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MAX3967D Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX3967D Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 14 page Detailed Description The MAX3967 provides a flexible current drive for the modulation of fiber optic light-emitting diodes (LEDs). The circuit is designed to be used with +3.3V or +5V power supplies. The IC provides up to 100mA of modu- lation current. An adjustable prebias current source sets the LED prebias voltage. An integrated resistor provides passive peaking and optical pulse-width com- pensation. Figure 2 shows a block diagram of the MAX3967, which comprises a reference-voltage generator, modulation- current generator, input buffer with disable, prebias- current generator, main output driver, complementary output driver, and LED-compensation network. Temperature Compensation The reference-voltage generator circuit provides two voltage sources that create modulation-current temper- ature compensation. A positive modulation-current tem- perature coefficient (tempco) is useful to compensate for the temperature characteristics of typical fiber optic LEDs. The first source has a temperature-stable output. The second source has a temperature-increasing out- put with a tempco of approximately 12,000ppm/°C (rel- ative to +25°C). A resistor-divider between the two reference generators programs the modulation-current tempco. For maximum modulation-current tempco, leave the TC pin disconnected. For a tempco of approximately 3600ppm/°C, connect TC to TCNOM. To obtain the minimum tempco, connect TCMIN to TC. Intermediate tempco values can be programmed by connecting an external resistor (RTC) between TCMIN and TC. Input Buffer The inputs are connected to the PECL-compatible dif- ferential input buffer. If left unconnected, IN+ is internal- ly pulled to a PECL low and IN- is pulled to a PECL high, causing low current at OUT+. The input imped- ance of IN+ and IN- is approximately 50k Ω. 270Mbps SFP LED Driver 6 _______________________________________________________________________________________ PIN NAME FUNCTION 1 TCNOM Shorting TC to TCNOM provides a modulation tempco of approximately 3600 ppm/°C. 2TC A resistor (RTC) connected between the TC and TCMIN pins sets the tempco of the modulation current. Leaving RTC unconnected provides the maximum tempco. 3, 4, 5 PB1, PB2, PB3 Programs the Prebias Voltage at the OUT+ Pin (Table 1) 6, 7 VEEOUT Ground for the Output-Current Drivers 8, 9 OUT+ Current Output Pins 10, 11 OUT- Complementary Current Output Pins 12, 16 N.C. Not Connected 13, 14 VCCOUT Supply Connection for the Output-Current Drivers 15, 19 VCC Provides Current to the Internal Amplifiers 17 MON The Current Sourced from the MON Pin is Proportional to the Modulator Current 18 MODSET A Resistor from MODSET to VEE Programs the LED Modulation Current 20 IN- Inverting Data Input 21 IN+ Noninverting Data Input 22 TX_DISABLE Transmit Disable. When high, the current at the OUT+ pins is in the low state. The transmitter is enabled when TX_DISABLE is open. 23 VEE Ground for internal amplifiers. 24 TCMIN Shorting TC to TCMIN provides the minimum modulation-current tempco. Pin Description |
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