Uc2834 connecting COMP pin to output of op-amp?

[Magnethicc]

Hello everyone,
I have a question if anyone have any application notes or schematic which shows it is possible to connect output of op-amp to COMP Pin of PWM Controllers?

I do this because I have a few PWM Controllers which used as slaves so I want to feed them the feedback of the master PWM.

So I buffer the feedback and feed the comp pin of PWM Controllers the output of the buffer op-amp (and the feedback pin of the slave controllers I short to ground)


Thank you.

 

[Easy peasy]

as these pins are generally current limited for output ( designed this way ) pulling them down for less power is an accepted design practice,

pushing current into them is an unknown - if the currents are kept low enough that heating does not occur - then the issues become more about other effects inside the chip, an 100 ohm ( or less ) R, in each output would allow you to measure this current - if you are forcing the pin high.

look at the internal schematic for the IC.

 

[Magnethicc]

Since I short FB Pin to ground the COMP pin wants to be high and I force it lower with the buffer op-amp --> I pull current from them.
so by your reply I understand it is a common practice.
Do you have any application notes stating this fact?

Thank you Easy Peasy

as these pins are generally current limited for output ( designed this way ) pulling them down for less power is an accepted design practice,

pushing current into them is an unknown - if the currents are kept low enough that heating does not occur - then the issues become more about other effects inside the chip, an 100 ohm ( or less ) R, in each output would allow you to measure this current - if you are forcing the pin high.

look at the internal schematic for the

 

[Easy peasy]

I don't really keep a store of app notes on hand for such requests . . .

 

[Magnethicc]

I don't really keep a store of app notes on hand for such requests . . .

Ok, well if someday you happen to find one Id surely appreciate it if you could drop it here
Thank you.

 

[cupoftea]

You can of course connect opamp to COMP pin when doing "external control" as such....just use a diode....anode to COMP pin.

You can demonstrate this on the LTspice sim using the LT1243 part, which is pin4pin with the uc3843 etc.

In fact, you can do this with almost all pwm controllers......you will find that the "compensation pin range of control" is usually "0.7V to 2V", or "1.4V to 5V" in the case of UC2843.......so the COMP pin range of control is literally lifted up so that the said diode can be put there and not mess it up.

 

[mtwieg]

I'm assuming the OP is referring to UC2843, not UC2834 (which doesn't have a COMP pin)

Generally I only overdrive if the error amplifier is an OTA with a high output impedance. That's not the case for the UC2843. The datasheet does say the COMP pin has limited source and sink current, and mentions pulling COMP low to force PWM to shut off.

But it doesn't mention overriding COMP for any other purpose. It might work, but should be tested carefully.

 

[Magnethicc]

I'm assuming the OP is referring to UC2843, not UC2834 (which doesn't have a COMP pin)

Generally I only overdrive if the error amplifier is an OTA with a high output impedance. That's not the case for the UC2843. The datasheet does say the COMP pin has limited source and sink current, and mentions pulling COMP low to force PWM to shut off.
View attachment 194564

But it doesn't mention overriding COMP for any other purpose. It might work, but should be tested carefully.

It does show an offline application which connects an opto-coupler from COMP to GND in-order to force the COMP voltage by sinking the sourcing current limit from the COMP pin.
Is it not the same as overriding the COMP voltage using an op-amp?

--- Updated Today at 3:30 PM ---

You can of course connect opamp to COMP pin when doing "external control" as such....just use a diode....anode to COMP pin.

You can demonstrate this on the LTspice sim using the LT1243 part, which is pin4pin with the uc3843 etc.

In fact, you can do this with almost all pwm controllers......you will find that the "compensation pin range of control" is usually "0.7V to 2V", or "1.4V to 5V" in the case of UC2843.......so the COMP pin range of control is literally lifted up so that the said diode can be put there and not mess it up.

Why diode is needed?is it to disconnect the buffer Incase output of buffer is higher than the maximum output voltage of the internal UC2842 error amplifier?

 

[cupoftea]

Is it not the same as overriding the COMP voltage using an op-amp?

...Yes diode is deffo needed...else you could damage the internal opamp.....its not really much of a hassle/restriction anyway.......so just using the diode is fine

 

[Magnethicc]

could you elaborate on the damaging mechanism?

...Yes diode is deffo needed...else you could damage the internal opamp.....its not really much of a hassle/restriction anyway.......so just using the diode is fine

 

[FvM]

UC2843 comp pin sink current is specified with 6 mA typically, there's IMHO no guarantee that it keeps absolute maximum of 10 mA if low comp output is externally driven high.

The obvious solution is to connect Vfb to GND to force comp high.

 

[Magnethicc]

Hi FvM,
Yes, indeed the connection is FB to GND and buffer output to COMP pin.
I suspect that the diode cupoftea recommends disconnect the buffer to block any sinking current of the internal error amplifier

UC2843 comp pin sink current is specified with 6 mA typically, there's IMHO no guarantee that it keeps absolute maximum of 10 mA if low comp output is externally driven high.

The obvious solution is to connect Vfb to GND to force comp high.

 

[dick_freebird]

Check the COMP pin I-V and find two things - the weaker side of the pin current (leading to best setup for FB) and how much, for selecting an op amp capable of doing what must be done.

My POL DC-DC designs were made with all key tap points looped via pads to make this kind of thing easy / no worries.

If you are especially lucky the slave FB path might be jammable to a place that delivers no current and then COMP:COMP could work. On mine the fully pinned out error amp os slaves was used as a local unity gain buffer driving ISET, easy as pie.