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| 型号： | LT3688HFE#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, PDSO24 |
| 封装： | LEAD FREE, PLASTIC, TSSOP-24 |
| 文件页数： | 11/28页 |
| 文件大小： | 415K |
| 代理商： | LT3688HFE#PBF |
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LT3688
19
3688f
age source. However, the full benet of the BIAS pin is not
realized unless it is at least 3V. Ensure that the maximum
voltage ratings of the BST and BIAS pins are not exceeded.
The minimum operating voltage of an LT3688 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle, as outlined in a previous section. For
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3688 is turned on with its EN/UVLO pin when the
output is already in regulation, then the boost capacitor may
not be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on input
and output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. Figure 7 shows a plot of minimum load
to start and to run as a function of input voltage. In many
cases, the discharged output capacitor will present a load
to the switcher, which will allow it to start. The plots show
the worst-case situation where VIN is ramping very slowly.
For lower start-up voltage, the boost diode can be tied to
VIN; however, this restricts the input range to one-half of the
absolute maximum rating of the BST pin. At light loads, the
inductor current becomes discontinuous and the effective
duty cycle can be very high. This reduces the minimum input
voltage to approximately 300mV above VOUT. At higher load
currents, the inductor current is continuous and the duty
cycle is limited by the maximum duty cycle of the LT3688,
requiring a higher input voltage to maintain regulation.
There is one particular issue to note if sequencing is
used. If the BIAS pin is tied to VOUT2, it will be low during
the startup of VOUT1. This will prevent the boost circuit
from working on VOUT1 until it has risen to 90% of its
programmed value, increasing the required startup volt-
age. Using circuit in Figure 6b for VOUT1 will reduce the
startup voltage to its normal value. An alternative is to tie
BIAS to VOUT1, if it is greater than 2.8V.
Soft-Start and Individual Channel Shutdown
The RUN/SS (Run/Soft-Start) pins are used to place the
individual switching regulators in shutdown mode. They
also provide a soft-start function. To shut down either
Figure 7. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
regulator, pull the RUN/SS pin to ground with an open-
drain or collector. Note that if CONFIG is tied high or low
(not open), shutting down Channel 1 will also shut down
Channel 2 because of the sequencing function (See the
Conguration and Sequencing section for more details).
2.5μA current sources pull up on each pin. If the RUN/SS
pin reaches ~0.2V, the channel will begin to switch
If a capacitor is tied from the RUN/SS pin to ground, then
the internal pull-up current will generate a voltage ramp on
this pin. This voltage clamps the VC pin, limiting the peak
switch current and therefore input current during start up.
A good value for the soft-start capacitor is COUT/10,000,
where COUT is the value of the output capacitor.
The RUN/SS pins can be left oating if the Soft-Start feature
is not used. They can also be tied together with a single
capacitor providing soft-start. The internal current sources
APPLICATIONS INFORMATION
LOAD (mA)
TO START
VOUT = 5V
TO RUN
1
10
100
1000
4
INPUT
VOL
TAGE
(V)
7
7.5
6
6.5
4.5
5
5.5
8
3688 F07a
LOAD (mA)
1
10
100
1000
3.0
INPUT
VOL
TAGE
(V)
6.0
6.5
5.0
5.5
3.5
4.0
4.5
7.0
3688 F07b
TO START
TO RUN
VOUT = 3.3V
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相关代理商/技术参数 |
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| LT3688IFE#PBF | 功能描述:IC REG BUCK ADJ 0.8A DL 24TSSOP 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 |
| LT3688IFE#TRPBF | 功能描述:IC REG BUCK ADJ 0.8A DL 24TSSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 开关稳压器 系列:- 设计资源:Design Support Tool 标准包装:1 系列:- 类型:升压（升压） 输出类型:固定 输出数:1 输出电压:3V 输入电压:0.75 V ~ 2 V PWM 型:- 频率 - 开关:- 电流 - 输出:100mA 同步整流器:是 工作温度:-40°C ~ 85°C 安装类型:表面贴装 封装/外壳:SOT-23-5 细型，TSOT-23-5 包装:剪切带 (CT) 供应商设备封装:TSOT-23-5 其它名称:AS1323-BTTT-30CT |
| LT3688IFEPBF | 制造商:LINER 制造商全称:Linear Technology 功能描述:Dual 800mA Step-Down Switching Regulator with Power-On Reset |
| LT3688IUF#PBF | 功能描述:IC REG BUCK ADJ 0.8A DL 24QFN 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 |
| LT3688IUF#TRPBF | 功能描述:IC REG BUCK ADJ 0.8A DL 24QFN RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 开关稳压器 系列:- 设计资源:Design Support Tool 标准包装:1 系列:- 类型:升压（升压） 输出类型:固定 输出数:1 输出电压:3V 输入电压:0.75 V ~ 2 V PWM 型:- 频率 - 开关:- 电流 - 输出:100mA 同步整流器:是 工作温度:-40°C ~ 85°C 安装类型:表面贴装 封装/外壳:SOT-23-5 细型，TSOT-23-5 包装:剪切带 (CT) 供应商设备封装:TSOT-23-5 其它名称:AS1323-BTTT-30CT |
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