42-VIN primary side regulation flyback controller with low IQ and low EMI

The LM25185 is a primary-side regulated (PSR) flyback controller with high efficiency over a wide input voltage range of 4.5 V to 42 V. The isolated output voltage is sampled from the primary-side flyback voltage, eliminating the need for an optocoupler, voltage reference, or third winding from the transformer for output voltage regulation.

The high level of integration results in a simple, reliable, and high-density design with only a transformer crossing the isolation barrier. Boundary conduction mode (BCM) switching enables a compact magnetic design and better than ±1.5% load and line regulation performance. BCM supports applications of up to 42-V input with enhanced headroom for line transients.

The LM25185 flyback controller simplifies implementation of isolated DC/DC supplies with optional features to optimize performance for the target end equipment. The output voltage is set by one resistor, while an optional resistor improves output voltage accuracy by negating the thermal coefficient of the flyback diode voltage drop. The LM25185 controller, with the integrated high-current MOSFET gate driver, extends the power range from less than 1 W to more than 50 W, making the device a scalable and easy-to-migrate design over a wide power range. The tunable compensation using the COMP pin and the adjustable peak current limit with the current-sensing resistor make the design flexible and an excellent choice towards different application requirements.

Additional features include an internally-fixed or externally-programmable soft start, optional external VCC for higher efficiency, precision enable input with hysteresis for adjustable line UVLO, peak current-mode control with shunt current sensing, hiccup-mode overload protection, a high-current MOSFET gate driver, and thermal shutdown protection with automatic recovery. A connection to the high-voltage MOSFET drain is not required, thus increasing safety and simplifying PCB clearance distance requirements.

The LM25185 flyback controller is available in a 14-pin, 5-mm × 4.4-mm, thermally-enhanced HTSSOP package with 0.65-mm pin pitch.