Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>On Fri, 04 Jul 2008 20:44:37 GMT, "JMini"
><j.minih...@minihane.tzo.com> wrote:

>>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?

>I guess the real question is, why? The light output will still be very
>nonlinear on any control input. Why not feedback on the light?

>John