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TLV2217: Backdriving

Jarom Christensen
Prodigy 20 points
Part Number: TLV2217


Hello,

I have a client that wants to power a device with battery voltages ranging from 3 V to 12 V. The required device voltage is 3-3.3 V, so we used the TLV2217-33. I designed the power circuit to have a battery plug-in that is regulated by the voltage regulator to 3.3 V, and then a separate battery input that powers the device directly at 3 V. There is a switch between the voltage regulator and higher-voltage battery. The schematic is shown below.

I realize there should be reverse voltage protection of the BATT battery, and something to avoid backdriving the BATT battery, but my client requested to leave those out. However, I'm not sure if I have designed the switch with the regulator correctly. It correctly turns off the battery before the voltage regulator to avoid power drain on the higher-voltage battery (J1) when the switch is disconnected. Assuming the switch is connected to the ground side and the 3V battery (BATT) is connected, what is the best way to avoid backdriving the voltage regulator? I grounded the input assuming it would power down the voltage regulator, but it got really hot when we tested it, which makes me think it backdrove the internal transistor.

I have two questions:

What happens when the voltage regulator is connected like this (3.3 V on output, input grounded)?

What is the best design to avoid backdriving the voltage regulator?

Thanks in advance,

Jarom

  • +1
    TI__Genius 14245 points

    Hi Jarom, 

    Your concern about having Vout higher than Vout is valid, TLV2217 does not have reverse current protection and so if Vout>Vin+0.3V then there is a risk that current flows from Vin to Vout through the LDO which could damage the LDO. We have more information on preventing reverse current in LDO applications here: https://e2e.ti.com/blogs_/b/powerhouse/posts/ldo-basics-preventing-reverse-current-in-ldos

    Note that this is always a concern unless an LDO datasheet explicitly states that it has reverse current protection. Implementing reverse current protection is requires a lot of extra die size and so it will be called out as a feature if it is present in a device. The link I included above has external protection topologies that can be used depending on the requirements of your system.  

  • 0 in reply to Kyle Van Renterghem
    Prodigy 20 points

    Thank you for the quick reply. So I can't connect the input to ground to turn the regulator "off", but can I leave it floating? Would the ambiguity of charge leave enough room for backdrive, or would it be alright?

  • +1 in reply to Jarom Christensen
    TI__Genius 14245 points

    Hi Jarom, 

    Correct, if Vout is high then bringing Vin down quickly could damage the LDO, however if you float the input (high impedance) then that would be ok, Vin will go down as Vout goes down. If there is no (or very low) output load, then the ground current used to run the LDO will slowly discharges Vin and as Vin<Vout that ground current will flow from Vout to Vin and then to GND and since this current is so small it shouldn't risk any damage to the LDO. 

    As a general rule LDOs without reverse current protection can sustain 5% of the nominal current in the reverse direction (that would be 25mA for TLV2217) before being at risk of damage.