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參數資料
| 型號： | FAN5068DDR |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | FAN5068 Component calculation and simulation tools |
| 中文描述： | FAN5068組件的計算和模擬工具 |
| 文件頁數： | 11/18頁 |
| 文件大小： | 159K |
| 代理商： | FAN5068DDR |
FAN5068
PRODUCT SPECIFICATION
REV. 1.0.1 9/9/04
11
Figure 7. SS Clamp and FB Open Protection
Figure 8. Current Limit / Summing Circuits
ISNS
RAMP
SS
FB
COMP
4
1
μ
A
I
SS
–
+
+
E/A
VREF
–
+
+
PWM
LDRV
PGND
ISNS
2.5V
ILIM det.
R
SENSE
I2 =
ILIM*9.6
ILIM
0.9V
R
ILIM
ILIM
mirror
in +
in –
V to I
ISNS
ISNS
S/H
SS/EN
C
SS
FB
Reference and
Soft Start
TO
PWM
COMP
4.41K
COMP
Current Processing Section
The following discussion refers to Figure 8.
The current through R
SENSE
resistor (ISNS) is sampled
shortly after Q2 is turned on. That current is held, and
summed with the output of the error amplifier. This effec-
tively creates a current mode control loop. R
SENSE
sets the
gain in the current feedback loop. For stable operation, the
voltage induced by the current feedback at the PWM com-
parator input should be set to 30% of the ramp amplitude at
maximum load currrent and line voltage. The following
expression estimates the recommended value of R
SENSE
as a
function of the maximum load current (I
LOAD(MAX
)) and the
value of the MOSFET’s R
DS(ON)
.:
RSENSE must, however, be kept higher than:
Setting the Current Limit
An ISNS is compared to the current established when a 0.9 V
internal reference drives the ILIM pin. R
ILIM
, the R
DS(ON)
of
Q2, and R
SENSE
determine the current limit:
Where ILIMIT is the peak inductor current. Since the toler-
ance on the current limit is largely dependent on the ratio of
the external resistors it is fairly accurate if the voltage drop
on the Switching Node side of R
SENSE
is an accurate repre-
sentation of the load current. When using the MOSFET as
the sensing element, the variation of R
DS(ON)
causes propor-
tional variation in the ISNS. This value not only varies from
device to device, but also has a typical junction temperature
coefficient of about 0.4%/°C (consult the MOSFET datasheet
for actual values), so the actual current limit set point will
decrease propotional to increasing MOSFET die tempera-
ture. A factor of 1.6 in the current limit setpoint should
compensate for all MOSFET R
DS(ON)
variations, assuming
the MOSFET’s heat sinking will keep its operating die
temperature below 125°C.
R
SENSE
I
30%
DS ON
V
IN MAX
)
4.1k
0.125
)
------------------------------------R
100
–
=
(5)
R
SENSE MIN
)
I
-----------------------------------------------------------
R
)
150
A
100
–
=
(6)
R
ILIM
I
LIMIT
---9.6
100
-------------+
R
R
DS ON
(
)
)
×
=
(7)
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