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参数资料
| 型号： | LT1123CZ#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 10/16页 |
| 文件大小： | 161K |
| 描述： | IC REG CTRLR SGL 5V TO-92-3 |
| 标准包装： | 2,000 |
| 类型： | 正，固定式 |
| 输出数： | 1 |
| 输出电压： | 5V |
| 电流 - 电源： | 700µA |
| 工作温度： | 0°C ~ 125°C |
| 封装/外壳： | TO-226-3、TO-92-3（TO-226AA）成形引线 |
| 供应商设备封装： | TO-92-3 |
| 包装： | 带卷 (TR) |
10
LT1123
1123fb
PNP. If the output of the regulator circuit can be shorted,
heat sinking must be adequate to limit the rate of tempera-
ture rise of the power device to approximately 50癈/
minute. This can be accomplished with a fairly small heat
sink, on the order of 3 to 4 square inches of surface area.
Design Example
Given the following operating requirements:
5.5V < V
IN
< 7V
I
OUTMAX
= 1.5A
Max ambient temperature = 70癈
V
OUT
= 5V
1. The first step is to determine the required drive current.
This can be found from the Maximum Dropout Voltage
curve. 50mA of drive current will guarantee 0.4V drop-
out at an output current of 2A. This satisfies our
requirements.
I
DRIVE
= 50mA
2. The next step is to determine the value of R
D
. Based on
50mA of drive current and a minimum input voltage of
5.5V, we can select R
D
from the graph of Figure 4. From
the graph the value of R
D
is equal to 50&, so we should
use the next lowest 5% value which is 47&.
R
D
= 47&
3. We can now look at the thermal requirements of the
circuit.
Worst-case power in the LT1123 will be equal to:
V
V
4R
IN(MAX) BE
2
D
(
)
Given: V
IN(MAX)
= 7V, V
BE
= 0.6V, R
D
= 47&
Then: P
MAX
(LT1123) = 0.22W.
APPLICATIO S I FOR ATIO
U
U
U
Assuming a thermal resistance of 150癈/W, the maximum
junction temperature rise above ambient will be equal to
(P
MAX
)(150癈/W) = 33癈. The maximum operating junc-
tion temperature will be equal to the maximum ambient
temperature plus the junction temperature rise above
ambient. In this case we have (maximum ambient = 70癈)
plus (junction temperature rise = 33癈) is equal to 103癈.
This is well below the maximum operating junction tem-
perature of 125癈 for the LT1123.
The power rating for R
D
can be found from the plot of
Figure 8 using V
IN
= 7V and R
D
= 47&. From the plot, R
D
should be sized to dissipate a minimum of 1/2W.
The worst-case power dissipation, for normal operation,
in the MJE1123 will be equal to:
(V
INMAX
V
OUT
)(I
OUTMAX
) = (7V 5V)(1.5A) = 3W
The maximum operating junction temperature of the
MJE1123 is 150癈. The difference between the maximum
operating junction temperature of 150癈 and the maxi-
mum ambient temperature of 70癈 is 80癈. The device
must be mounted to a heat sink which is sized such that the
thermal resistance from the junction of the MJE1123 to
ambient is less than 80癈/3W = 26.7癈/W.
It is recommended that the LT1123 be thermally coupled
to the MJE1123 so that the thermal limit circuit of the
LT1123 can protect both devices. In this case the ambient
temperature for the LT1123 will be equal to the tempera-
ture of the heat sink. The heat sink temperature, under
normal operating conditions, will have to be limited such
that the maximum operating junction temperature of the
LT1123 is not exceeded.
Refer to Linear Technologys list of Suggested Manufac-
turers of Specialized Components for information on
where to find the required heat sinks, resistors and capaci-
tors. This listing is available through Linear Technologys
marketing department.
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