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MAX3737EGJ Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX3737EGJ Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page 100 Ω resistor to ground at each monitor output gives the following relationships: VMC_MON = (IMOD / 268) ✕ 100 Ω VBC_MON = (IBIAS / 82) ✕ 100 Ω VPC_MON = IMD ✕ 100 Ω External sense resistors can be used for high-accuracy measurement of bias and photodiode currents. On-chip isolation resistors are included to reduce the number of components needed to implement this function. Design Procedure When designing a laser transmitter, the optical output is usually expressed in terms of average power and extinction ratio. Table 3 gives relationships that are helpful in converting between the optical average power and the modulation current. These relationships are valid if the mark density and duty cycle of the opti- cal waveform are 50%. For a desired laser average optical power (PAVG) and optical extinction ratio (re), the required bias and modula- tion currents can be calculated using the equations in Table 3. Proper setting of these currents requires knowl- edge of the laser to monitor transfer ( ρMON) and slope efficiency ( η). Programming the Monitor Diode Current Set Point The MAX3737 operates in APC mode at all times. The bias current is automatically set so average laser power is determined by the APCSET resistor: The APCSET pin controls the set point for the monitor- diode current. An internal current regulator establishes the APCSET current in the same manner as the MODSET pin. See the IMD vs. RAPCSET graph in the Typical Operating Characteristics and select the value of RAPC- SET that corresponds to the required current at +25°C: The laser driver automatically adjusts the bias to maintain the constant average power. For DC-coupled laser diodes: Programming the Modulation Current with Compensation Determine the modulation current from the laser slope efficiency: The modulation current of the MAX3737 consists of a static modulation current (IMODS), a current proportional to IBIAS, and a current proportional to temperature. The portion of IMOD set by MODSET is established by an internal current regulator, which maintains the reference voltage of VREF across the external programming resis- tor. See to the IMOD vs. RMODSET graph in the Typical Operating Characteristics and select the value of RMOD- SET that corresponds to the required current at +25°C: I Pr r MOD AVG e e =× × + 2 1 1 η - II I AVG BIAS MOD =+ 2 I V R MD REF APCSET =× 1 2 P I AVG MD MON = ρ Multirate Laser Driver with Extinction Ratio Control ______________________________________________________________________________________ 11 PARAMETER SYMBOL RELATION Average power PAVG PAVG = (P0 + P1) / 2 Extinction ratio re re = P1 / P0 Optical power of a 1 P1 P1 = 2PAVG re / (re + 1) Optical power of a zero P0 P0 = 2PAVG / (re + 1) Optical amplitude PP-P PP-P = P1 - P0 Laser slope efficiency ηη = PP-P / IMOD Modulation current IMOD IMOD = PP-P / η Threshold current ITH P0 at I ≥ ITH Bias current (AC-coupled) IBIAS IBIAS ≥ ITH + IMOD / 2 Laser to monitor transfer ρMON IMD / PAVG Table 3. Optical Power Relations Note: Assuming a 50% average input duty cycle and mark density. |
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