UCC28C53_V01 datasheet(32/48 Pages) TI

32 / 48 page

P1 =

:1 F D;

+ 1 + D

2 × N × R

OUT × COUT

(28)

P2 = N × fSW

(29)

P2 =

(30)

9.2.2.10.2 Slope Compensation

Slope compensation is the large signal subharmonic instability that can occur with duty cycles that may extend

beyond 50% where the rising primary side inductor current slope may not match the falling secondary side

current slope. The subharmonic oscillation would result in an increase in the output voltage ripple and may even

limit the power handling capability of the converter.

The target of slope compensation is to achieve an ideal quality coefficient (QP), equal to 1 at half of the switching

frequency. The QP is calculated with Equation 31.

QP =

× >MC × :1 F D; F 0.5?

(31)

where

• D is the primary side switch duty cycle

• MC is the slope compensation factor, which is defined with Equation 32

C =

+ 1

(32)

where

• Se is the compensation ramp slope

• Sn is the inductor rising slope

The optimal goal of the slope compensation is to achieve QP = 1; upon rearranging Equation 32 the ideal value

of slope compensation factor is determined:

ideal

+ 0.5

1 F D

(33)

For this design to have adequate slope compensation, MC must be 2.193 when D reaches it maximum value of

0.627.

The inductor rising slope (Sn) at the CS pin is calculated with Equation 34.

n =

INmin

× R

= 0.038

(34)

The compensation slope (Se) is calculated with Equation 35.

e = :MC

F 1; × Sn = 44.74

(35)

The compensation slope is added into the system through RRAMP and RCSF. The CRAMP is an AC-coupling

capacitor that allows the voltage ramp of the oscillator to be used without adding an offset to the current sense;

UCC28C53, UCC28C54, UCC28C55

UCC28C56H, UCC28C57H, UCC28C57L, UCC28C59

SLUSER8 – JUNE 2022

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