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TPS2115A: auto switching mode, not working for IN2 only

Kyran McGlasson
Prodigy 20 points
Part Number: TPS2115A
Other Parts Discussed in Thread: TPS2113A, TPS2114A

We use the TPS2115A in autoswitch mode as described on pg 15 (figure 13).

IN1 is the output from 9~36V DC to 5V DC converter (LMR23630A).

IN2 is the output from 5V PoE converter (Infomart PEM1305).

OUT is connected to a Raspberry Pi 3 Compute Module, via a 3V3 and 1V8 DC converter.

D0 is unconnected.

D1 is connected to GND.

ILIM has a 255 ohm resistor, to provide 2.5A cutoff (12.95W at 5V).

STAT is connected to LED and Resistor to the OUTput.

My issue is that the device works when powered by IN1, but not when powered by IN2.

I tried powering from both IN1 and IN2 and then remove IN1, but the device switches off.

I redesigned the board, swapping IN1 and IN2 around, so IN1 is from PoE, IN2 from DC converter, and the same again, IN1 works, IN2 only does not.

What am I missing?

I do NOT have 0.1uF capacitors next to IN1 or IN2, but I would not think this would be an issue? There are much bigger capacitors next to it from the output of other converters.

  • 0
    TI__Genius 15740 points

    Hi Kyran,

    Based on your description, I don't see anything that is wrong, but removing IN1 should result in a switchover to IN2 when the Dx logic pins are configured the way you have them. If you take a look at figure 6 (page 12), the circuit has the same configuration as yours, and it shows how a drop in IN1 results in the output being held up by IN2.

    One thing that does concern me is that both IN1 and IN2 are the same voltage, so it's not certain which supply would be chosen for the output. The TPS2113A has a sense pin that monitors the voltage on IN1 until it drops below a certain voltage, and this may be better for your application.

    Also, what is the output capacitance and output current load for the application? Perhaps the thermal shutdown is being engaged due to a large amount of power being dissipated during the switchover. From the datasheet:

    "High inrush currents can glitch the voltage bus and cause a system to hang up or reset. It can also cause reliability issues—like pit the connector power contacts, when hot-plugging a load such as a PCI card. The TPS2114A/5A slews the output voltage at a much faster rate when OUT switches between IN1 and IN2."

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    Prodigy 20 points

    Thankyou for your response, Alek, In the meantime I have replaced the TPS2113A with a manual jumper, until I figure this one out. I think we will be getting an evaluation board to work out what it is.

    After more testing I am suspecting that if the INput has a low frequency ripple, it will not turn on, and by ripple I mean 800mV at 250kHz (5.0V +/- 400mV).

    Kind Regards,

    Kyran.