ISL8840AABZ 데이터 시트보기 (PDF) - Renesas Electronics

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ISL8840AABZ

High Performance Industry Standard Single-Ended Current Mode PWM Controller

Renesas Electronics 

ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A

The current sense signal at the end of the ON time for CCM

operation is:

VCS = N-----S----N----RP-----C----S-- IO + ---1-----–----D------2----L--V-s---O--------f--s---w--

V

(EQ. 13)

VREF

where VCS is the voltage across the current sense resistor, Ls

is the secondary winding inductance, and IO is the output

current at current limit. Equation 13 assumes the voltage drop

across the output rectifier is negligible.

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

feedback signal plus the external ramp voltage must sum to

this value when the output load is at the current limit threshold.

Ve + VCS = 1

(EQ. 14)

Substituting Equations 12 and 13 into Equation 14 and solving

for RCS yields

RCS

=

-------------------------------------------------------------------------1---------------------------------------------------------------------------

-D--------f--s--L-w---p------V----I--N--



-1----1---+--–---0-D--.--5- –1





+

N-N----ps-









IO

+

---1-----–----D------2----L--V-s---O--------f--s---w-- 

(EQ. 15)

Adding slope compensation is accomplished in the ISL884xA

using an external buffer transistor and the RTCT signal. A

typical application sums the buffered RTCT signal with the

current sense feedback and applies the result to the CS pin as

shown in Figure 6.

R9

CS

R6

RTCT

C4

FIGURE 6. SLOPE COMPENSATION

Assuming the designer has selected values for the RC filter

(R6 and C4) placed on the CS pin, the value of R9 required to

add the appropriate external ramp can be found by

superposition.

Ve = 2----.R--0---65----D+-----R---R--9---6-

V

(EQ. 16)

The factor of 2.05 in Equation 16 arises from the peak

amplitude of the sawtooth waveform on RTCT minus a

base-emitter junction drop. That voltage multiplied by the

maximum duty cycle is the voltage source for the slope

compensation. Rearranging to solve for R9 yields:

R9 = ---2---.--0---5----D-----V-–---e-V-----e----------R----6-



(EQ. 17)

The value of RCS determined in Equation 15 must be rescaled

so that the current sense signal presented at the CS pin is that

predicted by Equation 13. The divider created by R6 and R9

makes this necessary.

RCS = R-----6--R---+--9--R-----9-  RCS

(EQ. 18)

FN6320 Rev 3.00

April 18, 2007

Page 12 of 15

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