參數(shù)資料
型號(hào)： FAN5236
廠商： Fairchild Semiconductor Corporation
英文描述： Dual Mobile-Friendly DDR / Dual-output PWM Controller
中文描述： 雙移動(dòng)友好的DDR /雙輸出PWM控制器
文件頁(yè)數(shù)： 12/20頁(yè)
文件大?。?/td> 199K
代理商： FAN5236
PRODUCT SPECIFICATION
FAN5236
12
REV. 1.1.7 4/4/03
Current Processing Section
The following discussion refers to Figure 11.
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 con-
nected 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 currrent
and line voltage. The following expression estimates the
recommended value of R
SENSE
as a function of the maxi-
mum load current (I
LOAD(MAX)
) and the value of the
MOSFET’s R
DS(ON)
:
R
SENSE
must, however, be kept higher than:
Setting the Current Limit
A ratio of ISNS is also compared to the current established
when a 0.9 V internal reference drives the ILIM pin. The
threshold is determined at the point
when the
. Since
therefore
,
Since the tolerance 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 representation of the load current. When using the
MOSFET as the sensing element, the variation of R
DS(ON)
causes proportional variation in the ISNS. This value not
only varies from device to device, but also has a typical junc-
tion 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 temperature. 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.
Figure 12. Improving current sensing accuracy
More accurate sensing can be achieved by using a resistor
(R1) instead of the R
DS(ON)
of the FET as shown in Figure
12. This approach causes higher losses, but yields greater
accuracy in both V
DROOP
and I
LIMIT
. R1 is a low value
(e.g. 10m
) resistor.
Current limit (I
LIMIT
) should be set sufficiently high as to
allow inductor current to rise in response to an output load
transient. Typically, a factor of 1.3 is sufficient. In addition,
since I
LIMIT
is a peak current cut-off value, we will need to
multiply I
LOAD(MAX)
by the inductor ripple current (we’ll
use 25%). For example, in Figure 5 the target for I
LIMIT
would be:
I
LIMIT
> 1.2
×
1.25
×
1.6
×
6A
≈
14A
(6)
Duty Cycle Clamp
During severe load increase, the error amplifier output can
go to its upper limit pushing a duty cycle to almost 100% for
significant amount of time. This could cause a large increase
of the inductor current and lead to a long recovery from a
transient, over-current condition, or even to a failure espe-
cially at high input voltages. To prevent this, the output of
the error amplifier is clamped to a fixed value after two clock
cycles if severe output voltage excursion is detected, limiting
the maximum duty cycle to
This circuit is designed to not interfere with normal PWM
operation. When FPWM is grounded, the duty cycle clamp
is disabled and the maximum duty cycle is 87%.
Gate Driver section
The Adaptive gate control logic translates the internal PWM
control signal into the MOSFET gate drive signals providing
necessary amplification, level shifting and shoot-through
protection. Also, it has functions that help optimize the IC
performance over a wide range of operating conditions.
Since MOSFET switching time can vary dramatically from
type to type and with the input voltage, the gate control logic
provides adaptive dead time by monitoring the gate-to-
source voltages of both upper and lower MOSFETs.
R
SENSE
I
0.30
DS ON
V
IN MAX
)
4.1K
0.125
)
------------------------------------R
100
–
=
(4a)
R
SENSE MIN
)
I
---------------------150
R
)
μ
A
100
–
=
(4b)
9
ISNS
ILIM
4
×
3
>
ISNS
I
R
DS ON
R
SENSE
)
×
+
---100
=
I
LIMIT
R
ILIM
0.9V
4
3
×
9
----------------------+
100
R
DS ON
SENSE
)
)
×
=
(5a)
or
R
ILIM
LIMIT
I
100
--------R
R
DS ON
+
(
)
)
×
=
(5b)
LDRV
PGND
ISNS
R
SENSE
R
Q2
DC
MAX
V
V
IN
--------------
V
IN
-2.4
+
=
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相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
FAN5236_04 制造商:FAIRCHILD 制造商全稱(chēng):Fairchild Semiconductor 功能描述:Dual Mobile-Friendly DDR / Dual-output PWM Controller
FAN5236_10 制造商:FAIRCHILD 制造商全稱(chēng):Fairchild Semiconductor 功能描述:Dual Mobile-Friendly DDR / Dual-Output PWM Controller
FAN5236MTC 功能描述:開(kāi)關(guān)變換器、穩(wěn)壓器與控制器 DC/DC RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開(kāi)關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel
FAN5236MTC_Q 功能描述:開(kāi)關(guān)變換器、穩(wěn)壓器與控制器 DC/DC RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開(kāi)關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel
FAN5236MTCX 功能描述:開(kāi)關(guān)變換器、穩(wěn)壓器與控制器 PWM Controller RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開(kāi)關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel
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