UC3846: Error Amplifier of UC3846

Satyavijay Durgule

Part Number: UC3846
Other Parts Discussed in Thread: UC2846,

I am testing Error Amplifier of UC 3846. Circuit diagram is given below. Pins not shown left open/not connected.

I am increasing voltage at pin no.6 (inverting of EA) expecting decrease in output voltage at pin no.7. But as voltage increases at pin no.6 (by changing pot R2), voltage at pin no.7 also increasing and decreasing voltage at pin 6 decreases voltage at pin 7.

As per my understanding any increase in voltage at pin-6 should decrease voltage at pin- 7. Secondly voltage at pin -6 should be same as voltage at pin-5 i.e. 5volts.

I have changed 3-4 ICs but the result is same. Please tell where I am making mistake.

Thanks.

Satyavijay Durgule.

over 7 years ago

0 Satyavijay Durgule over 7 years ago

Prodigy 130 points

0 Benjamin Lough over 7 years ago in reply to Satyavijay Durgule

TI__Mastermind 26695 points

Hi Satyavijay,

Thanks for your interest in UC2846. I suggest placing a resistor in between the R1/R2 divider and pin 6.

Best Regards,
Ben Lough

0 Satyavijay Durgule over 7 years ago in reply to Benjamin Lough

Prodigy 130 points

Hi,

I have already tried with placing a resistor of different values between voltage divider junction and pin6 but no success.

Regards.

Satyavijay.

0 John Griffin1 over 7 years ago in reply to Satyavijay Durgule

TI__Mastermind 34465 points

Hi
The error amplifier on this part can only source about 0.5mA. Increase the value of R4 to a value greater than 10k.
Also with a 1k pot the max voltage on V- = 2.79V and the voltage on V+ = 5V so the output voltage will remain high.
Regards
John

0 Satyavijay Durgule over 7 years ago in reply to John Griffin1

Prodigy 130 points

I tried R4 with 15k and 22k but the result is same that is output voltage is increasing with increase in voltage at pin-6.

Regards.

Satyavijay.

0 John Griffin1 over 7 years ago in reply to Satyavijay Durgule

TI__Mastermind 34465 points

In that case your schematic is not complete.
With 5V on V+ and a maximum of 2.79V on V-, the output voltage is going to be a fixed voltage equal to the saturation voltage of the amplifier.
I recommend that you check your schematic.
Do you have a decoupling cap on the ic ?
Regards
John

0 Satyavijay Durgule over 7 years ago in reply to John Griffin1

Prodigy 130 points

Out put voltage is not fixed it is changing - increasing with increase in voltage at pin -6 and decreasing with decrease in voltage at pin-6.
Earlier there was no decoupling cap but now it is added and tested but no change in result.
Thanks.
Satyavijay.

0 Satyavijay Durgule over 7 years ago in reply to Satyavijay Durgule

Prodigy 130 points

I found the solution and now increasing voltage at inverting pin-6 decreases voltage at pin-7. The modified circuit is attached other part of circuit as earlier.

Now the question is how the circuit works given on the first page in datasheet of UC3846/7.

Thanks.

Satyavijay.

0 Benjamin Lough over 7 years ago in reply to Satyavijay Durgule

TI__Mastermind 26695 points

Hi Satyavijay,

The schematic on the 1st page of the datasheet uses closed loop feedback with a fixed output voltage set by the resistor divider formed by the 3.2kΩ and 3.6kΩ resistors. As this is a current mode controller, duty cycle is determined by comparing the output of the error amplifier (COMP pin) and the current sense information derived from the C/S+ and C/S- pins. Does this answer your question?

Best Regards,
Ben Lough

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UC3846: Error Amplifier of UC3846