The two variable resistors in DS3902 are used to set bias and modulation currents. The settings are done through I2C* interface. Some modules may require additional EEPROM. This is typically used for serial ID information, and new modules may need to include this feature. DS3902 has programmable address, therefore connecting it to a single I2C bus line (with other devices) without any additional components.

Figure 1 shows connection details for using DS3902 and a serial EEPROM (ATmel AT24C02) on a common I2C interface. Also Figure1 illustrates connections to a laser driver.

Figure1. Schematic for LD, DS3902, and AT24C02.

The DS3902’s default address is A2h (Add_sel = 0). If an address different from A2h is required, Add_sel will be pulled high. Register 00h content is the device address when Add_sel = 1. In the above schematic AT24C02 is configured for A0h address, (A0 = A1 = A2 = 0).

The WP (write-protect) pin connects to ground using a link, allowing R/W access to memory locations. Once the memory is programmed, the WP pin can be pulled high through LK1, to prevent accidental write. DS3902 has S/W protection scheme, whereby access to memory is only possible with password.

The choice of laser driver depends on the specific application and there are a number of Maxim laser drivers to choose from.