Zintolo, hello.
I do not use pwm or any stepped current or voltage. Instead, my power supply is infinitely variable. I use feedback (better than 12-bit) to measure the current passing through the circuit and then direct very high speed pulses of power to the gate of a mosfet to control current through the mosfet which controls current through the LDR.
The duration of the pulse can be varied between continuous and as short as 60 micro seconds. That pulse is passed through a 1 megohm resistor to the high impedance gate of the mosfet which is loaded with a 1uF tantalum resistor.
The pulse is tri-state -- it can be above or below the gate capacitor state, or floating (causing no change) depending on whether the gate is where it needs to be or which direction I need to drive it.
The result is extremely accurate control of the current in the circuit between 10 milliamps and virtually unmeasureable. The power delivered in 60 micro seconds across a 1 megohm resistor with a few volts differential is vanishingly small and thus I can control the mosfet gate to very tight tolerance.
I cannot imagine any other way of achieving this kind of accuracy using a single IC.
Of course I could expand the control range to a higher impedance. However, the LDR itself is somewhat limited (unstable below a certain tiny current level) and I would say that 50K is the absolute maximum you can achieve with even the very finest of stable power supplies. I think 35K would be a much more reliable and achievable limit.
I do not use pwm or any stepped current or voltage. Instead, my power supply is infinitely variable. I use feedback (better than 12-bit) to measure the current passing through the circuit and then direct very high speed pulses of power to the gate of a mosfet to control current through the mosfet which controls current through the LDR.
The duration of the pulse can be varied between continuous and as short as 60 micro seconds. That pulse is passed through a 1 megohm resistor to the high impedance gate of the mosfet which is loaded with a 1uF tantalum resistor.
The pulse is tri-state -- it can be above or below the gate capacitor state, or floating (causing no change) depending on whether the gate is where it needs to be or which direction I need to drive it.
The result is extremely accurate control of the current in the circuit between 10 milliamps and virtually unmeasureable. The power delivered in 60 micro seconds across a 1 megohm resistor with a few volts differential is vanishingly small and thus I can control the mosfet gate to very tight tolerance.
I cannot imagine any other way of achieving this kind of accuracy using a single IC.
Of course I could expand the control range to a higher impedance. However, the LDR itself is somewhat limited (unstable below a certain tiny current level) and I would say that 50K is the absolute maximum you can achieve with even the very finest of stable power supplies. I think 35K would be a much more reliable and achievable limit.
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