Slope compensation ramp calculation for UCC38084

[cupoftea]

Hi,

Do you agree that equation (2) page 6 of the UCC38084D datasheet is nonsense.

UCC38084 datasheet
8-Pin Current Mode Push-Pull PWM ControllersWith Programmable Slope Compensation datasheet (Rev. E)

All they need to say is that ….
1…ramp current (ISLOPE) has a maximum of 150uA.
2…Ramp current (ISLOPE) is 5 x RSET current,
3….RSET pin has a maximum of 1.5V.
4……then use Ohms Law to get your ramp voltage by doing I(SLOPE) x R(series)
5....Remembering that CS threshold is 0.52V

Would you agree?

 

[FvM]

…ramp current (ISLOPE) has a maximum of 150uA.

No. It's a typical value, just to specify the current mirror gain. The datasheet states nowhere a maximal Islope or minimal Rset. Slope compensation has to be adjusted for the actual current slope, no matter if you vary Rset or Rf. The relation is described in equation 2.

 

[cupoftea]

The datasheet states nowhere a maximal Islope or minimal Rset.

....Yes i agree, and that seems very very bad of the datasheet.....i think i am going to assume that 150uA is the maximum islope allowable. These bias currents are usally in the microamps range, so 150uA sounds ok to use.

...Then i can adjust the slope by adjusting RF.

AYK, the Islope builds up throughout the switching period, to the peak...which i am assuming is 150uA.