Hi there,
The op amp circuit does work in some circuits, and i know for sure because
i have used it in at least a few designs already. There are a couple of
issues that must be thought about however when designing something like
this. The main issues are:
The op amp should be able to sense voltages near ground (LM358 comes to mind).
This is so 50mv will actually mean something to the input, and certainly not all
op amps can work down that low unless they have a negative supply which you
dont want to have to supply here.
The op amp should have low input offset voltage. This helps to insure that circuits
built with the same value external components will produce the same output. Keep
in mind even 5mv offset is already one-tenth of the full output signal level, and
a swing of +5mv to -5mv would change the output by a whopping 20 percent.
The op amp should have low input offset drift with temperature. This is to insure
output set point accuracy over the full expected temperature range. The full
expected temperature range doesnt necessarily mean the full range of the chip
however. With some op amps to reduce drift a third resistor is placed in series
with the non-inverting amplifier input whos value is equal to the parallel combination
of R1 and R2.
The op amp should be a fairly high frequency device, with GB at least 1MHz.
This is to keep the circuit fast enough to keep up with the regulator chip.
If the amp is too slow it will oscillate.
One other note here...
With 0.5 amp flowing though the sense resistor (0.1 ohm) there will be as you
said 50mv across this resistor. That means there will be 50mv across R2 too.
With 50mv across R2 there will be nine times that across R1 which is 0.45v .
The output voltage will be the sum of the voltages across R1 and R2, which comes
to 0.5 volts, as expected, so the resistor values are correct if the regulator
chip is expecting 0.5 volts input.
The only thing left then is to check for oscillations. Sometimes amps used like this
will oscillate. The way to prevent oscillations is to frequency compensate the
amplifier and/or regulator chip. Since this can be quite a complex process, it
sometimes works to purchase a set of capacitors of various values and try to
find two nodes in the circuit such that when you connect a small value cap there
the oscillations go away. One idea is to use a cap to slow down the regulator
chip a little so the op amp will be much faster than that. Remember without the
op amp the regulator chip expects to see a change in voltage (actuated by itself)
in zero time because the output normally connects directly to the FB pin. Since
the op amp will slow this FB signal down, slowing down the regulator should
reduce oscillations or use an RC filter with the high frequencies emphasized
in the op amp part of the circuit to help speed up transients getting to the
op amp.
Before worrying about this however, check the design by actually building up
the circuit. If you can, simulate it first.
