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RT7257EN

更新时间： 2022-06-24 15:44:50
品牌 Logo 应用领域
立锜 - RICHTEK /
页数 文件大小 规格书
15页 225K
描述
3A, 17V, 340kHz Synchronous Step-Down Converter

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中文翻译

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RT7257EN 数据手册

 浏览型号RT7257EN的Datasheet PDF文件第7页浏览型号RT7257EN的Datasheet PDF文件第8页浏览型号RT7257EN的Datasheet PDF文件第9页浏览型号RT7257EN的Datasheet PDF文件第11页浏览型号RT7257EN的Datasheet PDF文件第12页浏览型号RT7257EN的Datasheet PDF文件第13页 
Preliminary  
RT7257E  
Table 2. Suggested Inductors for Typical  
Application Circuit  
1
3
2
V
12V  
IN  
VIN  
EN  
BOOT  
V
OUT  
8V  
C
IN  
C
R
100k  
RT7257E  
10µF x 2  
BOOT  
L
EN1  
Component  
Supplier  
Dimensions  
(mm)  
7
SW  
Series  
R1  
R2  
C
R
OUT  
EN2  
TDK  
TDK  
VLF10045  
SLF12565  
10 x 9.7 x 4.5  
5
6
FB  
8
SS  
12.5 x 12.5 x 6.5  
C
C
C
SS  
R
C
4,  
COMP  
9 (Exposed Pad)  
TAIYO  
YUDEN  
GND  
C
P
NR8040  
8 x 8 x 4  
Figure 4. The Resistors can be Selected to Set IC  
Lockout Threshold  
CIN and COUT Selection  
The input capacitance, CIN, is needed to filter the  
trapezoidal current at the source of the high side MOSFET.  
To prevent large ripple current, a low ESR input capacitor  
sized for the maximum RMS current should be used. The  
RMS current is given by :  
Inductor Selection  
The inductor value and operating frequency determine the  
ripple current according to a specific input and output  
voltage. The ripple current ΔIL increases with higher VIN  
and decreases with higher inductance.  
V
OUT  
V
IN  
I
= I  
−1  
RMS  
OUT(MAX)  
V
IN  
V
OUT  
V
f ×L  
VOUT  
V
IN  
⎡
OUT ⎤ ⎡  
× 1−  
⎥ ⎢  
⎤
This formula has a maximum at VIN = 2VOUT, where  
IRMS = IOUT / 2. This simple worst case condition is  
commonly used for design because even significant  
deviations do not offer much relief.  
ΔIL =  
⎢
⎣
⎥
⎦
⎦ ⎣  
Having a lower ripple current reduces not only the ESR  
losses in the output capacitors but also the output voltage  
ripple. High frequency with small ripple current can achieve  
the highest efficiency operation. However, it requires a  
large inductor to achieve this goal.  
Choose a capacitor rated at a higher temperature than  
required. Several capacitors may also be paralleled to  
meet size or height requirements in the design.  
For the ripple current selection, the value of ΔIL= 0.24(IMAX  
)
For the input capacitor, two 10μF low ESR ceramic  
capacitors are recommended. For the recommended  
capacitor, please refer to Table 3 for more details.  
will be a reasonable starting point. The largest ripple  
current occurs at the highest VIN. To guarantee that the  
ripple current stays below the specified maximum, the  
inductor value should be chosen according to the following  
equation :  
The selection of COUT is determined by the required ESR  
to minimize voltage ripple.  
Moreover, the amount of bulk capacitance is also a key  
for COUT selection to ensure that the control loop is stable.  
Loop stability can be checked by viewing the load transient  
response as described in a later section.  
⎡
⎤ ⎡  
⎤
V
f × ΔI  
V
OUT  
V
IN(MAX)  
OUT  
L =  
× 1−  
⎢
⎥ ⎢  
⎥
L(MAX)  
⎣
⎦ ⎣  
⎦
The inductor's current rating (caused a 40°C temperature  
rising from 25°C ambient) should be greater than the  
maximum load current and its saturation current should  
be greater than the short circuit peak current limit. Please  
see Table 2 for the inductor selection reference.  
The output ripple, ΔVOUT , is determined by :  
1
⎡
⎤
ΔVOUT ≤ ΔIL ESR +  
⎢
⎣
⎥
⎦
8fCOUT  
10  
1...78910111213...15

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