I would like to build a high side current source that can source 0-5A with a 1mA resolution. The voltage source feeding into this current source is a variable 0-50V that is always ~3V above the required voltage for that current[1].
My initial thought on doing this was to use a current sense amplifier as the inverting input to an op amp (the non-inverting input is the reference voltage) that controls a p channel MOSFET which controls the current going into the load. Unfortunately, this means that the current sense amplifier is inside the feedback loop of the op amp, which seems to be a problem (I'm getting lots of oscillations).
However, This note from analog shows a low side current sink using a difference amplifier to measure the current through a current sense resistor. It is using (a small amount of) positive feedback, which I don't understand.
This note from linear shows a low voltage, lower current (2A vs the 5A I want), current source. I'm worried about the integrator causing the transient response of the current source to be too slow, and thus too inaccurate.
This note from TI talks about separating the unity gain bandwidths of two amplifiers by a "factor of 5" when using an amplifier inside a feedback loop of another amplifier. (They mention this as a rule of thumb, but don't talk much about how to make it more concrete). However this seems really hand wavy, and it doesn't go into more detail.
My questions:
What is the positive feedback for in the analog note?
Does this change the unity gain bandwidth of the op amp so the factor of 5 is obeyed? And does the amplifier INSIDE the feedback loop have to have a unity gain bandwidth greater than the one being fed-back?
What are other topologies for accurate current sources that can handle 5A?
[1]: For example, if I have a 1A current into 10Ω, the adjustable power supply will be at 13V, and 2A current into 10Ω will give me a power supply of 23V, etc. The transient response of this response is a problem for another day…
Best Answer
You say you want to have 1mA resolution. I take this to include settability as well as controlling ripple (e.g. from PWM) to less than 1mA.
For controlling a set point, there are numerous implementations to consider: single turn pots (with vernier upon vernier), multi-turn pots, digital thumbwheels or rotary switches, toggle switches with a ladder network, etc. I think you were favoring a micro controller with a D/A converter. Any of these options could be used to implement a 5000 interval setpoint.
As far as ripple current, depending on your application you may not want it to span a whole 1mA interval. I was able to achieve ripple of less than 1ma in simulation using as little as 500mH smoothing inductor, by using another 500mH downstream of the current monitor as an output filter. It is self-clocked at about 7kHz using an error integrator followed by a comparator with hysteresis. (Don't try to clock it with an external clock or the output will be erratic.) With the filter inductor ripple was ~500ua; without it ripple was ~2.7ma, but increasing the smoothing inductor to 800uH was enough to be able to eliminate the filter inductor. These are kind of BIG inductors, but not unreasonable. The switching frequency can also be increased, limited by the speed of your components (MOSFET, gate driver, comparators, op amps, etc.), and thermal considerations.