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參數資料
| 型號： | FAN5234 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | Terminal Block; Pitch Spacing:0.197"; Mounting Type:PC Board RoHS Compliant: Yes |
| 中文描述： | 移動友好的PWM / PFM控制器 |
| 文件頁數： | 7/15頁 |
| 文件大小： | 537K |
| 代理商： | FAN5234 |
FAN5234
PRODUCT SPECIFICATION
REV. 1.0.10 5/3/04
7
Figure 4. Current Limit / Summing Circuits
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
ISNS
S/H
TO PWM COMP
4.14K
300K
ISNS
0.17pf
I2 =
ILIM*11.2
ILIM
0.9V
R
ILIM
ILIM
The hysteretic comparator causes HDRV turn-on when the
output voltage (at VSEN) falls below the lower threshold
(10mV below VREF) and terminates the PFM signal when
VSEN rises over the higher threshold (5mV above VREF).
The switching frequency is primarily a function of:
1.
Spread between the two hysteretic thresholds
2.
I
LOAD
Output Inductor and Capacitor ESR
3.
A transition back to PWM (Continuous Conduction Mode or
CCM) mode occurs when the inductor current rises suffi-
ciently to stay positive for 8 consecutive cycles. This occurs
when:
where
V
HYSTERESIS
= 15mV and ESR is the equivalent
series resistance of C
OUT
.
Because of the different control mechanisms, the value of the
load current where transition into PWM operation takes
place is typically higher compared to the load level at which
transition into hysteretic mode occurs. Hysteretic mode
can be disabled by setting the FPWM pin HIGH.
Current Processing Section
The following discussion refers to Figure 4.
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 ISNS 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 rec-
ommended 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:
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:
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
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 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.
I
LOAD CCM
)
V
----------2 ESR
=
(3)
R
SENSE
I
)
R
DS ON
V
IN MAX
)
×
4.1K
×
0.125
×
)
×
--------30%
100
–
=
(4a)
R
SENSE
I
--------------------150
R
)
×
μ
A
100
–
=
(4b)
(5)
R
ILIM
I
LIMIT
---11.2
100
-------------+
R
R
DS ON
(
)
)
×
=
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相關代理商/技術參數 |
參數描述 |
|---|---|
| FAN5234_10 | 制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:Dual Mobile-Friendly PWM / PFM Controller |
| FAN5234MTC | 功能描述:開關變換器、穩壓器與控制器 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 開關頻率:1 MHz 工作溫度范圍: 安裝風格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
| FAN5234MTC_Q | 功能描述:開關變換器、穩壓器與控制器 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 開關頻率:1 MHz 工作溫度范圍: 安裝風格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
| FAN5234MTCX | 功能描述:開關變換器、穩壓器與控制器 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 開關頻率:1 MHz 工作溫度范圍: 安裝風格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
| FAN5234QSC | 功能描述:開關變換器、穩壓器與控制器 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 開關頻率:1 MHz 工作溫度范圍: 安裝風格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
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