How to calculate the load current control resolution?

Hello Team,

I have the existing circuit as like attached and we wanted to interface the solenoid valve with below requirement.


Solenoid Valve current control requirement :
Load current range 0-1.5A
The regulated load current with resolution of 0.78125mA/bit with a 0.2ohm sense resistor actuator
HS or LS can be controlled
Need current closed loop control

Hence i am doing the compatibility analysis.

My circuit will with stand the load current up to 4A. So no issue with maximum current requirement.
But i wanted to calculate the regulated load current control resolution.
Hence i first i calculated the current sensing resolutions like below.

Microcontroller ADC resolution – 12 bit, 2^12 = 4096
ADC reference voltage – 5V
amplifier gain – 14

I typ = (ADC_value * 1.221mV)/ Gain * shunt = 2.3815 mA

I one bit change = ( (ADC_VaLue-1) * 1.221mV)/ Gain * shunt = 2.3810 mA

Current Sensing Resolution / bit = I typ - I one bit change = 0.58mA

From the above calculation i came to conclusion that my HS current sensing Resolution can meet requirement

As a next step i wanted calculate the Load current control resolution?

Hence i would like to know whether my approach is correct on this compatibility study?
If so then how i should calculate the the Load current control resolution for the given circuit?

1. I can't reproduce the calculation
I think, with the given parameters the current measurement resolution would be 5V/(4096*14*0.2) = 0.436 mA
2. I don't understand the purpose of two driver transistors
3. What's the implemented control method? PWM or analog driver? In the latter case, there should be a DAC.
4. If the control method is PWM, you need to provide free-wheeling diodes and also calculate useful PWM frequency, ripple current etc.

FvM said:

1. I can't reproduce the calculation
I think, with the given parameters the current measurement resolution would be 5V/(4096*14*0.2) = 0.436 mA
2. I don't understand the purpose of two driver transistors
3. What's the implemented control method? PWM or analog driver? In the latter case, there should be a DAC.
4. If the control method is PWM, you need to provide free-wheeling diodes and also calculate useful PWM frequency, ripple current etc.

Click to expand...

Please find my feedback for each question.

Hi,

Please find my
1.Your calculation is correct.But you considered 0.2ohm. That is the requirement from resistor. In my circuit i have 0.15ohm resistor so = 5V/(4096*14*0.15)=0.58mA resolution.
2.It is already developed design and i do not have any options to change the layout and only the population change is possible for me. Hence it has both HS and LS control method.
3.The control method is PWM and i have freewheeling Diodes.Sorry that i did not shown in the picture.

As i am doing compatibility study. I wanted to check whether my existing circuit can meet the customer requirement. Now my question is how i can calculate the current control resolution?

Looks like you've already figured out the best case resolution for measuring the solenoid current. The control resolution will be limited by factors such as the PWM duty cycle resolution, the total resistance of the solenoid+FETs+sense resistor, and the driver voltage.

Can anyone let me know how to calculate the load current regulation resolution for my existing circuit?