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TPS2121: Can we add a diode to improve transient response and for noise immunity ?

Jomon
Intellectual 660 points
Part Number: TPS2121
Other Parts Discussed in Thread: LM3488

TPS2121 has been used to switch over from one 12V adapter source to 12V boost from battery, in the absence of adapter power.

Some adapters are noisy, and others are not. In some cases, the voltage spikes go down to 10.8V in the adapter, forcing the TPS2121 to oscillate between the 2 sources. To prevent this, we reduced the switch over voltage to 10.6V. When the switch over occur, the voltage boost regulator (LM3488) takes up to 20ms, with voltage dipping to 9.8V, to reach the 12V again. This time reduces to 2.6ms when the switch over is at 11.6V.

Is it OK to connect a diode from the battery boosted 12V (IN2) to the output of TPS2121 ? So that it works only when TPS2121 is using IN1, but has a DIP in voltage of Vf of the diode, with reference to Boosted voltage (IN2) to prevent oscillation of switching between supplies by TPS2121 ? When it works on battery only, the diode would be OFF as IN2 and Vout of TPS2121 would be less than Vf of the diode.

  • 0
    TI__Mastermind 32650 points

    Hi Jomon,

    Adding a diode from input to output is not a reccomended mode of operationg for this device.

    Is it possible to add input capacitance to the rails to make sure that the voltage on the rail is maintained?

    It is also possible to add a 10nF (at maximum) on the PR1 rail to further maintain the PR1 rail voltage if the VINx rail is noisy.

    Thank you for choosing TI Power MUXes for your designs!

  • 0 in reply to Shreyas Dmello
    Intellectual 660 points

    Hi,

    Thank you for the reply.

    We have 40uF on the input rail now in close proximity of TPS2121. We added a 0.1uF and 1uF at the input pin (1cm from the other capacitors)
    Would adding a bulk capacitor help ?
    Are you suggesting adding a capacitor from PR1 to GND (10nF max) ? Will that reduce the response time in anyway ?

    We have always used TI in all out designs so far :) Always happy with the support!

  • 0 in reply to Jomon
    TI__Mastermind 32650 points

    Hi Jomon,

    With a very small capacitor on PR1/CP2 to GND, the comparator rails will not be as susceptible to the noise that could be seen on the VINx rails.

    In your situation, this may help a little to avoid the spikes causing switchover. This will not help the situation where your boost regulator takes a long while to ramp up to a high voltage again. TPS2121 will respond to what is going on on its input rails so if the input rail rises slowly, there is no way for TPS2121 to make this faster. In my opinion, we need to have the switchover to happen at a higher voltage and clamp/maintain the voltage on the input rails as best we can.

    One more change you can make to the system is to use hysteresis. The purpose of hysteresis is to change the IN1->IN2 switching trigger point from the IN2->IN1 switching trigger point (effectively widening the switching window). Please take a look at this tech doc (Section 2.2.2) for more information: https://www.ti.com/lit/ug/tidue50/tidue50.pdf