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| 型号： | LT3991EMSE-5#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 10/24页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK 5V 1.2A 10-MSOP |
| 标准包装： | 50 |
| 类型： | 降压（降压） |
| 输出类型： | 固定 |
| 输出数： | 1 |
| 输出电压： | 5V |
| 输入电压： | 4.3 V ~ 55 V |
| PWM 型： | 电流模式，混合 |
| 频率 - 开关： | 200kHz ~ 2MHz |
| 电流 - 输出： | 1.2A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽）裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 10-MSOP 裸露焊盘 |
LT3991/LT3991-3.3/LT3991-5
APPLICATIONS INFORMATION
R1 = R2 ? OUT ? 1 ?
performanceoftheLT3991.Thefeedbackresistorsshould
preferably be on the order of MΩ and the Schottky catch
diode should have less than 1μA of typical reverse leak-
age at room temperature. These two considerations are
reiterated in the FB Resistor Network and Catch Diode
Selection sections.
It is important to note that another way to decrease the
pulse frequency is to increase the magnitude of each
single current pulse. However, this increases the output
voltage ripple because each cycle delivers more power to
the output capacitor. The magnitude of the current pulses
was selected to ensure less than 15mV of output ripple in
a typical application. See Figure 2.
V SW
5V/DIV
I L
500mA/DIV
V OUT
20mV/DIV
quiescent current will significantly increase to 1.5mA in
light load situations when synchronized with an external
clock. Holding the SYNC pin high yields no advantages in
terms of output ripple or minimum load to full frequency,
so is not recommended.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
? V ?
? 1.19V ?
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
The total resistance of the FB resistor divider should be
selected to be as large as possible to enhance low current
performance. The resistor divider generates a small load
on the output, which should be minimized to optimize the
low supply current at light loads.
V IN = 48V
V OUT = 3.3V
I LOAD = 20mA
5μs/DIV
3991 F02
When using large FB resistors, a 10pF phase lead capacitor
should be connected from V OUT to FB.
Figure 2. Burst Mode Operation
While in Burst Mode operation, the burst frequency and the
charge delivered with each pulse will not change with output
capacitance. Therefore, the output voltage ripple will be
inversely proportional to the output capacitance. In a typical
application with a 47μF output capacitor, the output ripple
is about 8mV, and with a 100μF output capacitor the output
ripple is about 4mV. The output voltage ripple can continue
to be decreased by increasing the output capacitance.
The LT3991-3.3 and LT3991-5 control an internal 10M FB
resistor divider as well as an internal lead capacitor.
Setting the Switching Frequency
The LT3991 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R T value for a desired switching
frequency is in Table 1.
At higher output loads (above 86mA for the front page
application) the LT3991 will be running at the frequency
programmed by the R T resistor, and will be operating in
standard PWM mode. The transition between PWM and low
ripple Burst Mode operation will exhibit slight frequency
jitter, but will not disturb the output voltage.
To ensure proper Burst Mode operation, the SYNC pin
must be grounded. When synchronized with an exter-
nal clock, the LT3991 will pulse skip at light loads. The
10
Table 1. Switching Frequency vs R T Value
SWITCHING FREQUENCY (MHz)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
R T VALUE (kΩ)
255
118
71.5
49.9
35.7
28.0
22.1
17.4
14.0
11.0
3991fa
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| LT3991IDD#PBF | 功能描述:IC REG BUCK ADJ 1.2A 10DFN RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 开关稳压器 系列:- 产品培训模块:MIC23xxx HyperLight Load™ Regulators 标准包装:5,000 系列:HyperLight Load® 类型:降压（降压） 输出类型:固定 输出数:1 输出电压:1.8V 输入电压:2.7 V ~ 5.5 V PWM 型:混合物 频率 - 开关:4MHz 电流 - 输出:2A 同步整流器:是 工作温度:-40°C ~ 125°C 安装类型:表面贴装 封装/外壳:8-VFDFN 裸露焊盘，8-MLF? 包装:带卷 (TR) 供应商设备封装:8-MLF?（2x2） 产品目录页面:1094 (CN2011-ZH PDF) 其它名称:576-3303-2 |
| LT3991IDD#TRPBF | 功能描述:IC REG BUCK ADJ 1.2A 10DFN RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 开关稳压器 系列:- 标准包装:2,500 系列:- 类型:降压（降压） 输出类型:固定 输出数:1 输出电压:1.2V，1.5V，1.8V，2.5V 输入电压:2.7 V ~ 20 V PWM 型:- 频率 - 开关:- 电流 - 输出:50mA 同步整流器:是 工作温度:-40°C ~ 125°C 安装类型:表面贴装 封装/外壳:10-TFSOP，10-MSOP（0.118"，3.00mm 宽）裸露焊盘 包装:带卷 (TR) 供应商设备封装:10-MSOP 裸露焊盘 |
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