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LM66100: Equal voltage power switch

Part Number: LM66100
Other Parts Discussed in Thread: TPS2116

Hi,

I have designed an equal voltage power switch with the LM66100 Ideal Diode based on the "Dual Ideal Diode ORing for Continuous Output Power" example from the Datasheet (https://www.ti.com/lit/ds/symlink/lm66100.pdf).

You can see my schematic appended to this post.

The intention was to be able to supply the Output (V_OUT) over either input (V_IN or V_USB). V_IN should be prioritized. Voltage on both inputs is 5V. In application the V_IN supply will probably be powered first and then the V_USB. Yet, I have no guarantee for it.

The datasheet states following about this schematic:

"The shortcoming of the previous implementation happens when both input voltages are the same for a long
period of time, then both devices will completely turn off, powering down the output load. To avoid this case, the
status output from the priority supply and a pull up resistor can be used causing both devices to switchover at the
same time."

Therefore I thought the device is capable to switch inputs of the same voltage level.

I now experienced undefined behavior in the assembled application: Whilst the pullup resistor should disable the lower LM66100 (at least I thought so), when a voltage is supplied to the upper one, the lower one stays active. Also when the upper one is already active and voltage is supplied to the lower one, the lower one takes over. The prioritization thus is completely inverse of the original intention. Also, when I switch the supply to both inputs on nearly simultaneously, I experience reverse current injection to the other supply.

I have now seen this thread: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/892906/lm66100-dual-ideal-diode-oring-for-continuous-output-power

There it is stated, that the LM66100 circuit cannot run on same voltage levels, it is said that the datasheet specifies a dropping voltage on one of the inputs. However, this is not stated in the datasheet and that is why I probably mistakenly decided on the LM66100.

This design is now in application, unfortunately, and I need to know when the device is operating in safe condition. It would be nice if you can give me more insight into this problem!

It should definitely be safe it I supply only over one input, as far as I can see.

But what happens exactly if I operate it from the upper input (V_IN) first and then additionally switch on the lower one (V_USB) and reverse?

What exactly happens if I switch on both supplies at the same time?

And what happens if I run the device for a long time on both supplies?

What is the difference of one voltage dropping to the same level, than the other voltage rising to the same level?

Thanks for your help!

  • Hi Thibault, 

    Ideal Diode ORing is strictly intended to choose the highest voltage supply. If priority of one supply is required, a power mux is needed to select and prioritize one input. I would recommend the TPS2116. Since an ideal diode ORing configuration will simply choose the highest supply, for your case, this means the LM66100 will be in a constant evaluation of which 5V supply is higher. 

    The source of confusion seems to be the condition mentioned in the datasheet in section 9.2.2.1. The intention was to explain how to counteract a transient case where the priority supply's voltage is dropping to a point where it is the same as the auxillary supply. However, I see where this can be misconstrued and I will look into rewording this paragraph to better describe the recommended design change intention and further clarify that the LM66100 can only be used to selected the highest voltage supply. 

    I unfortunately do not see a path forward knowing your intended application. Regardless of what sequencing changes can be made, we cannot change the fact the DC state involves both devices having equal voltages.

    • If both devices are on, due to the voltage supplies being inherently different, along with the Ron variability of each device, the supplies will attempt to balance themselves, likely triggering a reverse current event as you are seeing. By causing the FETs to run between their VIL and VIH, you will experience unpredictable, possibly on, possibly off states in your switches. 
    • This is why power muxing is needed for your application. With a power mux, you can select the priority input, even with two supplies at the same voltage. 

    Apologies for the confusion.

    Best Regards, 

    Elizabeth