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TPS73101-EP: TPS73101DBVR constant current source (in replacement of the LM317)

C-All
Prodigy 10 points
Part Number: TPS73101-EP
Other Parts Discussed in Thread: LM317, LM1086

I am looking to replace an LM317 in an existing design where it is used as a constant current source.

The constant current source provides a consant 10mA to a resistive fuel sensor.

The resistance varies from 33R to 240R depending on the fuel level.

This gives an output voltage of between 330mV and 2.4V into a microcontroller.

The adjust resistor was 120R.

This means the output votlage from the LM317 in the worst case conditions was 3.6V.

The VDO of the LM317 is recommended as 3V, so a supply voltage of over 7V is prefferred.

In the original design, there was a 12V rail available which meant that this device was suitable (although arguably being severely overated).

In the new design the only supply rail available is a 5V rail so this is no longer a suitable device.

In the LM317 datasheet using it for this kind of constant current source is documented, but it does not appear to be in any of the TPS devices.

Does anyone know if they can be used for this kind of appication? Or any alternative parts which can be recommended?

Proposed schematic below using TPS73101DBVR .

  • 0
    TI__Mastermind 37155 points

    Hello,

    There are a few things happening here.  First, the LM317 is easy to use as a current source because the internal reference is not connected directly to GND, it is floating.  Most LDO's have the reference connected to GND but not all of them.  You can still use a linear regulator which has the reference tied to GND as a current source, but something still needs to float.  It will either be your load or your LDO + source input supply.  I've created a diagram below which compares the two typologies and how they can be configured to generate a current source.  On the left is the LM317 and on the right is any LDO which uses a typology that ties the internal reference to ground. 

    The value of the current is set by connecting a resistor in parallel with the reference voltage, and the load in series with this resistor.  As you can see, if the load circuitry is tied to GND of the system then the LDO and input supply will need to be floating for this to work.

    The LM1086 might work, it has a dropout voltage of less than 1V at 100mA load current at room temperature or hotter.  It also uses a floating reference based topology like the LM317.

    Thanks,

    - Stephen