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I have this IC - Advance regulating PWM

I would like to understand how does the timing resistor and capacitor section works. Sorry for the beginner question but I am unable to understand this section.

The resistor in the RC circuit, will determine the rate of capacitor charging and discharging and that too when R and C are connected in series.

But in this IC, it is given as The timing capacitor provides the ramp signal. How does the charging and discharging of the capacitor take place? I dont see the timing resistor part involved in doing this. Can someone please explain clearly.

Thanks

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  • \$\begingroup\$ How does the charging and discharging of the capacitor take place? In the datasheet of the UC3524 on page 4 it is described how the IC works. As there is no more detailed information it is impossible to say how this exactly works on a transistor level. Anyway, why do you need to understand how this works? You use the IC as described in the datasheet and that is it. \$\endgroup\$ – Bimpelrekkie Jan 16 at 8:49
  • \$\begingroup\$ It's and RC Sawtooth generator circuit. Did you want to know how to make your own? \$\endgroup\$ – Sunnyskyguy EE75 Jan 16 at 9:15
  • \$\begingroup\$ I just wanted how this IC works on a transistor level. That's why I had doubt in this section \$\endgroup\$ – Electronic_Maniac Jan 16 at 9:22
  • \$\begingroup\$ A FET version of a programmable Unijunction with a current source and cap dump at some threshold. \$\endgroup\$ – Sunnyskyguy EE75 Jan 16 at 9:53
  • \$\begingroup\$ Examine the original datasheet (from Signetics Corporation; since bought by Philipps, and now part of NXP??) on their NE/SE555 timer IC. \$\endgroup\$ – analogsystemsrf Jan 16 at 15:16
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The datasheet says that

[…] the amplifier output voltage is then compared to the linear voltage ramp at CT.

"Linear" implies that the capacitor is charged with a constant current. So RT is not simply connected in series, but is used to program a constant current source.

We do not know how that chip implements this current source. But if we assume a current source like this:
Op-amp current source with pass transistor
(source: Wikipedia)

Then we could make it programmable by replacing the Zener diode with the RT resistor and letting a constant current flow through it; the varying resistance will then result in a varying reference voltage.

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