The VSC7938 is a single 3.3V or 5V supply laser diode driver specially designed for SDH/SONET applications up to 3.125Gb/s. External resistors set a wide range of bias and modulation currents for driving the laser. Data and clock inputs accept differential PECL signals. The automatic power control (APC) loop maintains a constant average optical power over temperature and lifetime. The dominant pole of the APC loop can be controlled with an external capacitor. Other features include enable/disable control, short-circuit protection for the modulation and bias inputs, short rise and fall times, programmable slow-start circuit to set laser turn-on delay, and failure-monitor output to indicate when the APC loop is unable to maintain the average optical power. The VSC7938 is available in die form or in a 48-pin TQFP package.