Linear Technology announces the LT3571, a fixed frequency, current mode step-up DC/DC converter with an integrated high-side avalanche photodiode (APD) current monitor. The device can deliver output voltages up to 75V from inputs ranging from 2.7V to 20V, making it ideal for a wide array of fiber optic applications. The LT3571 features a high-side fixed voltage drop APD current monitor, providing better than 10% relative accuracy over the -40°C to 125°C temperature range. The fixed 5V (±5%) drop across the current monitor is unique to the LT3571 and enables higher data bandwidths in many fiber optic applications. The power switch, Schottky diode and APD current monitor are integrated into a single IC that is packaged in a 3mm x 3mm QFN, providing a tiny solution footprint.
The LT3571's internal 75V, 350mA switch offers output voltages as high as 75V. Its switching frequency can be programmed between 250kHz and 2MHz, enabling the designer to optimize efficiency while minimizing the size of the external components. Constant frequency architecture is used to keep output ripple below 500uVPK-PK, which is critical in most APD applications. To further reduce noise concerns, the LT3571 is externally synchronizable. The LT3571 combines a traditional voltage loop and a unique current loop design to operate as a constant current source or constant voltage source, providing both excellent transient response as well as APD protection. It is equipped with both a 1.222V internal reference and an auxiliary reference input (via the CNTRL pin), enabling the output voltage to be adjusted while the chip is operating without any polarity inversion. Other features include soft-start, internal compensation and thermal shutdown.
The LT3571EUD is available in a 3mm x 3mm QFN-16 and is priced at $2.45 each in 1,000-piece quantities. An industrial grade version, the LT3571IUD, is tested and guaranteed to operate from a -40°C to 125°C operating junction temperature and is priced at $2.72 each in 1,000-piece quantities. Both versions are available from stock.