 has a protection circuit to prevent laser damage that can occur due to a dip of VDD(L). When a dip 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 CCFIL. After the dip a fully charged capacitor 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. τ -3dB C CFIL 16 10 6 ⋅ × G loop ------------------------------------------ = Fig.4 Automatic Laser Power Control (ALPC) loop. a. N-substrate monitor diode. b. P-substrate monitor diode. handbook, full pagewidth MGR519 DC-TO-DC CONVERTER CCFIL CCFIL VDD(L) 150 mV DC-TO-DC CONVERTER VDD(L) VDD VDD − 150 mV |
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