Post Go back to editing

LT3090 output voltage controlled by DAC

Dear Sir/Madam,

I used the TPS7A3301 in an application where i adjusted the feedback voltage using a DAC. Vin=-30 and Vout=-27 to -28. It worked nicely however i ran into a problem with this part at these voltages that made the solution non viable.

I would like to use the LT3090 in the same application where the output voltage can be changed by using a DAC or something similar. Vin = -30 and Vout=-27 to -28V.

Page 15 of the DS mentions the following:

For applications requiring higher accuracy
or an adjustable output voltage, the SET pin may
be actively driven by an external voltage source
capable of sourcing 50μA – the application limitations
are the creativity and ingenuity of the circuit designer.

Any thoughts how to achieve this without impacting on the noise performance of the LDO?

Regards Joe

  • The image below shows the basic concept of how to control an LDO output using a DAC.  The LT3082 in the image is a positive LDO and the LT3090 that you want to use is a negative LDO, but it should still be possible to drive the LT3090 SET pin in a similar fashion, even if it is necessary to invert your DAC output.  A capacitor from the SET pin to ground usually makes the SET pin quiet so noise from the DAC does not show up at the LDO output.  Try not to over-drive the SET pin in relation to the LDO output.  In other words, the LT3090 and other similar LDOs were designed so in most cases SET = OUT, and the LT3090 will work best if you drive SET so OUT has time to follow SET.

  • Dear Arad,

    Thank you for the reply. I was thinking about the arrangement you mention before posting. I had several concerns. Firstly, the op amp would be at -27V to -28V unless a secondary resistor is used. Secondly, the set input is high impedance and is susceptible to noise pickup. Lastly, the DS mentions not to drive SET more than 0V3 below VIN.

    Regards Joe

  • The image below shows one way to vary the output voltage of the LT3090 between -27V and -28V using the output of a DAC:

    LT3090 -27VOUT to -28VOUT_0.png

  • Hi ARad,

    Thank you.

    I took a slightly different approach by using a active filter to reduce DAC noise.
    The DAC is represented by a voltage source 0 to +2V048.
    The output of the OPA376 swings 0 to -2V048.
    The +-2V5 rails power up first and the DAC output powers up at 0V.

    Do you see any issues with this circuit?

    Regards Joe