ISL6742 Intersil Corporation, ISL6742 Datasheet - Page 15

• Manufacturer Part Number: ISL6742
• Description: Advanced Double-Ended PWM Controller
• Manufacturer: Intersil Corporation

If ∆V
slope compensation is needed and R
If ∆V
value of R
by the external ramp must be reduced by ∆V
Adding slope compensation is accomplished in the ISL6742
using an external buffer and the CT signal. A typical
application sums the buffered CT signal with the current
sense feedback and applies the result to the CS pin as
shown in Figure 14.
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.
Rearranging to solve for R9 yields
The value of R
rescaled so that the current sense signal presented at the
CS pin is that predicted by Equation 17. The divider created
by R6 and R9 makes this necessary.
R
V
R9
R ′
e
CS
CS
–
=
CS
CS
∆ V
=
=
(
------------------------------------------------------------ -
2D V
CS
------------------------------------------------------------------------------------------------------------------------------------- -
N
------- -
N
is less than Ve, then Equation 19 is still valid for the
FIGURE 14. ADDING SLOPE COMPENSATION
is greater than or equal to Ve, then no additional
R6
--------------------- - R
S
P
CS
–
R9
⋅
=
V
+



I
, but the amount of slope compensation added
e
R9
e
--------------------- -
R6
O
2D R6
–
+
CS
+
∆ V
∆ V
⋅
+
⋅
DT
---------------- -
R
2L
R9
determined in Equation 19 must be
CS
CS
CS
CS
SW
O
) R6
⋅
⋅



V
V
R9
IN
C4
R6
N
15
⋅
CT
N
------- -
N
S
P
Ω
–
V
O
CS






+
becomes
V
-------------------------------
IN
CT
1
2
3
4
5
6
7
8
CS
VREF
CT
CS
⋅
L
DT
m
.
ISL6742
SW
(EQ. 22)
(EQ. 23)
(EQ. 24)
(EQ. 25)
ISL6742
16
15
14
13
12
11
10
9
Example:
V
V
L
Np/Ns = 20
Lm = 2mH
I
Oscillator Frequency, Fsw = 400kHz
Duty Cycle, D = 85.7%
N
R6 = 499Ω
Solve for the current sense resistor, R
R
Determine the amount of voltage, Ve, that must be added to
the current feedback signal using Equation 16.
Ve = 153mV
Next, determine the effect of the magnetizing current from
Equation 21.
∆V
Using Equation 24, solve for the summing resistor, R9, from
CT to CS.
R9 = 13.2kΩ
Determine the new value of R
R’
Additional slope compensation may be considered for
design margin. The above discussion determines the
minimum external ramp that is required. The buffer transistor
used to create the external ramp from CT should have a
sufficiently high gain (>200) so as to minimize the required
base current. Whatever base current is required reduces the
charging current into CT and will reduce the oscillator
frequency.
Parallel Operation
Parallel operation of converters using the ISL6742 may be
accomplished using the average current signal, IOUT. IOUT
provides a very accurate representation of the output current
and may be used for active current sharing with many
sharing techniques commonly used including master-slave
and average current sharing methods.
Since IOUT represents the average inductor current (CCM
operation), sharing errors introduced by techniques using
peak inductor current are reduced. In particular, the current
sharing error introduced by mismatched switching
frequencies is eliminated.
O
O
IN
O
CT
CS
CS
= 55A
CS
= 2.0µH
= 12V
= 280V
= 50
= 15.1Ω.
= 15.7Ω
= 91mV
CS
, R’
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CS
CS
, using Equation 25.
, using Equation 19.
July 25, 2005
FN9183.1