Feedback for low frequency PWM regulator

JMini · 2008-07-04T20:44:37+00:00

I'm building a PWM regulator for an incandescent bulb. Some of thos wasdescribed in a thread called "RMS Approximation of PWM/Square wave". In anycase. Since there is no inductor/diode/cepacitor in the output stage I'll beusing 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 200-800Hzrange). 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 reallyjust a powerful error amplifier. The Output of that is sent to thenon-inverting input of the comparator. So if the feedback voltage drops, theNCP102 increases voltage ot the non-inverting input of the TLV7211, thusincreasing duty cycle. I've tested this type of layout on breadboard usingdifferent components. I got to thinking though.Question:Would it be possible to use a resistor divider between the MIC1557 andcomparator to reduce the voltage of the sawtooth and feed it to theNON-inverting comparator input and send the RMS converter output directly tothe INVERTING input of the comparator? That way, a falling output voltagewould cause a reduction in voltage to the INVERTING input and increase dutycycle? I could reduce the component count.I realize there is no true reference voltage in the system, but since the1557 is fed from a fixed 5V source, the sawtooth would be a constant 5*1/3 to5*2/3 V. Thoughts guys?

Jul 04, 2008 62 Replies

Rate this thread:

HarryD 18 years ago

You design like a TI apps. engineer, lots of silicon. Let's see a schematic so we can remove parts. The UCC2803 is a good choice. At your switching frequency, 250Hz, only

1.0mA is needed for supply current. So with input voltage from 30VDC to 50VDC, a 18K/0.25W resistor will suffice, no regulator is needed. 10K< Rt100mS. What happened to the other members of this NG? Do I have to do a complete design here? This is turning into my day job with no benefits. JF could do this with some 555s. Any more help will require good Tequila. Cheers, Harry

Vote

JMini 18 years ago

Ahh Harry, you're back. Just couldn't stand idly by and watch me wallow in incompetence. Thus far I've just been Looking for uc3843 chips like the 2803 that have the lower voltage range I 've been looking for. The ucc2803 has the lower UVLO that I need and you say it should be fine. Great. All I have are hand drawn schematics at the moment. I was going to order some components and build your design and start fiddling with it. For a softstart, using linear regulators at least, you can produce a softstart but holding down the Vref with a capacitor. But your design has the Vref switched by the BSS84. I'd have to figure out what's going on there. From your design, it looks like I can adjust output voltage by varying the values of the R5s (46.4K and 13.3K)

Vote

HarryD 18 years ago

Ok, ok, updated schematic posted on ABSE with the UCC2803 and soft start added. Q2 (BSS84) is not controlling VR, it is controlling Q3. That is how we generate the voltage ramp for VFF control.

Cheers, Harry

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required