11

FN9182.2

April 4, 2006

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.

cycle D and Lm is the primary magnetizing inductance. The

effect of the magnetizing current at the current sense

If

If

by the external ramp must be reduced by

Adding slope compensation is accomplished in the ISL6753

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 (0.4 - 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 7.

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

rescaled so that the current sense signal presented at the

CS pin is that predicted by Equation 16. The divider created

by R6 and R9 makes this necessary.

Example:

Np/Ns = 20

Lm = 2mH

Oscillator Frequency, Fsw = 400kHz

Duty Cycle, D = 85.7%

R6 = 499

Ω

Determine the amount of voltage, Ve, that must be added to

the current feedback signal using Equation 15.

Ve = 153mV

Next, determine the effect of the magnetizing current from

Equation 20.

Using Equation 23, solve for the summing resistor, R9, from

CTBUF to CS.

R9 = 30.1k

Ω

The above discussion determines the minimum external

ramp that is required. Additional slope compensation may be

considered for design margin.

I

P

∆

V

⋅

L

m

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

=

A

(EQ. 19)

∆V

CS

∆I

⋅

N

CT

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

=

V

(EQ. 20)

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. 21)

FIGURE 7. ADDING SLOPE COMPENSATION

R6

C4

R9

CTBUF

CS

1

2

4

3

5

6

7

8

R

CS

ISL6753

V

e

∆V

CS

–

DV

CTBUF

0.4

–

() 0.4

+

() R6

⋅

R6

R9

+

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

=

V

(EQ. 22)

R9

DV

CTBUF

0.4

–

() V

e

∆V

CS

0.4

++

–

() R6

⋅

V

e

∆V

CS

–

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

=

Ω

(EQ. 23)

R

′

CS

R6

R9

+

R9

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

CS

⋅

=

(EQ. 24)

ISL6753