STMPS2171STR

The STMPS2171STR from STMicroelectronics is a single-channel high-side power distribution switch designed for applications where heavy capacitive loads and short circuits are likely to be encountered, such as USB port power distribution, hot-swap power supplies, and general-purpose load switching.

The STMPS2171 integrates a 90 mOhm N-channel MOSFET power switch with comprehensive protection features (overcurrent, thermal shutdown, undervoltage lockout, reverse current blocking) and a fault reporting output, all in a tiny 5-pin SOT-23 package. This level of integration replaces what would otherwise require a discrete MOSFET, gate driver, current sense resistor, comparator, and thermal protection circuit, reducing component count, PCB area, and design complexity.

The STMPS2171 is the 1 A continuous current variant in the STMPS21x1 family. The family includes four members with different current ratings: STMPS2141 (0.5 A), STMPS2151 (0.5 A, different current limit threshold), STMPS2161 (1 A, different current limit threshold), and STMPS2171 (1 A). All share the same pinout and package, allowing different current limit options without PCB changes.

The N-channel MOSFET switch provides 90 mOhm typical on-resistance (120 mOhm maximum), which results in only 90 mV of voltage drop at 1 A load current. This low voltage drop ensures that the load receives nearly the full input voltage, which is critical for USB applications where the voltage at the device must remain above 4.75 V (USB 2.0 specification minimum) from a 5 V source.

The high-side switch topology means the switch is placed between the power supply and the load (in the positive rail), rather than in the ground return path. This is the preferred topology for USB and hot-swap applications because it completely disconnects the load from the supply when the switch is off, preventing any sneak current paths. The N-channel MOSFET is used with an internal charge pump to generate the gate voltage above the input voltage, ensuring the MOSFET can be fully enhanced even when VIN equals VOUT.

The overcurrent protection limits the output current to approximately 120 percent of the rated current (approximately 1.2 A for the STMPS2171) when a fault occurs. The constant-current limiting mode prevents the MOSFET from being damaged by excessive current, while the thermal shutdown provides a secondary protection layer if the current limit condition persists and the junction temperature rises above 135 degrees C.

The 10 ms fault-blanking feature is particularly important for hot-swap applications. When a load with large input capacitors is connected to the output, the inrush current can momentarily exceed the current limit threshold. Without fault blanking, this would cause a false FAULT assertion and potentially shut down the port. The 10 ms blanking period allows the capacitive inrush current to settle before the FAULT output is asserted, preventing nuisance fault signals during normal hot-swap events.

The reverse current blocking feature prevents current from flowing backward from the output to the input when the switch is disabled (EN = LOW) or when the device is unpowered (VIN = 0 V). This is important in battery-powered applications where the battery may be disconnected while the output is still connected to a powered external device, or in multi-source systems where one supply may backfeed through the switch.

The UL recognition (file E354278) is important for equipment that must meet UL safety standards, as it simplifies the certification process for the overall system.

At $0.32 per unit in volume (1000+), the STMPS2171STR is one of the most cost-effective 1 A high-side power switches available, making it practical to add protection and switching to every USB port and power rail in a system.

The STMPS2171STR operates as a high-side N-channel MOSFET power switch with integrated charge pump, current limiting, thermal protection, and fault reporting.

N-Channel MOSFET Switch: The power switching element is an N-channel MOSFET connected between the VIN and VOUT pins. To fully enhance an N-channel MOSFET (turn it on with minimum RDS(on)), the gate voltage must be higher than the source voltage by at least the threshold voltage (VGS(th), typically 1-2 V) plus the desired overdrive. Since the source is connected to VIN, the gate must be driven above VIN. An internal charge pump generates this gate drive voltage from the input supply. The charge pump operates continuously when the switch is enabled, maintaining the gate voltage approximately 5 V above VIN to ensure the MOSFET is fully enhanced. When the switch is disabled (EN = LOW) or during UVLO, the charge pump stops and the gate is pulled to ground, turning off the MOSFET.

Enable Control: The EN pin is a logic input with CMOS and TTL compatible thresholds. When EN is HIGH (above approximately 1.5 V), the charge pump operates and the MOSFET is turned on. When EN is LOW (below approximately 0.8 V), the charge pump stops and the MOSFET is turned off. The enable input can be driven directly by a microcontroller GPIO or by logic circuits. The standby current when disabled is only 12 uA maximum.

Undervoltage Lockout (UVLO): The UVLO circuit monitors the input voltage (VIN). When VIN drops below the UVLO threshold (approximately 2.0-2.5 V), the switch is forced off regardless of the EN input state. This prevents the MOSFET from operating with insufficient gate drive voltage, which could cause it to operate in the linear region with high RDS(on) and excessive power dissipation. The UVLO has hysteresis (approximately 100-200 mV) to prevent chattering near the threshold. The FAULT output is asserted immediately when UVLO activates.

Current Limiting: The current limiting circuit monitors the current through the MOSFET using an internal sense element (a scaled replica of the power MOSFET). When the output current exceeds the current limit threshold (approximately 1.2 A for the STMPS2171), the circuit reduces the gate drive voltage to limit the current to the threshold value. The MOSFET operates in the constant-current mode, maintaining the current at the limit value while the output voltage drops. If the current limit condition persists and the MOSFET power dissipation (VIN – VOUT) x ILIMIT causes the junction temperature to rise above 135 degrees C, the thermal shutdown activates.

Thermal Shutdown: The thermal sensor on the die monitors the junction temperature. When the junction temperature exceeds approximately 135 degrees C, the MOSFET is turned off completely to prevent damage. The FAULT output is asserted immediately (no blanking). When the junction temperature drops below approximately 125 degrees C (10 degree hysteresis), the switch automatically restarts with the current limit active, allowing it to safely drive the load again if the fault has been cleared.

Fault Blanking: The 10 ms fault blanking timer prevents false FAULT assertions during transient events. When the output current exceeds the current limit, an internal counter starts. The FAULT output is not asserted until the counter reaches the 10 ms timeout. If the overcurrent condition clears before 10 ms (e.g., capacitive inrush current that decays quickly), the FAULT output is never asserted. Only current limit faults are blanked; thermal shutdown and UVLO faults assert FAULT immediately without blanking.

Reverse Current Blocking: When the switch is OFF (EN = LOW or UVLO active), the body diode of the N-channel MOSFET would normally conduct current from VOUT to VIN (reverse direction). The STMPS2171 includes a circuit that disconnects the body diode path when the switch is off, preventing any reverse current flow. When the device is unpowered (VIN = 0 V), the switch presents a high impedance at the output pin.

FAULT Output: The FAULT pin is an open-drain N-channel MOSFET output that pulls LOW when a fault is detected. The FAULT output is asserted in the following conditions: overcurrent (after 10 ms blanking), thermal shutdown (immediate), and UVLO (immediate). The FAULT output can be wire-ORed with other FAULT outputs from additional power switches, creating a single fault signal for the system. An external pull-up resistor (10-100 kOhm) is required.