Comments on Dual ratiometric power scaler design review and issues
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Dual ratiometric power scaler design review and issues
I need to make a circuit board that has the following inputs and outputs:
Inputs and outputs
Inputs:
- $V_{\text{in1}}$ (0-10VDC) PLC controlled signal
- $V_{\text{in2}}$ (0-10VDC) PLC controlled signal
- $V_{\text{batt}}$ (4.8-5.2VDC) battery power. This fluctuates around 5V based on the charge level.
The input signals will be scaled very slowly. Ramps from 0-10V will happen over 100's of milliseconds to seconds. The inputs will sustain their level for a while. Changes are infrequent (30 seconds to > 10 minutes)
The relation between the input and output scale could probably be within a tolerance of 10% max.
The device outputting the signal and the battery will have their ground terminals connected for a shared reference.
Outputs:
- $V_{\text{out1}}$ (2.5-0VDC*) $\implies$ ($\frac{V_{\text{batt}}}{2}$ - GND)** power output
- $V_{\text{out2}}$ (2.5-5VDC*) $\implies$ ($\frac{V_{\text{batt}}}{2}$ - $V_{\text{batt}}$)** power output
Both power outputs must draw current from the battery, up to 1A sustained. The load is a solenoid with continuous positions based on voltage. 3.3v will be a different position than 3.4v and etc.
* This is with respect to their input voltage signal. See the image below
** The output must scale with the battery voltage as a reference. For the rest of the post I will use the 2.5 and 5V values for simplicity.
High level diagram
This is the higher level design for my circuit. The only difference here is the PWM system highlighted in green.
Circuit Diagram
NOTE: The 2 systems ($V_{\text{in1}}$ to $V_{\text{out1}}$ and $V_{\text{in2}}$ to $V_{\text{out2}}$) only differ by the configuration at the LTC6992-1. The circuit does not show duplicated circuitry for the sake of simplicity.
Questions
- Is this design okay? This is my first time making something like this
- From my simulations, I haven't been able to achieve the upper bounds of both functions. How do I get closer to my bounds? and more accurate in general?
- Any general improvements or redesign suggestions?
- I haven't picked an exact op amp model yet. Suggestions for specifications to look out for would be appreciated. Notes on what to look for with MOSFETs and BJTs for this load switch would also be helpful
Datasheets
Post
Partial answer awaiting more specs
Your question can't really be answered until we know how accurate the resulting solenoid drive levels need to be relative to the 0-10 V input signals, and how fast the system must respond. However, here are a few observations in the mean time:
- Use a microcontroller already! There are lots of good reasons analog circuits like this went out 20 years ago. There are many cheap and available micros that come with 12 bit A/Ds and PWM generators built in. There would be three analog inputs to the micro: The two control signals and a reference voltage. The micro would be running directly from the battery, which would also be the positive voltage reference for the A/D. The rest is math deciding what the pulse width needs to be each PWM cycle.
- A TL081 is totally inappropriate in this application. See its datasheet for power supply range and headroom.
- The coils of solenoids are inherently floating. You are going thru some trouble to convert a low side PWM signal to a high side drive. The solenoid will work just fine with one input tied to power and the other to a low side PWM drive. Of course if someone decided to tie one side of the solenoid to ground externally where you can't control it, then you have to deal with this. However, that should at least prompt a discussion of the higher level architecture.
- If the digital PWM signal goes from ground to the battery voltage, then you can use that to drive a P-channel FET as a high side driver directly.
- D1 makes no sense as shown. The anode should go to ground, not the right side of L1. Also, at these voltages it's a no-brainer to make it Schottky.
- Depending on the accuracy you really need, you might want to use synchronous rectification. There are plenty of micros with PWM generators that have complementary outputs with the appropriate dead time controls. Synchronous rectification removes the diode drop from the off-time output voltage as a source of error. You could compensate for some of that in the math when the solenoid is run in continuous mode. At low drive levels it will go to discontinuous mode, in which case it will be much more difficult to know the off-time solenoid drive voltage open-loop.
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