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| 型号： | LTC3405ES6#TRMPBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/16页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ .3A SOT23-6 |
| 标准包装： | 1 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.8 V ~ 6 V |
| 输入电压： | 2.5 V ~ 5.5 V |
| PWM 型： | 电流模式，混合 |
| 频率 - 开关： | 1.5MHz |
| 电流 - 输出： | 300mA |
| 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-23-6 细型，TSOT-23-6 |
| 包装： | 标准包装 |
| 供应商设备封装： | TSOT-23-6 |
| 产品目录页面： | 1334 (CN2011-ZH PDF) |
| 其它名称： | LTC3405ES6#TRMPBFDKR |
LTC3405
APPLICATIO S I FOR ATIO
The basic LTC3405 application circuit is shown in Figure 1.
External component selection is driven by the load require-
Table 1. Representative Surface Mount Inductors
MAX DC
ment and begins with the selection of L followed by C IN and
C OUT .
MANUFACTURER PART NUMBER
Taiyo Yuden LB2016T3R3M
Panasonic ELT5KT4R7M
VALUE CURRENT DCR HEIGHT
3.3 μ H 280mA 0.2 ? 1.6mm
4.7 μ H 950mA 0.2 ? 1.2mm
Inductor Selection
Murata
LQH3C4R7M34
4.7 μ H
450mA
0.2 ?
2mm
For most applications, the value of the inductor will fall in
the range of 3.3 μ H to 10 μ H. Its value is chosen based on
the desired ripple current. Large value inductors lower
Taiyo Yuden
Panasonic
Panasonic
LB2016T4R7M
ELT5KT6R8M
ELT5KT100M
4.7 μ H
6.8 μ H
10 μ H
210mA
760mA
680mA
0.25 ? 1.6mm
0.3 ? 1.2mm
0.36 ? 1.2mm
ripple current and small value inductors result in higher
Sumida
CMD4D116R8MC 6.8 μ H
620mA
0.23 ? 1.2mm
ripple currents. Higher V IN or V OUT also increases the ripple
current as shown in equation 1. A reasonable starting point
for setting ripple current is ? I L = 120mA (40% of 300mA).
C IN and C OUT Selection
In continuous mode, the source current of the top MOSFET
V OUT ? 1 ? OUT ?
? I L =
1
( f )( L )
? V ?
? V IN ?
(1)
is a square wave of duty cycle V OUT /V IN . To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 360mA rated
inductor should be enough for most applications (300mA
C IN required I RMS ? I OMAX
[ V OUT ( V IN ? V OUT ) ] 1 / 2
V IN
+ 60mA). For better efficiency, choose a low DC-resistance
inductor.
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
100mA. Lower inductor values (higher ? I L ) will cause this
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Tor-
oid or shielded pot cores in ferrite or permalloy materials
are small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
electrical characteristics. The choice of which style induc-
tor to use often depends more on the price vs size require-
ments and any radiated field/EMI requirements than on
This formula has a maximum at V IN = 2V OUT , where
I RMS = I OUT /2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further derate
the capacitor, or choose a capacitor rated at a higher
temperature than required. Always consult the manufac-
turer if there is any question.
The selection of C OUT is driven by the required effective
series resistance (ESR). An ESR in the range of 100m ? to
200m ? is necessary to provide a stable loop. For the
LTC3405, the general rule for proper operation is:
0.1 ? ≤ C OUT required ESR ≤ 0.6 ?
ESR is a direct function of the volume of the capacitor; that
is, physically larger capacitors have lower ESR. Once the
ESR requirement for C OUT has been met, the RMS current
rating generally far exceeds the I RIPPLE(P-P) requirement.
The output ripple ? V OUT is determined by:
? V OUT L ? ESR +
? ? I
8 fC OUT ?
whattheLTC3405requirestooperate.Table1showssome
typical surface mount inductors that work well in LTC3405
applications.
?
?
1 ?
?
3405fa
8
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