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参数资料
| 型号： | LT3748EMS#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, PDSO12 |
| 封装： | LEAD FREE, PLASTIC, MSOP-16/12 |
| 文件页数： | 13/30页 |
| 文件大小： | 284K |
| 代理商： | LT3748EMS#TRPBF |
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LT3748
20
3748fa
DESIGN EXAMPLE: 12VIN TO 5V, 2A OUT
The first example is an automotive application shown on
the back page of this data sheet—a nominal 12VIN, 5VOUT
at 2A with an operating input voltage range of 6V to 45V
with a design focus of maximizing efficiency.
1. Select Transformer Turns Ratio
Transformer turns ratio will affect the requirements for the
MOSFET switch VDS rating, the output diode reverse bias
rating, the output power capability, and the efficiency of
the overall converter. Because the output voltage is low
compared to the forward drop on the output diode and
the currents are high in this application, efficiency can be
optimized by minimizing the RMS diode current. Typical
efficiency in a variety of applications will be 85% to 90%
and due to compromises made for the wide input voltage
range and the low output voltage in this specific applica-
tion, an efficiency of 85% is assumed for calculating
output power. This assumption can be revised once the
application is tested. Equations for evaluating each of the
important criteria are:
NPS = NP/NS
VDS(MAX) ≥ VIN(MAX) + VOUT NPS
VR(DIODE) ≥ VIN(MAX)/NPS + VOUT
IOUT(MAX) ≈ 0.85 (1 – D) NPS ILIM/2
D = (VOUT + VF(DIODE)) NPS/(VIN + (VOUT + VF(DIODE))
NPS)
IDIODE(RMS) = √(ILIM NPS)2 (1 – D)/3
The equation for output power can be rearranged to solve
for the current limit, ILIM, which can be solved at the
nominal or the minimum VIN depending on application
requirements. In this application the 2A load requirement
will be set at VIN = 7.5V to reduce operating stresses at
higher input voltages. The results of the aforementioned
equations in this application are found in Table 2.
APPLICATIONS INFORMATION
Evaluating the results of the table, the 1:2 turns ratio looks
demanding in terms of diode reverse-voltage requirements
(a diode with higher reverse bias capability generally will
have a larger forward drop and therefore lower application
efficiency) and primary side currents and only decreases
the output diode RMS current by 13% from the 1:1 case.
However, on evaluating the minimum and maximum in-
ductance requirements in Step 3, even the 1:1 case does
not allow for enough on-time from maximum VIN for the
range of inductance that provides sufficient off-time.
For that reason, a 2:1 turns ratio is selected, easing the
requirement on the output diode reverse voltage rating
in the process.
2. Calculate Sense Resistor Value
The sense resistor can be calculated by the following
equation:
RSENSE =
100mV
ILIM
The desired 5.8A current limit leads to an unusual value of
0.0172Ω, so the current limit is increased to use a more
standard 0.016Ω value and ILIM of 6.25A.
3. Select a Transformer Based on Inductance and
Saturation Current Requirements
The transformer in this application will be selected to
optimize efficiency at a 80kHz minimum switching fre-
quency at maximum load from the nominal input voltage.
In applications where transformer size is the primary
requirement, reducing the current limit or increasing the
switching frequency may be required. The following equa-
tions select the inductance required for a given switching
frequency at max load and then verify that the inductance
is large enough to satisfy the minimum on and off times
of the LT3748.
Table 2. Voltage Stresses, Output Capability and Diode Current vs Turns Ratio in 12VIN to 5V, 2A Application
NPS
VDS(MAX)
VR(DIODE)
D (VIN = 12V)
D (VIN = 7.5V)
ILIM (2A OUT AT VIN = 7.5V)
IDIODE(RMS) (VIN = 12V)
0.5
47.5
95
0.19
0.27
12.9
3.3
1
50
0.31
0.42
8.2
3.9
2
55
27.5
0.48
0.59
5.8
4.8
3
60
20
0.58
0.69
5.0
5.6
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LT3748HMS#PBF | 功能描述:IC REG CTRLR FLYBK ISO CM 16MSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 特色产品:LM3753/54 Scalable 2-Phase Synchronous Buck Controllers 标准包装:1 系列:PowerWise® PWM 型:电压模式 输出数:1 频率 - 最大:1MHz 占空比:81% 电源电压:4.5 V ~ 18 V 降压:是 升压:无 回扫:无 反相:无 倍增器:无 除法器:无 Cuk:无 隔离:无 工作温度:-5°C ~ 125°C 封装/外壳:32-WFQFN 裸露焊盘 包装:Digi-Reel® 产品目录页面:1303 (CN2011-ZH PDF) 其它名称:LM3754SQDKR |
| LT3748HMS#TRPBF | 功能描述:IC REG CTRLR FLYBK ISO CM 16MSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 V 降压:是 升压:无 回扫:无 反相:无 倍增器:无 除法器:无 Cuk:无 隔离:无 工作温度:-40°C ~ 125°C 封装/外壳:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
| LT3748HMSPBF | 制造商:LINER 制造商全称:Linear Technology 功能描述:100V Isolated Flyback Controller |
| LT3748HMSTRPBF | 制造商:LINER 制造商全称:Linear Technology 功能描述:100V Isolated Flyback Controller |
| LT3748IMS#PBF | 功能描述:IC REG CTRLR FLYBK ISO CM 16MSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:2,500 系列:- PWM 型:电流模式 输出数:1 频率 - 最大:500kHz 占空比:96% 电源电压:4 V ~ 36 V 降压:无 升压:是 回扫:无 反相:无 倍增器:无 除法器:无 Cuk:无 隔离:无 工作温度:-40°C ~ 125°C 封装/外壳:24-WQFN 裸露焊盘 包装:带卷 (TR) |
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