Secondary side drivers protect power MOSFETs in isolated DC/DC converters

Milpitas, CA – December 15, 2003 – Linear Technology Corporation introduces two secondary side synchronous N-channel MOSFET drivers, the LTC3900 and LTC3901. Both devices have timer and current sense comparators to protect the external MOSFETs during normal and light load operation, power up/down, and when the driver supply voltage drops too low.

A programmable timeout function disables the synchronous drivers when the synchronization signal between the primary and secondary sides is missing or incorrect. This protects the MOSFETs from overvoltage and electrical overstress. Additionally, the ICs turn off the MOSFETs if the negative inductor current exceeds a user-programmable level, thereby preventing the buildup of excessive inductor energy and possible damage. Moreover, to ensure proper power up and power down, these drivers shut off the MOSFETs when the driver input voltage is below 4V.

The LTC3900 is designed for use in forward converter topologies and the LTC3901 is suited for use in push-pull and full-bridge converters. Applications include 48V input isolated DC/DC converters, isolated telecom power supplies, industrial control and automotive equipment.

The LTC3900 and LTC3901 are rectifier drivers that drive two N-channel MOSFETs and accept pulse synchronization from the primary side controller via a pulse transformer. The timer circuit and VDS current sense comparator in these ICs are used to prevent excessive reverse current buildup in the output inductor. The timer, which is adjustable, operates using an external RC network to program the time-out period. Strong synchronous gate drivers guarantee 10ns rise/fall times measured at VCC=5V with 4700pF capacitive load. Synchronous operation improves efficiency when compared to solutions using diodes as secondary side rectification. On every negative transition at the SYNC input, the ICs generate a 200ns pulse to reset the timer cap. VCC supply range is from 4.5V to 11V.

The LTC3900 is offered in an 8-lead SO package. The LTC3901 is housed in a 16-lead SSOP. Both are rated for operation from -40?C to 85?C. The LTC3900 and LTC3901 1,000-piece prices are $2.50 and $2.60, respectively.

Linear Technology Corporation, 1630 McCarthy Boulevard, Milpitas, CA 95035-7417. Tel: 408-432-1900

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