MAX40203AUK+T

The MAX40203AUK+T from Analog Devices (Maxim Integrated) is a nanoPower ideal diode current switch that replaces Schottky diodes in power routing applications with dramatically lower forward voltage drop. In a compact 5-pin SOT-23 package, it delivers the functionality of a near-perfect diode with only 90mV of forward drop at 1A, compared to 300-500mV for a typical Schottky diode at the same current. This 3-5x reduction in voltage drop translates directly to longer battery life and cooler operation in portable devices.

The ideal diode concept is straightforward: instead of using a passive P-N junction (Schottky or conventional diode) for power routing, the MAX40203 uses an active MOSFET switch with intelligent control circuitry. An internal comparator monitors the voltage difference between the input (VDD) and output (OUT). When VDD exceeds OUT by a small threshold (indicating forward-bias condition), the MOSFET turns on, allowing current to flow with very low resistance. When OUT exceeds VDD (reverse-bias condition), the MOSFET turns off within 80us, blocking reverse current flow. This behavior mimics a diode but with much lower forward voltage drop and much faster reverse recovery.

The device is particularly valuable in ORing applications where two or more power sources (e.g., main battery and backup battery, or USB and battery) are connected to a common load through separate ideal diodes. The low forward drop ensures that the higher-voltage source supplies the load, while the reverse blocking prevents current from flowing back into the lower-voltage source. With a Schottky diode, the voltage drop at 1A would be 300-500mV, which significantly reduces the usable voltage at the load and wastes power as heat. The MAX40203 reduces this loss by an order of magnitude.

The quiescent current of 300nA is among the lowest for any ideal diode controller, making it suitable for always-on battery-powered devices where even microamp-level quiescent current can drain the battery over time. When disabled (EN=LOW), the device draws even less current and blocks voltages up to 6V in both directions, providing complete isolation of the power path.

The integrated current limiting of 2A protects the device and downstream circuitry during short circuits. If the output is shorted to ground while the device is enabled, the internal current-limit circuit reduces the MOSFET gate drive to limit the output current to approximately 2A. The resulting power dissipation (VDD x 2A) causes the junction temperature to rise, and the thermal protection circuit shuts down the device when the junction temperature exceeds the thermal shutdown threshold (approximately 145C with 14C hysteresis). The device automatically restarts when it cools down.

The EN (enable) pin provides direct control over the ideal diode function. When EN is HIGH (above approximately 1.0V), the ideal diode function is enabled and the device operates normally. When EN is LOW (below approximately 0.4V), the MOSFET is turned off regardless of the VDD-OUT voltage difference, blocking current in both directions. This allows system-level control over power routing, such as disabling a backup battery path when the main supply is active.

The MAX40203AUK+T is the SOT-23-5, tape-and-reel packaged version (2,500 units per reel). The MAX40203AUK+ is the cut-tape version. The MAX40203ANS+T is the 4-bump WLP version (0.77 x 0.77mm) with even lower forward voltage drop, suitable for ultra-compact designs where PCB area is at a premium.

The MAX40203AUK+T operates as an active ideal diode using an internal P-channel MOSFET with intelligent control circuitry.

Internal MOSFET and Controller: The core element is a P-channel MOSFET connected between the VDD (input) and OUT (output) pins. A P-channel MOSFET is used because it can be turned on with a gate voltage lower than the source (VDD), which is naturally available without a charge pump. The MOSFET is sized to carry 1A continuous current with minimal voltage drop. The on-resistance is designed such that at 1A forward current, the voltage drop (VDD – OUT) is approximately 90mV (SOT-23 package).

Forward-Bias Detection: An internal comparator continuously monitors the voltage difference between VDD and OUT. When VDD exceeds OUT by more than the forward turn-on threshold (approximately 10-20mV), the comparator output signals the gate driver to turn on the MOSFET. The MOSFET gate is driven to a voltage that allows the desired forward current to flow. As the load current increases, the gate drive strengthens to maintain the MOSFET in the linear region with controlled voltage drop. At low currents (1mA), the drop is only 14mV; at 1A, the drop increases to approximately 90mV due to the MOSFET on-resistance.

Reverse-Bias Blocking: When OUT exceeds VDD (reverse-bias condition), the comparator detects this within 80us and turns off the MOSFET gate drive. The MOSFET turns off, blocking reverse current flow. The reverse blocking threshold is approximately 26mV (VOUT – VDD), meaning the device turns off when the output voltage exceeds the input by 26mV. Once the MOSFET is off, the reverse leakage current is less than 10nA typical, which is far lower than a Schottky diode (typically 1-100uA at rated voltage). The device blocks voltages up to 6V in the reverse direction.

Enable Control: The EN pin provides an external control input. When EN is driven HIGH (above the enable threshold, approximately 1.0V), the ideal diode function is enabled and the device operates normally. When EN is driven LOW (below the disable threshold, approximately 0.4V), the gate driver is forced off regardless of the VDD-OUT voltage, and the MOSFET is turned off. In the disabled state, the device blocks up to 6V in both directions and draws minimal current from both VDD and EN pins.

Current Limiting: The internal current-limit circuit monitors the MOSFET current. When the forward current exceeds approximately 2A (such as during an output short circuit or a fast power-up into a large capacitive load), the gate drive is reduced to limit the current. The current limit operates in a constant-current mode, maintaining approximately 2A through the MOSFET. The resulting power dissipation (VDD x 2A) causes the junction temperature to rise.

Thermal Protection: When the junction temperature exceeds the thermal shutdown threshold (approximately 145C), the MOSFET is turned off to prevent damage. The thermal shutdown has 14C of hysteresis, meaning the device does not restart until the junction temperature drops to approximately 131C. This thermal cycling continues as long as the overcurrent condition persists, providing self-protection without requiring external intervention. The thermal protection also protects downstream circuitry by limiting the power that can be delivered into a fault.

Low Quiescent Current: The 300nA quiescent current is achieved through the use of sub-threshold CMOS design techniques in the control circuitry. The comparator, bias circuits, and gate driver are all designed to operate at nanoampere current levels. The quiescent current is relatively constant over the load current range because the control circuitry operates independently of the load current path. The low quiescent current ensures minimal battery drain in always-on applications such as backup power systems and portable devices in sleep mode.