0-10v input and 10-0v output for LTM8025 3A driver module

[JMG]

I am trying to design a 0 - 10v analog adjust circuit for the LTM8025 3A driver module from Linear. I have a working prototype where I can either use an op amp or voltage divider to get my 0-10v to equal the 0 to 800mV the adj pin needs.
My problem is the device is full on at 0 volts on the adj pin and full off at 800mV.

I need a circuit that gives the exact opposite operation. I need 10v at the input to be 0 volts at the adj pin. Any ideas? I tried using the first half of a dual opamp to invert the 0-10v into the second opamp that does the 12.5:1 reduction, but this just sends the device crazy with noise and has poor current stablity. I've only simulated the dual opamp circuit in LTspice and not actually built it. Maybe my numbers are off on the opamps resistors? I thought this would work.

 

[Prototyp_V1.0]

0-10v input circuit

If you have propriate voltage supply for an opamp, you could use a simple inverting amplifier with gain=1. But you have to make gnd fixed at 5 volts then.

[edit]
Posting a simple schematic here

 

[dick_freebird]

voltage dvider 0-10v

Inverting-configured A=-1 op amp, with the "+" input at 5V and run the divider off its output?

 

[JMG]

potentiometer 0-10v

Hey guys, thanks that helped. It was the 5v connection that made all the difference!

 

[JMG]

Re: 0-10v in, 10-0v out

So this design got a bit more interesting. The initial requirement was 0 - 10V in gives as close to 0 - 100% current out as possible. Max current is set by the current limit resistor on the cathode side of the LED's.

The new requirement is to have a pot on the pcb to adjust max possible current and have the 0- 10V in adjust 0 - 100% of the current limit set by the pot.
There is still the max current limit resistor on the cathode side of the LED's as a fail safe.

So basically the output from the opamp needs to be limited by the pot and the 0-10v in needs to work within that window.

Any ideas? I've tried a pot between the adj pin and the 12K resistor (R4) and tried a second opamp but I'm not having much success.

I'm simulating this in LTSpice 4 and adding the resistor between adj and R4 kills the speed. So far it's been about an hour and we are a couple of nano seconds in.

 

[FvM]

0-10v in, 10-0v out

You don't need to limit the OP output voltage. Max. current is achieved at OP output of 0. The OP output can't go lower, because it's single supply operated. An OP output above 10V could be achieved, but it's no harm, it just shuts down the switcher completely.

You can design precision limiter with OP and R-D circuits, but they aren't required in this case.

 

[JMG]

0-10v in, 10-0v out

I think you may have misunderstood or I didnt explain very well. The opamp is inverting the Vin signal so 10v is 0v at the adj pin. I need a way of setting how low the voltage at the adj pin will go, thus setting the max current. So, lets say currently 10v is a max current of 2A or 20mV. I want a pot on the pcb to set the max current to be 1.8A or 25mV at 10v in. Now 10v is 1.8A, not the 2.0A previously. Basically the 0-10v in is 0 - 100% of what ever max current I set with the pot on the pcb.

 

[senthilkumar.b]

0-10v in, 10-0v out

Hi,

First you should make clear about Vcontrol and Icontrol.

You can do both the things. Dont confue with each other.

If your requirement is current (1.8A) variation, dont worry about voltage(x/y).

Regards,
senthil

 

[JMG]

0-10v in, 10-0v out

It is current control I need but I still need to reference a voltage on the adj pin.

 

[FvM]

0-10v in, 10-0v out

O.K., I didn't read your description exactly. My point was, that the output current is basically limited to the maximum value with OP output at zero.

I understand now, that you want to scale down the o-10 V control voltage, which is different from limiting it to a cdertain value. Scaling could be achieved by a divider behind the 0-10 V source, unfortunately a buffer OP may be needed, because the inverter circuit needs a low impedance source.

 

[JMG]

0-10v in, 10-0v out

I solved this quite simply. I placed a 200K pot between the inverting i/p of the opamp and ground.