0

I am having a few confusions regarding the PWM IC UC3842:

  1. I am not able to understand why we have two sensing pins on the PWM IC. The current sense pin uses the current to set up the PWM while the feedback takes the voltage, although both are voltages but isn't a single pin enough to have feedback. If the voltage is false on the output then ultimately the current will be false.
  2. I have heard there are two modes, one is voltage mode and the other is the current mode, is this IC saying that it has both the current and voltage modes that's why it has two pins for feedback?? Or the voltage mode control and current mode control is something different than this??
kam1212
  • 404
  • 8
  • The isense pin terminates the current pwm cycle. It is for overload protection. See table 6-1. – Kartman Aug 19 '23 at 13:42
  • You have one pin for the inductor current, the ISENSE pin and another one, VFB, for the regulated variable you want to keep constant (usually the output voltage). In current-mode control, there are two loops, current (inner) and voltage (outer). The outer loop regulates the voltage by setting a peak current setpoint monitored by the inner current loop. Please have a look at the response I gave on SE [here](https://electronics.stackexchange.com/questions/660684/idea-behind-current-mode-control-in-dc-dc-converter/660695#660695) and let me know if you have questions. – Verbal Kint Aug 19 '23 at 13:42
  • @VerbalKint I've understood the voltage mode but the current mode is a bit unclear. If the voltage at out decreases then the inductor current decreases. This means that out reference ramp(inductive voltage) is not constant, then how can it be a reference? – kam1212 Aug 19 '23 at 13:56
  • The inductor peak current setpoint is adjusted by the error amplifier observing the output voltage. It is connected in an *inverting* mode: if the output voltage is below it's regulation target, then the error voltage at COMP pin increases (higher peak current) but if the output voltage is higher than the regulated value, the loop reduces the error voltage and the peak current goes down. The peak current setpoint and error voltage are linked by a div of 3, clamped to 1 V max: if you have 2 V at the COMP pin, then you have a peak of (2-1.2)/3 = 0.27 V, divided by \$R_{sense}\$ for the current. – Verbal Kint Aug 19 '23 at 14:04
  • @VerbalKint I have seen that the open loop gain of the error amplifier is very large then even a small change will give alarge output error of approximately digital considering -5/+5. Is it actually such digital values or they are analog? They can only my analog if the gain is low or the difference is very less giving AVdiff. How is it contstructed? – kam1212 Aug 19 '23 at 15:00
  • 1
    You can have a look at this other [response](https://electronics.stackexchange.com/questions/658103/dc-dc-converter-error-amplifier-output/658123#658123) I gave on SE. The error voltage continuously varies between roughly 0 V and 5 V. Actually, considering the two diodes in series, when the error voltage is below 1.2 V, the duty ratio is zero with this part and it skips cycles so the COMP voltage truly varies between 1.2 V and 5 V in a pure analog way. – Verbal Kint Aug 19 '23 at 15:14
  • I can see a resistor acting at the error amplifier acting as compensator. If reference voltage is 2.5 then the opamp maintains this voltage at the other terminal and thus the error is dropped to zero? Is this method used in this? – kam1212 Aug 19 '23 at 15:47
  • Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/147969/discussion-between-kam1212-and-verbal-kint). – kam1212 Aug 19 '23 at 16:49

1 Answers1

0

  1. No because the current sense pin is used to terminate each individual PWM cycle when current of inductor has reached the required level. Then the voltage sense pin is used to regulate the output voltage, for example by defining how often each PWM cycle happens to keep the output voltage at required level.

  2. current mode means that coil current is used to terminate the PWM cycle. Some other devices use other methods like having constant on time or some other means of determining the PWM length.

Justme
  • 127,425
  • 3
  • 97
  • 261