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ISL8845AMBEPZ Datasheet(PDF) 8 Page - Intersil Corporation

Part # ISL8845AMBEPZ
Description  High Performance Industry Standard Single-Ended Current Mode PWM Controller
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Manufacturer  INTERSIL [Intersil Corporation]
Direct Link  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

ISL8845AMBEPZ Datasheet(HTML) 8 Page - Intersil Corporation

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FN6792.0
September 29, 2008
where D is the percent of on-time during a switching cycle.
Setting Q = 1 and solving for Se yields Equation 10:
Since Sn and Se are the on time slopes of the current ramp
and the external ramp, respectively, they can be multiplied
by tON to obtain the voltage change that occurs during tON.
where Vn is the change in the current feedback signal (ΔI)
during the on-time and Ve is the voltage that must be added
by the external ramp.
For a flyback converter, Vn can be solved for in terms of
input voltage, current transducer components, and primary
inductance, yielding
where RCS is the current sense resistor, fSW is the switching
frequency, Lp is the primary inductance, VIN is the minimum
input voltage, and D is the maximum duty cycle.
The current sense signal at the end of the on-time for CCM
operation is:
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.
Substituting Equations 12 and 13 into Equation 14 and
solving for RCS yields Equation 15:
Adding slope compensation is accomplished in the
ISL884xAMBEPZ 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.
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.
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:
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.
Example:
VIN = 12V
VO = 48V
Ls = 800µH
Ns/Np = 10
Lp = 8.0µH
IO = 200mA
Switching Frequency, fSW = 200kHz
Duty Cycle, D = 28.6%
S
e
S
n
1
π
---
0.5
+
⎝⎠
⎛⎞ 1
1D
-------------
1
⎝⎠
⎛⎞
=
(EQ. 10)
V
e
V
n
1
π
---
0.5
+
⎝⎠
⎛⎞ 1
1D
-------------
1
⎝⎠
⎛⎞
=
(EQ. 11)
V
e
Dt
SW VIN RCS
⋅⋅
L
p
--------------------------------------------------
1
π
---
0.5
+
⎝⎠
⎛⎞ 1
1D
-------------
1
⎝⎠
⎛⎞
=
V
(EQ. 12)
V
CS
N
S RCS
N
P
------------------------ I
O
1D
() V
O f
⋅⋅
SW
2L
s
----------------------------------------------
+
⎝⎠
⎜⎟
⎛⎞
=
V
(EQ. 13)
V
e
V
CS
+
1
=
(EQ. 14)
R
CS
1
Df
sw VIN
⋅⋅
L
p
-------------------------------
1
π
---
0.5
+
1D
------------------ 1
⎝⎠
⎜⎟
⎜⎟
⎛⎞
N
s
N
p
-------
I
O
1D
() V
O fsw
⋅⋅
2L
s
--------------------------------------------
+
⎝⎠
⎜⎟
⎛⎞
+
-----------------------------------------------------------------------------------------------------------------------------------------------------
=
(EQ. 15)
CS
RTCT
R6
C4
R9
VREF
FIGURE 6. SLOPE COMPENSATION
V
e
2.05D R
6
R
6
R
9
+
----------------------------
=
V
(EQ. 16)
R
9
2.05D
V
e
() R
6
V
e
----------------------------------------------
=
Ω
(EQ. 17)
R
CS
R
6
R
9
+
R
9
--------------------- R
CS
=
(EQ. 18)
ISL8840AMBEPZ, ISL8841AMBEPZ, ISL8842AMBEPZ, ISL8843AMBEPZ, ISL8844AMBEPZ, ISL8845AMBEPZ


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