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[FAQ] TPS2121: How do I Achieve a Seamless Power Switchover using TI’s Power Mux Devices

Part Number: TPS2121
Other Parts Discussed in Thread: TPS2116,

Redundant power is critical in systems that require uninterrupted sources of power. To complement this redundancy, a power selection and transition between multiple sources needs to be made, and quickly, to ensure there is no loss of power. This is accomplished through seamless fast switchover.

What is seamless fast switchover?

The switchover time, or tSW, is the period of time where no power is being delivered from the supply to the output. Seamless switchover can be achieved if the output voltage remains within the switch’s operating range. Devices detailed in the next section provide this feature by comparing a reference to the input voltage and an internal voltage reference. Once the input voltage drops to a voltage determined by external design, the device will wait until the other supply reaches that voltage to avoid a reverse current condition, while ensuring power is still delivered to the load.

What devices provide fast switchover?

Power Mux Device

Switchover Time

TPS2116

8us

TPS2121

5us

 

How to achieve fast switchover?

  • TPS2121

In order to achieve the seamless switchover with the TPS2121, there must be a voltage greater than Vref (1.06V) applied to the PR1 and CP2 pins of the device. This can be done with voltage divider configurations going from VIN1 and VIN2 to PR1 and CP2 respectively.

We denote this fast switchover mode of operation as XCOMP/XREF mode in the datasheet. The TPS2121 will compare both voltages and will output VIN1 if PR1>CP2 or VIN2 if PR1<CP2.

For an example of this switchover, let’s assume PR1 is greater than CP2 and VIN1 is the output. At the point PR1 drops lower than CP2, the TPS2121 will start the switchover between VIN1 to VIN2. The switchover in this mode is 5us typical. The switchover has a break-before-make functionality to protect the sources from channel to channel conduction, so during the 5us switchover, neither channel will be conducting to the output.

Examples and more details on the XCOMP/XREF configurations can be found in the TPS2121 datasheet on section "10 Application and Implementation."

  • TPS2116

There are no special conditions to trigger the fast switchover for the TPS2116. The typical switchover time specifications are denoted in the switching characteristics section of the datasheet for select test conditions.

A typical application walkthrough and design considerations can be found in the TPS2116 datasheet in section "8 Application and Implementation."

Even though my device is set up for fast switchover, during switchover, why do I not experience fast switchover?

Please read through the following steps to walk you through your debug:

  • If your CP2 and PR1 voltages are continuously crossing one another, see the next question for more detail
  • How stable is your Vin rail? (if it is not stable, please add input capacitance, preferably balanced on each input)
  • If you're expecting your load to have a high current draw, we recommend having an output capacitor to hold the voltage up

 

How do I stop my CP2 an PR1 from getting stuck in a constant evaluation stage?

The power mux device is always responding to its inputs. During fast switchover, there is no soft start or current clamping. Therefore, when the device switches from one supply to another, the unselected supply will rise slightly in voltage as the source is unloaded while the loaded supply will slightly drop in voltage as current is being drawn. Since a switchover event occurs when CP2 and PR1 intersect, it is possible for the device to bounce between the inputs during this switchover point as the voltage of each chosen input drops while the other raises. The PR1 and CP2 comparator is quite sensitive which could cause a change the PR1 and CP2 comparison over and over again, especially so if the Vin rail is not stable.

To address this, a hysteresis resistor can be added between the ST and PR1 pins. This will lower the sensitivity of the CP2 – PR1 comparison. However, it’s important to note that this resistor will jointly affect the voltage divider on PR1 when IN2 is selected and the transition voltages will need to be set accordingly.

For further information on adding hysteresis, please refer to tech note, “Seamless Switchover for Backup Power Reference Design.”

If you are still experiencing issues, please reach out to E2E https://e2e.ti.com/support/power-management-group/