LT3090 output voltage controlled by DAC

Question

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

ARad · Accepted Answer

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.)

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LT3090 output voltage controlled by DAC