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MAX3766EEP Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX3766EEP Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 20 page Automatic Power Control Transmitters employing a laser with monitor photodiode can use the APC circuit to maintain constant power, regardless of laser threshold changes due to temperature and aging. The APC circuit consists of the POWERSET current mirror and the monitor diode amplifier. The POWERSET current mirror provides an accurate method of programming the back facet monitor photo- diode current, which is assumed to be proportional to laser output power. An external resistor from REF1 to POWERSET programs the current in the unity-gain cur- rent mirror. RPOWERSET can be estimated as follows: The monitor-diode amplifier senses the current from the monitor photodiode at MD, provides gain, and adjusts the laser bias programming current (ILBP). The monitor- diode amplifier forces the monitor-diode current to equal the current programmed at POWERSET. The monitor-diode amplifier can reduce the laser bias pro- gramming current, but cannot increase it. Therefore, the APC circuit can adjust laser bias current between 0 and the setting determined by RBIASMAX. When the APC feedback loop is closed, the voltage at MD is approximately 2V below VCC. If the loop cannot close due to excess or insufficient photocurrent, a fail- ure is detected by the failure-detection circuit. Internal circuitry prevents the voltage at MD from dropping below VCC - 3.2V. The stability and time constant of the APC feedback loop is determined by an external compensation capacitor (CMD) of at least 0.1µF. Connect the com- pensation capacitor from VCC to MD, as shown in Typical Application Circuits, to ensure a smooth start- up at power-on or transmitter enable. If a monitor diode is not available, the APC feature can be disabled by connecting RPOWERSET to GND and leaving MD unconnected. Failure Detection Figure 3 shows a simplified schematic of the failure- detection circuit. The failure-detection circuit senses two conditions. First, if the APC control loop cannot control the monitor current due to laser undercurrent, overcurrent, or a fault condition, a window comparator detects that VMD is above or below VCC - 2V and asserts the failure signal. Second, if REF1 is shorted to the positive supply (or any another voltage above the normal operating level), a comparator detects this con- dition and asserts the failure signal. If left undetected, the reference voltage would rise, the current at POWER- SET would increase, and the APC loop would attempt to add laser current beyond the intended value. Either failure condition causes the FAIL output to assert TTL low. The FAIL output buffer is an open-collector output and is designed to operate with a 5.1k Ω external pull-up resistor. Safety/Start-Up Circuit The safety circuit includes the digital logic needed to provide a latched internal shutdown signal (SHDN) for disabling the laser if a failure condition exists. The MAX3766 produces less than 20µA of total laser cur- rent when disabled by safety features or by the ENABLE input. Figure 4 is a simplified schematic of the safety circuit. If ENABLE is low or open, the laser bias and modula- tion outputs are disabled by SHDN, regardless of the state of the safety logic. The TTL-compatible ENABLE input is internally pulled low with a 100k Ω resistor. There are two useful safety configurations: failure indication and latched shutdown. Failure-Indication Configuration Select the failure-indication configuration by connecting SAFETY to ground. In this configuration, a failure condi- tion is reported at FAIL, but does not cause a latched shutdown. This configuration requires no additional cir- cuitry for start-up. R = 1.55V I POWERSET MOD − 300Ω 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown _______________________________________________________________________________________ 9 VMD FAILURE (INTERNAL) 200mV 200mV 2V VCC 0.5V VREF1 VBANDGAP Figure 3. Failure-Detection Circuit (Simplified) |
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