TPS2061CDBVR

The TPS2061CDBVR from Texas Instruments is a single-channel current-limited power-distribution switch designed for USB and other hot-swap applications where heavy capacitive loads and short circuits are likely to be encountered. Housed in a tiny 5-pin SOT-23 package, it provides a complete power switching solution with minimal external components.

The TPS2061C belongs to the TPS20xxC family, which differs from the earlier TPS20xx (non-C) family by adding output discharge functionality. When the switch is disabled (EN pin high), an internal discharge resistor pulls the output voltage to ground, ensuring a controlled power-down sequence. This feature is critical in USB applications where the host must detect device disconnection reliably, and in systems where residual output voltage could cause unexpected behavior on subsequent power-up.

The device integrates an 86mOhm N-channel MOSFET as the power switch, controlled by an internal charge pump that operates from supplies as low as 2.7V. The charge pump also controls the switch rise and fall times to minimize inrush current surges during turn-on and turn-off, eliminating the need for external soft-start circuitry. The typical rise time is 0.6ms, which meets the USB specification for power switching.

The current-limiting circuit operates in a constant-current mode when the output load exceeds the current-limit threshold of 1.55A (typical). This provides a predictable fault current under all conditions, which is essential for designing the power supply and protection circuitry. The fast overload response time of 2us eases the burden on the main 5V supply by quickly limiting the current before it can cause significant voltage droop. The current limit accuracy is within +/-20% of the nominal value.

The enable (EN) pin is active-low, meaning the switch turns on when EN is driven low. This is the default configuration for the TPS2061C. The active-low enable is convenient for USB applications where the power switch should be off by default (EN pulled high) and enabled only when the host controller asserts control. Internal pull-up circuitry ensures the switch remains off when the EN pin is left floating.

The fault (FLT) output is an active-low open-drain pin that is asserted during overcurrent or overtemperature conditions. The fault signal is deglitched with a typical delay of 8ms to prevent false triggering from transient events. This deglitch period ensures that only sustained fault conditions are reported, reducing nuisance fault interrupts. A 10kOhm external pull-up resistor to the logic supply is typically used to read the fault status.

Reverse current blocking prevents current from flowing from the output back to the input when the output voltage exceeds the input voltage. This protects the input power supply from backfeeding and is particularly important in systems with multiple power sources or battery backup. The undervoltage lockout (UVLO) circuit disables the switch when the input voltage drops below the threshold, preventing unreliable operation during power-up and power-down transients.

The SOT-23-5 (DBV) package is the smallest available option for this device family, making it ideal for space-constrained applications such as USB port protection in laptops, tablets, and handheld devices. Despite its small size, the package provides adequate thermal performance for the 1A current rating when properly laid out on a PCB with a ground plane for heat spreading.

Other package options for the TPS2061C include the HVSSOP-8 (DGN) with PowerPAD for improved thermal performance, and the SOIC-8 (D) for larger assemblies. For applications requiring higher current, the TPS2068C (1.5A) and TPS2069C (2A) offer the same pinout and functionality with higher current-limit thresholds.

The TPS2061CDBVR operates as a high-side N-channel MOSFET power switch with integrated current limiting, fault protection, and control circuitry.

Power Switch Architecture: The core element is an N-channel MOSFET connected between the IN (input) and OUT (output) pins. As a high-side switch, the MOSFET is placed between the power supply and the load, allowing the load to be completely disconnected from the supply when the switch is off. The N-channel MOSFET is used instead of a P-channel device because it offers lower on-resistance (86mOhm) for a given die area, resulting in lower power dissipation and a smaller package. However, N-channel MOSFETs require a gate voltage above the source voltage to turn on, which is provided by the internal charge pump.

Charge Pump: The internal charge pump generates a gate drive voltage that is approximately 5V above the input voltage (VIN + 5V). This elevated gate voltage ensures the N-channel MOSFET is fully enhanced, minimizing the on-resistance. The charge pump operates from the input supply and requires no external capacitors or components. The charge pump also controls the gate voltage rise and fall times to implement the soft-start function, limiting the inrush current when the switch turns on into a capacitive load.

Current Limiting: The current-limit circuit senses the MOSFET current and compares it to the fixed threshold (1.55A typical). When the load current exceeds this threshold, the circuit operates in a constant-current mode by reducing the gate drive voltage to limit the MOSFET current. The output voltage drops as a result, and the switch dissipates power equal to (VIN – VOUT) x ILIMIT. If the overcurrent condition persists, the junction temperature rises until the thermal protection circuit activates (typically at 140C). The thermal protection cycles the switch on and off to maintain a safe junction temperature while continuing to limit the current.

Fault Reporting: The FLT (fault) output is driven low by an internal open-drain MOSFET when an overcurrent or overtemperature condition is detected. The fault signal is deglitched by an 8ms timer to prevent false triggering from transient spikes. During the deglitch period, the FLT output remains high (not asserted) even if the current limit is active. Only if the fault condition persists beyond the deglitch period does the FLT output assert. This deglitch feature is particularly important in USB applications where momentary current spikes (e.g., during hot-plug events) should not trigger a fault interrupt.

Output Discharge: When the switch is disabled (EN = high), an internal discharge resistor (approximately 250Ohm) is connected between the OUT pin and GND. This resistor discharges the output capacitance, ensuring the output voltage decays to a low level in a controlled manner. The discharge time depends on the total output capacitance: t_discharge = R_discharge x C_out. For a 100uF output capacitor, the discharge time is approximately 25ms. The output discharge feature ensures that the host USB controller reliably detects device disconnection and that downstream circuitry is fully reset before the next power-up.

Reverse Current Blocking: The reverse current blocking circuit monitors the voltage difference between IN and OUT. When VOUT > VIN, the circuit disables the MOSFET to prevent current from flowing from the output back to the input. This protection is important in systems where the output may be driven by another power source (e.g., a battery backup) while the input supply is off.

Undervoltage Lockout (UVLO): The UVLO circuit monitors the input voltage and disables the switch when VIN falls below the UVLO threshold (approximately 2.5V rising, 2.2V falling). This prevents the switch from operating with insufficient gate drive, which could result in excessive on-resistance and power dissipation. The UVLO hysteresis (approximately 300mV) prevents chattering during slow input voltage ramps.

Enable Control: The EN (enable) pin is active-low for the TPS2061C. When EN is low (below 0.6V), the switch is enabled and the MOSFET turns on. When EN is high (above 1.4V) or left floating (internal pull-up), the switch is disabled and the MOSFET turns off. The EN pin has a maximum input voltage of 5.5V and can be directly driven by 3.3V or 5V logic.