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参数资料
| 型号： | 750108 |
| 厂商： | Wurth Electronics Inc |
| 文件页数： | 20/30页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR LT3748 |
| 设计资源： | LT3748 Design Schematic/Tips |
| 标准包装： | 1 |
| 主要目的： | DC/DC，步降 |
| 输出及类型： | 1，隔离 |
| 功率 - 输出： | 30W |
| 输出电压： | 12V |
| 电流 - 输出： | 2.5A |
| 输入电压： | 22 ~ 75 V |
| 稳压器拓扑结构： | 回扫 |
| 板类型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | LT3748 |
| 相关产品： | LT3748HMS#TRPBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748HMS#PBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748IMS#TRPBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748IMS#PBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748EMS#TRPBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748EMS#PBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP |
| 其它名称： | 732-3308 |
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LT3748
APPLICATIONS INFORMATION
DESIGN EXAMPLE: 12V IN TO 5V, 2A OUT
The first example is an automotive application shown on
the back page of this data sheet—a nominal 12V IN , 5V OUT
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 V DS 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:
N PS = N P /N S
V DS(MAX) ≥ V IN(MAX) + V OUT ? N PS
V R(DIODE) ≥ V IN(MAX) /N PS + V OUT
I OUT(MAX) ≈ 0.85 ? (1 – D) ? N PS ? I LIM /2
D = (V OUT + V F(DIODE) ) ? N PS /(V IN + (V OUT + V F(DIODE) )
? N PS )
I DIODE(RMS) = √(I LIM ? N PS ) 2 ? (1 – D)/3
The equation for output power can be rearranged to solve
for the current limit, I LIM , which can be solved at the
nominal or the minimum V IN depending on application
requirements. In this application the 2A load requirement
will be set at V IN = 7.5V to reduce operating stresses at
higher input voltages. The results of the aforementioned
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 V IN 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:
100mV
R SENSE =
I LIM
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 I LIM 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.
equations in this application are found in Table 2.
Table 2. Voltage Stresses, Output Capability and Diode Current vs Turns Ratio in 12V IN to 5V, 2A Application
N PS
0.5
1
2
3
V DS(MAX)
47.5
50
55
60
V R(DIODE)
95
50
27.5
20
D (V IN = 12V)
0.19
0.31
0.48
0.58
D (V IN = 7.5V)
0.27
0.42
0.59
0.69
I LIM (2A OUT AT V IN = 7.5V)
12.9
8.2
5.8
5.0
I DIODE(RMS) (V IN = 12V)
3.3
3.9
4.8
5.6
3748fa
20
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