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.


  • \$\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 '19 at 8:49
  • \$\begingroup\$ It's and RC Sawtooth generator circuit. Did you want to know how to make your own? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 16 '19 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\$ – Freshman Jan 16 '19 at 9:22
  • \$\begingroup\$ A FET version of a programmable Unijunction with a current source and cap dump at some threshold. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 16 '19 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 '19 at 15:16

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.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.