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TPS2124: How to set IN1 and IN2 switch voltages

Leonard Jenski
Prodigy 50 points
Part Number: TPS2124
Other Parts Discussed in Thread: TPS2121

I am having some issues understanding how to set up the TPS2124 to switch between 2 supplies based on voltage levels.

I reviewed the documentation and I did not see anything that helps describe how to set the IN1 and IN2 levels for switching.

I assume it has to do with the PR1 input but do not see how to set this level for desired function.

Here is what my intent was for the IC when I first reviewed the data sheet.

 

Conditions:

12V into IN1

4.5V into IN2

Out set to IN1 (12V) initially

 

On loss of In1 (12V) to say less than 6V or lower, the OUTPUT switches over to IN2 (4.5V).

If IN1 increases to something > 6V the OUTPUT switches back to IN1.

 

How do I set the voltage that determines when IN1 switches to IN2 and back IN2 to IN1.

I understand the OVP requirements but not sure how to set these switch points.

 

  • 0
    TI__Genius 10652 points

    Hi Leonard,

    Good question. Basically you want to design a resistor divider from IN1->PR1->GND in order to tell the device you want to give PR1 PRIORITY until it drops below a certain, which is built-in use case of this family.

    Now, you can use TPS2124 to sort of approximate this but it will not do exactly what you want. I recommend to use TPS2121 which can be configured according to your need. 

    Regardless, first lets look at how you can get close to that functionality with TPS2124. I will also explain why TPS2124 can't do exactly this, then i will explain about it with TPS2121 which i recommend to realize your switchover condition. 

    TPS2124 Priority (VREF Mode)

    If we look at the truth table for the device, we basically  want it to operate in VREF mode which ensures VIN1 is output until PR1 <VREF. At that point, it will switch to VIN2 or stay to VIN1.

    If you look at the VCOMP/VREF modes, you can see that once PR1<VREF, its not guaranteed the device will switch to the IN2. If IN2 is less than IN1 like in your case, where VIN1<6V & VIN2=4.5V, it will stay in VIN1. This is not what you want and it is the reason i am suggesting TPS2121

    The basic schematic is shown below for PR1 resistor divider, as stated in the truth table we need PR1 > VREF in order for the device to prioritize VIN1.

    VREF=1.06V , and as you said you want the point where it stops prioritizing VIN1 to be VIN1<6V, so pick 6V as the point and calculate the resistor divider values as below:

    6V * R2 / (R1+R2) = VREF = 1.06V

    TPS2121 XCOMP/XREF Mode

    TPS2121 has a functionality of XREF/xcomp modes which basically take care of the highlighted condition with TPS2124 and can be setup to switchover to VIN2 at an arbitrary VIN1 voltage, and regardless of whether VIN1 > VIN2 which can't be done in TPS2124. I recommend you to take a look at this device. 

    Please see the below table for XREF function (from datasheet). Basically look at XCOMP column which is highlighted. TPS2121 has a pin called CP2 which functionas as an external reference for the crossover point. When you put a voltage on CP2 pin, CP2 basically replaces VREF. You can also add hysterisis for falling/rising of VIN1 via an extra resistor to ST. 

    I could explain it here but the datasheet has both fantastic explanation, as well as detailed design procedure for xcomp/xref modes, but this is pretty much what you are aiming to do in your descripotion. Please find eblow the links to the datasheet for xcomp / xref modse. 

    XCOMP mode (sec 10.3)

    XREF mode (sec 10.4)

    Please let me know if you have any additional questions on this, you can let me know. Please help me know if i've answered your questions by pressing the green button. 

    Best

    DImitri

    www.ti.com/.../tps2121.pdf