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IC-NZP Datasheet(PDF) 11 Page - IC-Haus GmbH |
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IC-NZP Datasheet(HTML) 11 Page - IC-Haus GmbH |
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11 / 15 page  iC-NZP P-TYPE LASER DIODE DRIVER Rev C1, Page 11/15 LASER CURRENT LIMITATION OverTemp. i(RSI)x540 100 nF.. NSLP CVDDA Overcurrent VDD-0.5V 0.68..9 kΩ 0.5V VDDA NERR 1 CVDD VDD 100 nF.. RSI RSI RSI @ VDD RSI 1 Figure 6: iC-NZP VDDA current limitation iC-NZP monitors the average laser current flowing from pin VDDA (Figure 6). The DC current limit is set by means of a resistor at pin RSI. When dimensioning resistor RSI the following applies (cf. Electrical Characteristics No. 107): Imax (VDDA) = 540 × 0.5 V RSI Short pulses at VDDA with higher currents are possi- ble as only the DC current is monitored and capacitor CVDDA supplies the current for short pulses. BURST MODE In controlled burst mode iC-NZP can pulse with up to 155 MHz. Controlled here means that a pre-set oper- ating point is maintained during the burst phase. Therefore an operating point is settled first, for which pin REGE has to be high and the laser must be switched on. Once the operating point has been reached the laser can be switched off again. The oper- ating point is stored in an on-chip capacitor and when REGE is set to low, the burst mode is activated. The pre-set operating point is maintained. To prevent the laser current from rising due to residual currents, the capacitor is discharged with a maximum of 150 nA (cf. Electrical Characteristics No. 405). For a longer burst mode, an external capacitor can be connected to pin CI. As the capacitor is discharged gradually, the out- put level must be re-settled again after a certain period, depending on the admissible degradation of the laser output power. CURRENT CONTROL The iC-NZP also supports laser current control, when no monitor diode is present. For that purpose, a frac- tion of the current flowing trough the laser is provided at IMON pin (ILDA / 280, cf. Electrical Characteristics No. 406). The laser current is set by means of resistor RMON (= RMD + PMD). RMON = V (MD) IMON Figure 7 shows the typical set-up for current control. External capacitor mode In applications where an external capacitor is required (see best performance recommendations below), the external capacitor mode must be enabled (NCID pin set to low). Best performance recommendations The operating point for the laser diode is stored in an on-chip capacitor. This permits a fast start-up but can lead to an unstable control circuit under certain condi- tions such as inadequate PCB layout or laser with very low monitor current. In that cases, an external capaci- tor is to be connected as close as possible to the chip, across pin CI and CIS. This will prevent instability of the control circuit. Setting AVG to hi, keeps the control loop always on and settled, thus improving the dynamic performance. |
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