Linear Technology designed its new LT3905 fixed-frequency, current-mode, step-up dc-dc converter to bias avalanche photodiodes (APDs) in optical receivers. High-side APD current monitoring features over five decades of dynamic range and better than 2% relative accuracy over a 3-µA to 3-mA range. A single resistor programs maximum APD current, and a fast current limiter with indicator protects the APD during overload conditions. Adjustable output voltage delivers dynamic bias control and a programmable loss-of-signal indicator flags low APD current. An internal 65-V, 350-mA high-efficiency switch offers output voltages up to 60 V. Meanwhile, the converter’s 2.7- to 12-V input voltage range accommodates a wide range of input sources. Switching frequency is user-selectable at 1 or 2 MHz. The single IC, which integrates a Schottky diode, comes in a 3- by 3-mm QFN package. An industrial-grade version, the LT3905IUD, operates from −40 to 125°C.