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參數(shù)資料
| 型號： | LTC3775EMSE#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 575 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | LEAD FREE, PLASTIC, MSOP-16 |
| 文件頁數(shù)： | 11/34頁 |
| 文件大小： | 534K |
| 代理商： | LTC3775EMSE#TRPBF |
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LTC3775
19
3775fa
High Duty Cycle Operation
The maximum duty cycle is limited by the LTC3775 internal
oscillator reset time, the propagation delay of the PWM
comparator and the BOOST pin supply refresh rate. The
minimum off-time is typically 300ns.
The top MOSFET driver is biased from the oating bootstrap
capacitor, CB, which normally recharges during each off
cycle through an external diode when the top MOSFET turns
off. If the input voltage, VIN, decreases to a voltage close to
VOUT, the controller will enter dropout and attempt to turn
on the top MOSFET continuously. To avoid depleting the
charge on the bootstrap capacitor, CB, the LTC3775 has an
internal counter that turns on the bottom MOSFET every
eight cycles for 200ns to refresh the bootstrap capacitor.
Figure 13 shows maximum duty cycle operation with the
200ns BOOST pin supply refresh.
step-down VIN to VOUT ratios, another consideration is
the minimum on-time of the LTC3775 (see the Minimum
On-Time Considerations section). A nal consideration for
operating frequency is that in noise-sensitive communica-
tions systems, it is often desirable to keep the switching
noise out of a sensitive frequency band.
The LTC3775 uses a constant frequency architecture that
can be programmed over a 250kHz to 1MHz range with a
single resistor from the FREQ pin to ground, as shown in
Figure 14. The nominal voltage on the FREQ pin is 1.22V,
and the current that ows from this pin is used to charge
and discharge an internal oscillator capacitor. The value of
RSET for a given operating frequency can be chosen from
Figure 14 or from the following equation:
RSET(k ) =
19500
f(kHz)
The oscillator can also be synchronized to an external clock
applied to the MODE/SYNC pin with a frequency in the
range of ±20% of the programmed free-running frequency
set by the FREQ pin. In this synchronized mode, pulse-
skipping mode operation is disabled. The clock high level
must exceed 1.5V for a minimum of approximately 25ns
to engage the feature. The bottom MOSFET will turn-on
following the rising edge of the external clock.
APPLICATIONS INFORMATION
Figure 13. Maximum Duty Cycle Waveforms
RSET VALUE (kΩ)
0
FREQUENCY
(kHz)
1000
900
800
700
600
500
400
300
200
80
3775 F14
20
40
60
100
70
10
30
50
90
Figure 14. Frequency Set Resistor (RSET) Value
BG
5V/DIV
TG
5V/DIV
2μs/DIV
3775 F13
VIN = 4.5V
VOUT = 4.2V
LOAD = 0A
SW FREQ = 500kHz
OSCILLATOR
RESET
BOOST PIN
SUPPLY REFRESH
EXTERNAL COMPONENTS SELECTION
Operating Frequency
The choice of operating frequency and inductor value is
a trade-off between efciency and component size. Low
frequency operation improves efciency by reducing
MOSFET switching losses and gate charge losses. However,
lower frequency operation requires more inductance for a
given amount of ripple current, resulting in a larger induc-
tor size and higher cost. If the ripple current is allowed
to increase, larger output capacitors may be required to
maintain the same output ripple. For converters with high
Top MOSFET Driver Supply
An external bootstrap capacitor, CB, connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET. This capacitor is charged through diode DB from
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| LTC3775EUD#PBF | 功能描述:IC REG CTRLR BUCK PWM VM 16-QFN RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:96% 電源電壓:4 V ~ 36 V 降壓:無 升壓:是 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:24-WQFN 裸露焊盤 包裝:帶卷 (TR) |
| LTC3775EUD#PBF | 制造商:Linear Technology 功能描述:IC SYNC STEP-DOWN DC/DC CONTROLLER QFN16 |
| LTC3775EUD#TRPBF | 功能描述:IC REG CTRLR BUCK PWM VM 16-QFN RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:96% 電源電壓:4 V ~ 36 V 降壓:無 升壓:是 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:24-WQFN 裸露焊盤 包裝:帶卷 (TR) |
| LTC3775IMSE#PBF | 功能描述:IC REG CTRLR BUCK PWM VM 16-MSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:96% 電源電壓:4 V ~ 36 V 降壓:無 升壓:是 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:24-WQFN 裸露焊盤 包裝:帶卷 (TR) |
| LTC3775IMSE#TRPBF | 功能描述:IC REG CTRLR BUCK PWM VM 16-MSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:96% 電源電壓:4 V ~ 36 V 降壓:無 升壓:是 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:24-WQFN 裸露焊盤 包裝:帶卷 (TR) |
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