参数资料
| 型号： | LT1777IS#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 10/24页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 0.7A 16SOIC |
| 标准包装： | 2,500 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 1.24 V ~ 30 V |
| 输入电压： | 12 V ~ 48 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 100kHz |
| 电流 - 输出： | 700mA |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 16-SOIC |
LT1777
APPLICATIO N S I N FOR M ATIO N
Deciding upon a value for the sense inductor involves
evaluating the trade-off between overall efficiency (P OUT /
P IN ) and switch current slew rate. Larger sense inductors
yield lower current slew rates which offer reduced high
frequency RFI emissions, but at the expense of poorer
efficiency.
The question is “What is the allowed range of values for a
sense inductor in a given application?” There is really no
minimum limit to the sense inductor, i.e., its value is
allowed to be zero. (In other words, the physical sense
inductor ceases to exist and is replaced by a short circuit.)
This will yield the highest efficiency possible in a given
situation. Although an explicit current slew rate no longer
exists, the naturally less aggressive nature of the LT1777
will often yield quieter supply operation than other stan-
dard switching regulators.
As far as the maximum allowable value for the sense
inductor, this is dictated by the current ramp rate in the
main inductor during the conventional part of the switch-
ing cycle. It is generally overconservative to limit the
switch current slew rate to that exhibited by the main
inductor. This would potentially yield a triangular current
waveform. Efficiency would be greatly reduced at little
As an example, a maximum input voltage of 36V, an output
voltage of 5V and a main inductor value of 220 μ H yields a
maximum suggested sense inductor value of 3.5 μ H.
Circuit behavior versus sense inductor value is shown in
the oscilloscope photos in Figure 2. The circuit and oper-
ating conditions are similar to the Typical Application on
the first page of this data sheet with the exception that the
sense inductor is allowed to assume the series of values:
0 μ H, 0.47 μ H, 1 μ H and 2.2 μ H. Figure 2a shows a close-up
of the leading edge (turn-on) of the current waveform.
Values of 0 μ H and 0.47 μ H are found to yield a dI/dt of
about 2.2A/ μ s, while 1 μ H yields 1.4A/ μ s and 2.2 μ H yields
0.6A/ μ s. Figure 2b shows the trailing edge (turn-off) of the
further gain in noise performance. Stated mathematically,
maximum slew rate in the main inductor occurs at maxi-
mum input voltage as:
200ns/DIV
(a) Leading Edge
1777 F02a
dI
dt
=
Max V IN – V OUT
L MAIN
The sense inductor experiences 2V BE of applied voltage.
This is perhaps 1.0V at a maximum hot condition. If we use
an additional factor of two to be conservative, this yields
a maximum sense inductor value as follows:
=
0 . 5 V Max V IN – V OUT
Max L SENSE MAIN ?
?
L SENSE L MAIN
10
= L
or ,
? 0 . 5 V ?
? Max V IN – V OUT ?
1777 F02b
200ns/DIV
(b) Trailing Edge
Figure 2. V SW Node Current Behavior vs L SENSE Value.
L SENSE = 0 μ H, 0.47 μ H, 1.0 μ H and 2.2 μ H
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