LM5050MK-1/NOPB

The LM5050MK-1/NOPB from Texas Instruments is a high-side OR-ing FET controller designed to operate in conjunction with an external N-channel MOSFET as an ideal diode rectifier when connected in series with a power source. This OR-ing controller allows MOSFETs to replace diode rectifiers in power distribution networks, significantly reducing both power loss and voltage drops compared to Schottky diode solutions.

The controller provides a charge pump gate drive for the external N-channel MOSFET, generating a gate voltage above the source (MOSFET drain) to fully enhance the MOSFET and achieve the lowest possible RDS(on). A fast response comparator monitors the voltage across the MOSFET (VDS = VIN – VOUT) and turns off the FET within 50ns when current flows in the reverse direction (VDS polarity reverses), preventing reverse current from flowing from the output back to the input. This fast turnoff is critical in redundant power supply systems where a failed supply must be quickly isolated.

The LM5050-1 variant (as opposed to LM5050-2) provides N+1 OR-ing functionality with reverse current blocking. When the forward voltage across the MOSFET drops below the regulation point (22mV typical), the controller reduces the gate drive to regulate the voltage drop at this minimum level, maintaining the MOSFET in the linear region. When the voltage drop exceeds this regulation point, the gate drive increases until the gate-to-source voltage reaches the 12V Zener clamp level, fully enhancing the MOSFET for minimum voltage drop.

Key specifications include: operating input voltage range of 5V to 75V (with external VBIAS, operation down to 1V is possible), 100V transient capability, charge pump gate driver, 2A peak gate turnoff current for rapid MOSFET discharge during reverse current events, and OFF pin for logic-level MOSFET shutdown. The quiescent supply current is 130uA typical, drawn from the VS pin. The OFF pin, when driven HIGH, pulls the GATE pin low and turns off the external MOSFET independent of sensed voltages.

The device is available in a compact SOT-23-6 Thin (DDC) package measuring 2.90 x 1.60 mm, suitable for space-constrained power supply designs. An AEC-Q100 qualified version (LM5050Q1MK-1) is available for automotive applications with operating junction temperature up to 125C. The device is also qualified for functional safety, with documentation available to support functional safety system design. The /NOPB suffix indicates Pb-free (RoHS compliant) packaging in Digi-Reel format.

The LM5050MK-1/NOPB operates as a high-side OR-ing controller, regulating an external N-channel MOSFET to emulate an ideal diode with minimal forward voltage drop and fast reverse current blocking.

Charge Pump Gate Drive: The controller uses an internal charge pump to generate a gate drive voltage above the MOSFET source (IN pin) voltage. The VS pin provides power for the charge pump and all internal biasing. The charge pump output drives the GATE pin, which connects to the external MOSFET gate. When the MOSFET should conduct (forward current detected), the charge pump raises VGS until either the MOSFET is fully enhanced or the VDS regulation point is reached. The gate-to-IN Zener clamp limits VGS to 12V maximum to protect the MOSFET gate oxide.

Forward Conduction (Ideal Diode Mode): When forward current flows through the MOSFET (VIN > VOUT), the controller senses a positive VDS voltage (IN pin voltage higher than OUT pin voltage). If VDS exceeds the regulation threshold of 22mV (typical), the charge pump increases VGS to reduce RDS(on), driving VDS toward the minimum. If the MOSFET is fully enhanced and VDS is still above 22mV, the MOSFET remains in ohmic (triode) region with VDS = I*RDS(on). If the load current decreases such that VDS falls below 22mV, the controller reduces VGS to increase RDS(on) and maintain the 22mV regulation point. This minimum VDS clamp ensures the MOSFET operates at the lowest possible forward drop while maintaining a positive VDS for reverse current detection.

Reverse Current Blocking: If the input voltage drops below the output voltage (e.g., input supply failure), the VDS polarity reverses (VOUT > VIN). The fast comparator detects this within 50ns and triggers a rapid gate discharge using a 2A peak current sink, turning off the MOSFET and blocking reverse current. After the MOSFET turns off, the body diode of the MOSFET blocks further reverse current. The fast 50ns response time minimizes the reverse current transient, which is critical for preventing output bus voltage droop in redundant power systems.

OFF Pin Control: The OFF pin provides logic-level control over the MOSFET state. When driven HIGH (above the OFF pin threshold), the controller pulls the GATE pin low regardless of the sensed IN and OUT voltages, forcing the MOSFET off. When the OFF pin is LOW or floating, normal OR-ing operation resumes. Note that when the MOSFET is turned off via the OFF pin, current can still flow through the MOSFET body diode from source to drain (forward direction), but not in the reverse direction.

VS Pin Supply: The VS pin is the main power supply for the controller. It can be connected to VOUT (the common bus), VIN (the input supply), or a separate auxiliary supply. When connected to VOUT, the controller remains powered even if the input supply fails. When connected to VIN, the controller powers down if the input fails but has slightly faster response. For low-voltage operation (VIN < 5V), VS can be connected to a higher voltage auxiliary supply (5V or higher) to maintain adequate gate drive. Redundant OR-ing Application: In a typical N+1 redundant power supply system, each power supply feeds the common output bus through an LM5050-1 and an external MOSFET. Under normal conditions, all MOSFETs conduct with minimal voltage drop, sharing the load current. If one supply fails (output drops), the corresponding LM5050-1 detects reverse current and turns off its MOSFET within 50ns, isolating the failed supply while the remaining supplies continue to power the load uninterrupted.