參數(shù)資料
型號(hào)： FAN5026MTCX
廠(chǎng)商： FAIRCHILD SEMICONDUCTOR CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： Dual DDR/Dual-output PWM Controller
中文描述： DUAL SWITCHING CONTROLLER, 345 kHz SWITCHING FREQ-MAX, PDSO28
封裝： 4.40 MM, MO-153AB, TSSOP-28
文件頁(yè)數(shù)： 10/17頁(yè)
文件大小： 192K
代理商： FAN5026MTCX
PRODUCT SPECIFICATION
FAN5026
10
REV. 1.0.2b 9/2/03
Initialization and Soft Start
Assuming EN is high, FAN5026 is initialized when VCC
exceeds the rising UVLO threshold. Should VCC drop below
the UVLO threshold, an internal Power-On Reset function
disables the chip.
The voltage at the positive input of the error amplifier is lim-
ited by the voltage at the SS pin which is charged with a 5
μ
A
current source. Once C
SS
has charged to VREF (0.9V) the
output voltage will be in regulation. The time it takes SS to
reach 0.9V is:
where T
0.9
is in seconds if C
SS
is in
μ
F.
When SS reaches 1.5V, the Power Good outputs are enabled
and hysteretic mode is allowed. The converter is forced into
PWM mode during soft start.
Current Processing Section
The following discussion refers to Figure 10.
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. The resistor
connected to ISNSx pin (R
SENSE
) sets the gain in the current
feedback loop. For stable operation, the voltage induced by
the current feedback at the PWM comparator input should be
set to 30% of the ramp amplitude at maximum load current
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)
:
R
SENSE
must, however, be kept higher than:
Figure 10. Current Limit
/
Summing Circuits
T
0.9
0.9
----------5
C
×
=
(1)
R
SENSE
I
30%
DS ON
V
IN MAX
)
4.1K
0.125
)
------------------------------------R
100
–
=
(2a)
R
SENSE MIN
)
I
---------------------150
R
)
μ
A
100
–
=
(2b)
LDRV
PGND
ISNS
in +
in –
2.5V
ILIM det.
R
SENSE
SS
1.5M
C
SS
VSEN
V to I
Reference and
Soft Start
17pF
I1B =
ISNS
9
I2 =
4 * ILIM
3
ILIM
0.9V
R
ILIM
ILIM mirror
S/H
TO PWM COMP
4.14K
300K
I1A =
ISNS
0.17pF
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