Hello, I'm using TPS2121 to switch from 5V power supply to battery 3.7V. Unfortunately I see drop in the output even-though the battery level is solid.
Hi Timothy,
IN1 nominal voltage is 5V (DC), IN2 is 3.7V (BAT). CP2 is connected to BAT.
Thanks,
Chris
Chris,
Doing the calculations here with your voltage dividers we get that:
The way that the TPS2121 works is that it will compare PRI to CP2 and then switch over to IN2 if PRI falls below CP2. The discrepancy I see in your schematic is the note on the top that says that the device should switchover at 3.3V.
Your resistor divider for PRI and CP2 are the same 10K/4.7K configuration which gives the values I listed above. This means when the PRI pin goes to 1.18V the device will switchover to IN2. If we plug this value into the resistor divider formula...
And you get a switching voltage of 3.7V which is also the same voltage of IN2. This might run into some unexpected behavior as you do not have any configured hysteresis on your schematic. Any sort of slight variation here could cause the device to switch between rails relatively rapidly and may run into unexpected behavior.
Was a 3.7V switchover your target or did you want 3.3V? If you did want 3.7V I would recommend connecting the ST pin to the PRI pin through a 1MΩ resistor. This way when the device switches to IN2 the PRI pin will be pulled slightly down and prevent this rapid switch back and forth.
We also have a reference design here that illustrates the seamless switchover design that might be useful if you haven't already seen it:
