Terry Given said:
thats an understatement. for voltages that low, you need to use a FET
spec'd for operation at Vgs = 2.7V. "ordinary" FETs have Vth = 3-4V or so,
and are seriously unimpressive when Vgs isnt much larger than Vth. I
recently had to replace a 2N7002 with an NDS355AN for this very reason
(Vcc = +3.3V).
In my experience with using a 555 in a self boosting configuration, even a
low gate drive will invoke enough drain current to get things going, from
that point onwards the Vcc rises rapidly. The circuit I built used a MOSFET
liberated from the SMPSU of a scrap monitor - I can't remember the type but
I think it needed about 6 - 8V for full conduction (the circuit was powered
by a 4.8V NiCd pack), the first prototype didn't have any regulation and the
Vcc shot up to about 30V, the spec sheet for the Hitachi 555 I used said Vcc
max 18V so its a miracle it survived. The boosted Vcc was brought under
control by tying the reset pin to Vcc via a 4k7 resistor and shunting it
with a control transistor who's base was fed from the boosted Vcc via a
zener and current limiting resistor. This crude regulation was adequate for
my application so I left it at that, but if the OP is driving LEDs there
might be a noticeable fluctuation in the brightness, so a little extra
circuitry driving the CV pin might be needed to provide continuous smooth MS
ratio control. Since LED brightness is directly related to current, I'd
suggest using an OP-AMP to compare the voltage across a current sensing
resistor with a reference voltage and directly control the CV pin.