UCC28950, SLUA560C

hi, Francesc

do you meet any problem with SLUA560B?

regards,
yunsheng

The equation (140) in SLUA560B says that 12,9K will give Tmin = 100 ns but in fact Tmin is 500 ns, as you can check in the evaluation module, which has  a12K resistor, and it is not possible to decrease this value, as other people in this blog has already stated.

And what is amazing is that the data sheet SLUSA16B gives a different equation (16) and a graph (figure 10) that does not match with the equation.

Other errors in the AB an CD delays graphs have already been reported. It is not possible to unload the newer version SLUA560C: the old one appears under the new name. The data sheet has not been corrected.

Hello Francesc
Thanks for pointing this out, we value these inputs. I am collecting your inputs, along with those from others so that they can be included in any future datasheet update.
Regards
Colin

Hi All,

Another question from Japan customer.

SLUA560C page 3 Equation 9

Why VIN* (1-Dtyp) ?

Customer said, It should be VIN*Dtyp  or VIN* Dmax.

Please explain for me why VIN*(1-Dtyp) is correct.

Best regards,

H.DOI

I agree with your customer. Somone from TI should answer.

Francesc Casanellas

Hello Doi-san

This inequality is used to ensure that the contribution to the slope of the Current Sense signal due to magnetizing current was not more than half that due to the inductor ripple current down-slope, reflected to the primary of the transformer.

The Vin term is the voltage applied to the magnetizing inductance - the rest of the expression reduces to a calculation of the recriprocal (dt/dI) of the output inductor current down-slope during the off time reflected through the transformer turns ratio. The off time is defined by the (1-D) term along with the switching frequency as seen by the output inductor which of course is twice the switching frequency seen on the primary.

Of course L = V dt/dI and we use this expression to limit Lmag > Vin dt/dI

(1-Dtyp) was chosen simply because it reflects normal operating conditions - it is not a worst case condition.

Hopefully this answers your question -

regards
Colin

Colin-san,

Thank you for your advise.
My customer requested last question below and ordered EVM.

Control mode depends on down-slope current only?
And then the formula use down-slope current except the over current limit?

Best Regards,
H.DOI

Hello Doi-san

The UCC28950 can be configured to operate in either Voltage control mode or Current control mode depending on the connection at the RSUM pin. Placing a resistor from the RSUM pin to ground allows the controller to operate in Peak Current Mode control. Connecting a resistor from RSUM to VREF switches the controller to voltage mode control (VMC) with the internal PWM ramp.

Slope compensation is not needed in Voltage Mode Control - EXCEPT when the controller enters current limit operation -.

Slope compensation is needed in current mode control - if the duty cycle is greater than 50%. Slope compensation is set by the resistor at the RSUM pin and there are full details of how to calculate it in section 8.3.11 and Figure 38 of the datasheet.

Please let me know if you have any further questions.

Regards

Colin