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參數(shù)資料
| 型號： | ISL6742AAZA-T |
| 廠商： | INTERSIL CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Advanced Double-Ended PWM Controller |
| 中文描述： | 0.1 A SWITCHING CONTROLLER, 2000 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 0.150 INCH, ROHS COMPLIANT, PLASTIC, SSOP-16 |
| 文件頁數(shù)： | 15/18頁 |
| 文件大?。?/td> | 422K |
| 代理商： | ISL6742AAZA-T |
15
FN9183.1
July 25, 2005
If
V
CS
is greater than or equal to Ve, then no additional
slope compensation is needed and R
CS
becomes
N
S
N
P
O
If
V
CS
is less than Ve, then Equation 19 is still valid for the
value of R
CS
, but the amount of slope compensation added
by the external ramp must be reduced by
V
CS
.
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
CS
determined in Equation 19 must be
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.
Example:
V
IN
= 280V
V
O
= 12V
L
O
= 2.0
μ
H
Np/Ns = 20
Lm = 2mH
I
O
= 55A
Oscillator Frequency, Fsw = 400kHz
Duty Cycle, D = 85.7%
N
CT
= 50
R6 = 499
Solve for the current sense resistor, R
CS
, using Equation 19.
R
CS
= 15.1
.
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
CS
= 91mV
Using Equation 24, solve for the summing resistor, R9, from
CT to CS.
R9 = 13.2k
Determine the new value of R
CS
, R’
CS
, using Equation 25.
R’
CS
= 15.7
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.
R
CS
-------
I
O
----------------
V
IN
S
N
P
-------
V
O
–
+
V
DT
m
-------------------------------
+
-------------------------------------------------------------------------------------------------------------------------------------
=
(EQ. 22)
FIGURE 14. ADDING SLOPE COMPENSATION
R6
C4
R9
R
CS
CT
CT
CS
1
2
4
3
5
6
7
8
9
10
11
12
13
14
15
16
ISL6742
VREF
V
e
V
CS
–
R6
+
R9
----------------------
=
V
(EQ. 23)
R9
2D
-----------------------–
V
e
V
CS
+
–
(
)
R6
=
(EQ. 24)
R
′
CS
--------+
R
CS
=
(EQ. 25)
ISL6742
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