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參數(shù)資料
| 型號： | LTC1708-PG |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Dual Adjustable 5-Bit VID High Efficiency, 2-Phase Current Mode Synchronous Buck Regulator Controller |
| 中文描述： | 可調(diào)式雙5位VID高效率，2相電流模式同步降壓穩(wěn)壓控制器 |
| 文件頁數(shù)： | 22/32頁 |
| 文件大小： | 358K |
| 代理商： | LTC1708-PG |
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LTC1708-PG
APPLICATIU
normal operation automatically resumes. A shorted top
MOSFET will result in a high current condition which will
open the system fuse. The switching regulator will regu-
late properly with a leaky top MOSFET by altering the duty
cycle to accommodate the leakage.
W
U
U
The Standby Mode (STBYMD) Pin Function
The Standby Mode (STBYMD) pin provides several choices
for start-up and standby operational modes. If the pin is
pulled to ground, the RUN/SS pins for both controllers are
internally pulled to ground, preventing start-up and thereby
providing a single control pin for turning off both control-
lers at once. If the pin is left open or decoupled with a
capacitor to ground, the RUN/SS pins are each internally
provided with a starting current enabling external control
for turning on each controller independently. If the pin is
provided with a current of >3
μ
A at a voltage greater than
2V, both internal linear regulators (INTV
CC
and 3.3V) will
be on even when both controllers are shut down. In this
mode, the onboard 3.3V and 5V linear regulators can
provide power to keep-alive functions such as a keyboard
controller. This pin can also be used as a latching “on” and/
or latching “off” power switch if so designed.
Frequency of Operation
The LTC1708 has an internal voltage controlled oscillator.
The frequency of this oscillator can be varied over a 2 to 1
range. The pin is internally self-biased at 1.19V, resulting
in a free-running frequency of approximately 220kHz. The
FREQSET pin can be grounded to lower this frequency to
approximately 140kHz or tied to the INTV
CC
pin to yield
approximately 310kHz. The FREQSET pin may be driven
with a voltage from 0 to INTV
CC
to fix or modulate the
oscillator frequency as shown in Figure 5.
Minimum On-Time Considerations
Minimum on-time t
ON(MIN)
is the smallest time duration
that the LTC1708 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
charge required to turn on the top MOSFET. Low duty cycle
applications may approach this minimum on-time limit
and care should be taken to ensure that
t
V
V
f
ON MIN
(
OUT
( )
IN
)
<
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC1708 will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple voltage and current will increase.
The minimum on-time for the LTC1708 is generally less
than 200ns. However, as the peak sense voltage decreases
the minimum on-time gradually increases up to about
300ns. This is of particular concern in forced continuous
applications with low ripple current at light loads. If the
duty cycle drops below the minimum on-time limit in this
situation, a significant amount of cycle skipping can occur
with correspondingly larger current and voltage ripple.
FCB Pin Operation
The FCB pin can be used to regulate a secondary winding
or as a logic level input. Continuous operation is forced
when the FCB pin drops below 0.8V. During continuous
mode, current flows continuously in the transformer pri-
mary. The secondary winding(s) draw current only when
the bottom, synchronous switch is on. When primary load
currents are low and/or the V
IN
/V
OUT
ratio is low, the
synchronous switch may not be on for a sufficient amount
of time to transfer power from the output capacitor to the
secondary load. Forced continuous operation will support
secondary windings providing there is sufficient synchro-
nous switch duty factor. Thus, the FCB input pin removes
the requirement that power must be drawn from the
inductor primary in order to extract power from the
auxiliary windings. With the loop in continuous mode, the
auxiliary outputs may nominally be loaded without regard
to the primary output load.
The secondary output voltage V
SEC
is normally set as
shown in Figure 6a by the turns ratio N of the transformer:
V
SEC
(N + 1) V
OUT
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
SEC
will droop. An external resistive divider from
V
SEC
to the FCB pin sets a minimum voltage V
SEC(MIN)
:
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相關代理商/技術參數(shù) |
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| LTC1709EG | 功能描述:IC REG CTRLR BUCK PWM CM 36-SSOP RoHS:否 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標準包裝:4,500 系列:PowerWise® PWM 型:控制器 輸出數(shù):1 頻率 - 最大:1MHz 占空比:95% 電源電壓:2.8 V ~ 5.5 V 降壓:是 升壓:無 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 125°C 封裝/外殼:6-WDFN 裸露焊盤 包裝:帶卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名稱:LM1771SSDX |
| LTC1709EG#PBF | 功能描述:IC REG CTRLR BUCK PWM CM 36-SSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標準包裝:4,000 系列:- PWM 型:電壓模式 輸出數(shù):1 頻率 - 最大:1.5MHz 占空比:66.7% 電源電壓:4.75 V ~ 5.25 V 降壓:是 升壓:無 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 85°C 封裝/外殼:40-VFQFN 裸露焊盤 包裝:帶卷 (TR) |
| LTC1709EG#TR | 功能描述:IC REG CTRLR BUCK PWM CM 36-SSOP RoHS:否 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標準包裝:4,500 系列:PowerWise® PWM 型:控制器 輸出數(shù):1 頻率 - 最大:1MHz 占空比:95% 電源電壓:2.8 V ~ 5.5 V 降壓:是 升壓:無 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 125°C 封裝/外殼:6-WDFN 裸露焊盤 包裝:帶卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名稱:LM1771SSDX |
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