Our op-amp PCB is currently stuck somewhere in Finnish customs (classic…), so instead of waiting, we kicked off Mockup #1 testing without it.

The first experiment focuses on the ATRK input for output voltage regulation on the LM51772EVM. The datasheet isn’t very clear about how this feature behaves, so let’s figure it out ourselves.

Setup

• STM32 DAC connected directly to the ATRK input of LM51772EVM

• Sweeping DAC voltage while measuring Vout 

• Three different Vout setpoints, setup over I²C

Observations

At first glance, the DAC seems a bit nonlinear — maybe due to bias current? So we checked linearity directly.

Result: DAC looks fine. That means the nonlinearity is a quirk of the LM51772 itself. And that’s actually fine for us. All we need from ATRK is the ability to slightly trim Vout (no more than ±50 mV).

Results

Near the maximum output of 32 V, the trim resolution looks promising:

• ~200 µV/step for the last 10 mV of Vout

• ~600 µV/step for the last 60 mV of Vout

With an additional op-amp stage to amplify the DAC voltage into the desired range, resolution will only improve.

Next steps

Once the op-amp PCB finally arrives, we’ll move on to testing ISET regulation, comparing direct ATRK control vs. buffered DAC inputs with op-amps.