Linear Technology LT3752/LT3752-1 synchronous forward controller

The Linear Technology LT3752/LT3752-1 is a high-input, voltage-capable synchronous forward controller with active clamp transformer reset. A controlled VOUT start-up and shutdown is maintained using an integrated housekeeping controller to bias the primary and secondary ICs.

The LT3752 operates over a 6.5 to 100 V input voltage range. For higher inputs, the LT3752-1 allows RC start-up from the input voltage with the maximum voltage limited only by external components, making it suitable for HV/HEV automotive applications.

An output voltage regulation of ±5% can be attained without the use of an optocoupler. When an optocoupler is used, ±1.5% regulation can be realised. The LT3752/-1 sends a control signal via a pulse transformer to a secondary-side MOSFET driver for the synchronous rectification timing. It can be used in self-driven applications where the secondary-side MOSFETs are controlled by the power transformer pulses. With the LT3752/-1, secondary side ICs no longer require start-up circuitry to operate when the output voltage is 0 V.

The LT3752/-1 contains a programmable volt-second clamp which provides a duty-cycle guardrail to limit primary switch reset voltage and prevent transformer saturation during load transients. The clamp also limits maximum VOUT if the opto path breaks open or defines VOUT in ‘no-opto’ applications. For non-isolated high step-down ratios applications, each IC contains a voltage-error amplifier.

Additional features include programmable overcurrent protection; adjustable input undervoltage and overvoltage lockout; and built-in thermal shutdown. The LT3752/-1 has a programmable 100 to 500 kHz operating switching frequency and can be synchronised to an external clock, allowing the use of a wide range of output inductor values and transformer sizes.