I'm building a PWM regulator for an incandescent bulb. Some of thos was
described in a thread called "RMS Approximation of PWM/Square wave". In any
case. Since there is no inductor/diode/cepacitor in the output stage I'll be
using an RMS converter (LTC1968). For the PWM section I'm using the MIC1557
(SOT-23 size 555 equiv) for a R-C sawtooth to a comparator (TLV7211a)
inverting input. I can choose the frequency (probably in the 200-800Hz
range). The feedback is sent through the LTC1968 RMS converter to the FB pin
(0.8V) of a tiny (SC-70) 5mA voltage regulator (OnSemi NCP102). It's really
just a powerful error amplifier. The Output of that is sent to the
non-inverting input of the comparator. So if the feedback voltage drops, the
NCP102 increases voltage ot the non-inverting input of the TLV7211, thus
increasing duty cycle. I've tested this type of layout on breadboard using
different components. I got to thinking though.
Question:
Would it be possible to use a resistor divider between the MIC1557 and
comparator to reduce the voltage of the sawtooth and feed it to the
NON-inverting comparator input and send the RMS converter output directly to
the INVERTING input of the comparator? That way, a falling output voltage
would cause a reduction in voltage to the INVERTING input and increase duty
cycle? I could reduce the component count.
I realize there is no true reference voltage in the system, but since the
1557 is fed from a fixed 5V source, the sawtooth would be a constant 5*1/3 to
5*2/3 V. Thoughts guys?