Hi Analog engineers
First something info about the circuit. I'm designing a Current control for a switched mode oversupply with a feedback (FB) voltage of 1.215 Volt ( the U5 reference has a little more 1.23V but for the simulation that doesn't matter). The current control is done by a microcontroller using a 12bit DAC. The idea is that the DAC sets the set point of the current and when the set point is reached by the increasing current the SMPS FB will start reducing it's duty cycle till it is equal or below the set point again. The circuit is based on a AD8220 Differential amplifier with a G=62.5 where the Uin+ input voltage generated by the Current of a Rs=0.004Ohm is subtracted with the Uin- voltage with is the DAC voltage divided by 62.5 using a resistor divider till the output reaches 1.215Volt (here 1.23V)..
I have simulated all this in LTSpice and no problem with simulation. When I match the specs of the AD8220 with the simulation results, I see some weird things. For example, the data sheet specifies an output swing way better than what the simulation shows. The Voltage swing is between 0.15V and 4.85V with Vcc=5V with a 10K load. The simulation show that after 3.10Volts output the amplification curve knees and continues with a very different gain compared with the calculated gain of 62.5. This knee point you see at 7.59A or at 0.42 seconds. I simulated the same circuit in Multisim with the same results.
Can anyone tell me what I'm doing wrong here or if there is some kind of error in the spice model of this AD8220.
What is your experience with LTspice, does it normally simulate close to reality.
Can you comment on the schematic, any better ideas here?.
Hope you can help thanks
LTspice Simulation results