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Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
Someone might find this useful. I worked out a good way to set the duty cycle on the SG3525 manually. You certainly won't see this in the datasheet. It tested well at 50 kHz and at several hundred kHz.
Tie the noninverting input to Vref. Interestingly, as you adjust the duty cycle up and down, the voltage across the 10k compensation resistor stays at a constant 1 volt. With these component values, you can adjust down to about 15% duty cycle. If you require the ability to adjust all the way down to 0% duty cycle, use 6k2 for the compensation resistor.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well you can do it with just a single potentiometer. Set the output of the error amp to low (inverting to vref, noninverting to gnd) and then use ~50k pot from pin 9 to vref to change your duty.
How is this possible? Because the error amp only uses single pull-up transistor on its output, and there is a 0.1mA current sink pulldown on pin 9, so by varying the resistance, you bring the voltage up and down
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
How did you suss out the fact that the error amp has a pullup resistor on its output? The error amp diagram in the datasheet shows what looks like a current mirror there.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
kell wrote ...
How did you suss out the fact that the error amp has a pullup resistor on its output? The error amp diagram in the datasheet shows what looks like a current mirror there.
The transistor on the output of the error amp has a pull-down 100uA current sink/mirror. That's why you can connect a "pullup" pot to it to vary the voltage. I have done that and it works.
Registered Member #503
Joined: Sun Nov 19 2006, 06:37PM
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Posts: 59
Hi guys,
Sorry for piling on this thread,but I did not want to start a new one for a similar topic :). I have this circuit from Dr.Kilovolt , I think. Can this be used to generate lower frequencies ( about 25~50Hz ) and duty cycles down to about 1% ? Any modifications would be required ? I would be running the entire setup from a 10V supply.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Probably yes, you would need to increase the tuning resistor value as well as timing capacitor. Duty cycle of 1% should be theoretically possible, but I think you will have a really hard time setting that value precisely. Maybe you could connect a second, much smaller pot in series with the big one for precise control. There's also the problem of pulse width drift with temperature, but I think it shouldn't be that bad.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
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Posts: 600
Addressing the original topic, that's what the discharge resistor in the timing portion of the circuit. 0ohms is zero dead time, more resistance adds more dead time, which for all intents and purposes is duty cycle. One pot.
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sg3525 manual duty cycle adjust
