ISL6752DBEVAL1Z datasheet(12/18 Pages) RENESAS

12 / 18 page

ISL6752

FN9181 Rev 4.00

Page 12 of 18

August 1, 2016

Where RCS is the current sense burden resistor, NCT is the

current transformer turns ratio, LO is the output inductance, VO

is the output voltage, and NS and NP are the secondary and

primary turns, respectively.

The inductor current, when reflected through the isolation

transformer and the current sense transformer to obtain the

current feedback signal at the sense resistor yields in

Equation 14:

Where VCS is the voltage across the current sense resistor and

IO is the output current at current limit.

Since the peak current limit threshold is 1.0V, the total current

feedback signal plus the external ramp voltage must sum to

this value.

Substituting Equations 13 and 14 into Equation 15 and solving

for RCS yields in Equation 16:

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.

Where VIN is the input voltage that corresponds to the duty

cycle D and Lm is the primary magnetizing inductance. The

effect of the magnetizing current at the current sense resistor,

RCS, is expressed in Equation 18:

VCS is greater than or equal to Ve, then no additional slope

compensation is needed and RCS becomes Equation 19:

VCS is less than Ve, then Equation 16 is still valid for the

value of RCS, but the amount of slope compensation added by

the external ramp must be reduced by

VCS.

Adding slope compensation may be accomplished in the

ISL6752 using the CTBUF signal. The 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 9.

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:

The value of RCS determined in Equations 16 must be 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:

VIN = 280V

VO = 12V

LO = 2.0µH

Np/Ns = 20

Lm = 2mH

IO = 55A

Oscillator Frequency, fSW = 400kHz

Duty Cycle, D = 85.7%

NCT = 50

R6 = 499Ω

VCS

NS RCS



NP NCT



------------------------ IO



2LO

------------------- VIN

--------



–













(EQ. 14)

Ve VCS

(EQ. 15)

RCS

NP NCT



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

--------tSW



---

----





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





(EQ. 16)



VIN DtSW



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

(EQ. 17)

V

I

P RCS



NCT

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

(EQ. 18)

RCS

NCT

--------

DtSW

2LO

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

VIN

--------



–

















VIN DtSW



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

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

(EQ. 19)

FIGURE 9. ADDING SLOPE COMPENSATION

CTBUF

RCS

ISL6752

Ve VCS

–

DVCTBUF 0.4

–

 0.4

 R



R6 R9

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

(EQ. 20)

DVCTBUF 0.4

–

 V

V

0.4

–

 R



Ve VCS

–

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



(EQ. 21)



R6 R9

--------------------- RCS



(EQ. 22)

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