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# UCC28610: FB input current calculation

Part Number: UCC28610

Hi Team,

UCC28610's FB pin sinks current from opto-coupler, so the FB current should highly depend on opto-coupler's CTR, is there any way to calcaute FB pin current?

Besides, according to datasheet, the absolute maximum rating of FB pin input current is 1mA, is it possible to exceed 1mA with improper opto-coupler or improper external R-C selection?

Regards,

• Hi, Charles:

Actually the IFB signal is controlled by the feedback loop, not only CTR.

Please refer the schematic as below. Assume IFB=CTR * IFB2nd

As you can see, once Vo varies,  IFB = IFB' to maintain output voltage.

So

If the lower CTR opto is used, IFB2nd needs to be higher to achieve IFB'

If the higher CTR opto is used, IFB2nd needs to lower to meet IFB'

If would like to calculate IFB, you may calculate IFB2nd and get IFB through IFB=IFB2nd*CTR.

May I know why you would like to calculate IFB?  IFB is controlled by feedback loop. So generally IFB would be stable once out is stable, and also it would adjust itself to meet different parameters design in each converter.

Besides, according to datasheet, the absolute maximum rating of FB pin input current is 1mA, is it possible to exceed 1mA with improper opto-coupler or improper external R-C selection?

The IFB might exceed over 1mA by improper feedback loop design, but it is almost not able to make it.

As you already understand the IFB comes from IFB2nd*CTR, and IFB2nd is related to Voutput, and DRV off once IFB > 280uA

So if you would like to get IFB = 1mA. you have to keep Vo is higher than Vo,regulated to get enough IFB2nd and enough CTR.

However, in real case, once IFB > 280uA, DRV stops and than Voutput drops. Also, there is usually a capacitor in parallel of FB pin (C2), the surge current would charge C2 first and then flows into IFB. So it could be but not easy to make IFB over 1mA.

It seems you meet some trouble as FB design. Please let me know if you need any assistance.

Regards,

Wesley