A device can still reset when its main supply disappears, even when the design includes a fast power multiplexer. The useful purchasing question is not simply whether the chip switches quickly, but whether the complete system keeps the load within its operating limits.

The TPS2121RUXR product page provides an inquiry entry for this TI part. Use TI's ordering information and the TPS212x datasheet for engineering decisions.

1. What does TPS2121 do?

TPS2121 is a two-input, one-output power multiplexer. It helps select between supplies using the configured operating mode and priority conditions. TI lists adjustable current limiting, reverse-current blocking and protection features. The RUX ordering package is a 12-pin VQFN-HR.

It is not a voltage regulator or a battery charger. If the selected source voltage changes, downstream circuitry must tolerate that change or use a suitable regulator. Do not connect a battery expecting the mux to provide a charging algorithm.

2. Fast switchover does not guarantee zero droop

TI specifies a typical fast output switchover time of 5 microseconds under the applicable conditions. A typical value is not a maximum interruption guarantee for every circuit. Input decay, priority thresholds, load current, capacitor behavior and the downstream reset threshold affect the outcome.

Ask engineering to define the lowest permitted load voltage and maximum disturbance duration. Then measure the actual output during source removal and restoration. A successful no-load demonstration is not sufficient evidence for a loaded product.

3. Estimate hold-up capacitance before testing

A useful first approximation is C = I × Δt / ΔV, assuming the capacitor supplies a roughly constant load over the interval.

For a hypothetical 1 A load, a 20-microsecond hold-up interval and 0.2 V permitted droop, the ideal estimate is 100 microfarads. The 20-microsecond assumption is a design example, not a TPS2121 timing specification.

The final capacitor choice must account for effective capacitance under DC bias, tolerance, temperature, ESR, transient response and startup current. The simple calculation excludes these effects and must be checked on the real board. Increasing capacitance also affects inrush and source loading.

4. Check heat and current limit separately

TI lists a typical on-resistance of 56 milliohms. Using that typical value, a 3 A conduction example gives P = I²R = 0.504 W, before other losses. Actual resistance varies with operating conditions, so this is a screening estimate rather than a thermal qualification.

TI lists an adjustable current-limit range up to 4.5 A for TPS2121. A current-limit setting is not an unconditional continuous-current rating for any PCB. Evaluate tolerances, transient current, voltage drop, copper layout and temperature. Consult datasheet limits instead of relying on a catalog headline.

5. Define the power-selection behavior

Write down the intended response before buying samples:

  • Which source has priority when both are present?

  • At what input condition should the design change sources?

  • Can the backup supply support the peak load?

  • What happens when the preferred source returns?

  • How should the system behave if neither source is valid?

Two inputs do not mean the device adds their currents together. Treat it as a source-selection design, not a promise of load sharing or increased available power.

6. Sample acceptance should reproduce faults

Test main-source removal and return, slow input collapse, startup under load, load steps and relevant source combinations. Observe both inputs and the output with suitable measurement equipment, and monitor downstream resets or communication interruptions.

Repeat at the project's relevant load and temperature conditions. Record the board revision, capacitor part numbers, control-component values and load profile. Engineering should define voltage, timing and temperature acceptance limits. Protection features do not remove the need to validate the application.

7. Purchasing checklist

Use the full code TPS2121RUXR. Confirm the RUX footprint against the approved PCB; do not assume another power mux or similar suffix is interchangeable.

  • Sample quantity, production quantity and target arrival date.

  • Packing format and whether partial reels are acceptable.

  • Lot and traceability documentation requirements.

  • Approved substitutions, if any, and required engineering review.

  • Agreed sample-test conditions and acceptance criteria.

Request a quotation through the WOMAiELEC TPS2121RUXR page. Confirm quantity, price and delivery in the actual quote. Component specifications and calculated examples do not constitute a guarantee of uninterrupted operation for an untested system.