11

FN9181.3

October 31, 2008

Since the peak current limit threshold is 1.00V, the total

current feedback signal plus the external ramp voltage must

sum to this value.

Substituting Equations 13 and 14 into Equation 15 and

For simplicity, idealized components have been used for this

discussion, but the effect of magnetizing inductance must be

considered when determining the amount of external ramp

to add. Magnetizing inductance provides a degree of slope

compensation to the current feedback signal and reduces

the amount of external ramp required. The magnetizing

inductance adds primary current in excess of what is

reflected from the inductor current in the secondary.

effect of the magnetizing current at the current sense

If

If

by the external ramp must be reduced by

Adding slope compensation may be accomplished in the

ISL6752 using the CTBUF signal. CTBUF is an amplified

representation of the sawtooth signal that appears on the CT

pin. It is offset from ground by 0.4V and is 2x the peak-to-peak

amplitude of CT (0.4V to 4.4V). A typical application sums this

signal with the current sense feedback and applies the result

to the CS pin, as shown in Figure 6.

Assuming the designer has selected values for the RC filter

placed on the CS pin, the value of R9 required to add the

appropriate external ramp can be found by superposition.

Rearranging to solve for R9 yields Equation 21:

rescaled so that the current sense signal presented at the

CS pin is that predicted by Equation 14. The divider created

by R6 and R9 makes this necessary.

Example:

Np/Ns = 20

Lm = 2mH

Duty Cycle, D = 85.7%

R6 = 499

Ω

V

e

V

CS

+

1

=

(EQ. 15)

R

CS

N

⋅

N

S

------------------------

1

I

O

V

O

L

O

-------- t

SW

1

π

---

D

2

----

+

⎝⎠

⎛⎞

+

----------------------------------------------------

⋅

=

Ω

(EQ. 16)

I

P

Δ

V

⋅

L

m

-----------------------------

=

A

(EQ. 17)

ΔV

CS

ΔI

⋅

N

CT

--------------------------

=

V

(EQ. 18)

R

CS

N

CT

N

S

N

P

--------

I

O

Dt

SW

2L

O

---------------

V

IN

N

S

N

P

--------

⋅

V

O

–

⎝⎠

⎜⎟

⎛⎞

⋅

+

⎝⎠

⎜⎟

⎛⎞

⋅

V

⋅

L

m

-----------------------------

+

----------------------------------------------------------------------------------------------------------------------------------

=

(EQ. 19)

FIGURE 6. ADDING SLOPE COMPENSATION

R6

C4

R9

CTBUF

CS

1

2

4

3

5

6

7

89

10

11

12

13

14

15

16

ISL6752

V

e

ΔV

CS

–

DV

CTBUF

0.4

–

() 0.4

+

() R6

⋅

R6

R9

+

-------------------------------------------------------------------------------

=

V

(EQ. 20)

R9

DV

CTBUF

0.4

–

() V

e

ΔV

CS

0.4

++

–

() R6

⋅

V

e

ΔV

CS

–

-------------------------------------------------------------------------------------------------------------------

=

Ω

(EQ. 21)

R

′

CS

R6

R9

+

R9

---------------------- R

CS

⋅

=

(EQ. 22)

ISL6752