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IC-WJZ Datasheet(PDF) 7 Page - IC-Haus GmbH |
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IC-WJZ Datasheet(HTML) 7 Page - IC-Haus GmbH |
7 / 12 page iC-WJ, iC-WJZ LASER DIODE DRIVER Rev C1, Page 7/12 For the monitor diode current of 0.25 mA the resistor RSET is calculated as: RSET = CR1 ∗ V (ISET ) Iav (AMD) = 3 ∗ 1.22 V 0.25 mA = 14.6 k Ω with Electrical Characteristics No. 301 for V(ISET) and No. 108 (iC-WJZ) for current ratio CR1. Averaging control The control of the average optical laser power requires a capacitor at pin CI. This capacitor is used for aver- aging and must be adjusted to the selected pulse rep- etition frequency and the charging current preset with RSET. The ratios are linear in both cases, i.e. the ca- pacitor CI must be increased in size proportionally as the pulse repetition frequency slows or the current from ISET increases: CI ≥ 440 ∗ I(ISET ) f ∗ V (ISET ) = 440 f ∗ RSET Example Pulse repetition frequency 100 kHz, RSET = 10 kΩ: CI = 440 nF, chosen 470 nF. Otherwise the charging of the capacitor CI during the pulse pauses (with I(ISET) = 1.22 V / RSET) will create an excessive mean value potential and may destroy the laser diode during the next pulse. The capacitor CI is correctly dimensioned when the current through the laser diode and the optical output signal do not show any overshots following the rising edge. I(AMD) V(CI) V(IN) Time 2.552 V 2.550 V 0 A 250 uA 0 V 5.0 V Figure 3: Steady-state averaging control, f(IN) = 100 kHz (1:1), CI = 470 nF, RSET = 10 kΩ In steady-state condition and for a pulse duty factor of 50 % (pulse/pause 1:1), signals as shown in Figure 3 are present at the IC pins. Figure 4 shows the corresponding signals for a pulse duty factor of 20 %. The influence of the pulse duty factor on the peak value of the monitor current pro- portional to the laser current is apparent. The average kept constant by the control (RSET unchanged) means a peak value increased by the factor 2.5. The pulse duty factor for which RSET was dimensioned should therefore be kept constant if possible. Time I(AMD) 600 uA 0 A 3.120 V 3.118 V V(IN) 5.0 V 0 V V(CI) Figure 4: Steady-state averaging, f(IN) = 100 kHz (1:4), CI = 470 nF, RSET = 10 kΩ 0 s 2 ms 4 ms 6 ms 8 ms 10 ms 12 ms I(KLD) 0 A V(CI) 3.0 V 2.0 V 1.0 V 0 V Figure 5: Turn-on behavior, f(IN) = 100 kHz (1:1), CI = 470 nF, RSET = 10 kΩ Turn-on and turn-off behavior Capacitor CI also determines the starting time from switching on the supply voltage VCC to steady-state laser pulse operation or after a discharge of CI by the watchdog. The following applies to estimating the starting time (Figure 5): Ton = 2.5 V ∗ CI I(ISET ) = 2.5 V ∗ CI ∗ RSET 1.22 V |
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