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參數資料
| 型號： | LMS8117AMPX-ADJ |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | 1A Low-Dropout Linear Regulator |
| 中文描述： | 1.25 V-13.8 V ADJUSTABLE POSITIVE LDO REGULATOR, 1.25 V DROPOUT, PDSO4 |
| 封裝： | SOT-223, 3 PIN |
| 文件頁數： | 8/16頁 |
| 文件大小： | 777K |
| 代理商： | LMS8117AMPX-ADJ |
Application Note
(Continued)
When the adjustable regulator is used (
Figure 3
), the best
performance is obtained with the positive side of the resistor
R1 tied directly to the output terminal of the regulator rather
than near the load. This eliminates line drops from appearing
effectively in series with the reference and degrading regu-
lation. For example, a 5V regulator with 0.05
resistance
between the regulator and load will have a load regulation
due to line resistance of 0.05
x I
. If R1 (=125
) is con-
nected near the load, the effective line resistance will be
0.05
(1+R2/R1) or in this case, it is 4 times worse. In
addition, the ground side of the resistor R2 can be returned
near the ground of the load to provide remote ground sens-
ing and improve load regulation.
4.0 PROTECTION DIODES
Under normal operation, the LMS8117A regulators do not
need any protection diode. With the adjustable device, the
internal resistance between the adjust and output terminals
limits the current. No diode is needed to divert the current
around the regulator even with capacitor on the adjust ter-
minal. The adjust pin can take a transient signal of
±
25V with
respect to the output voltage without damaging the device.
When a output capacitor is connected to a regulator and the
input is shorted to ground, the output capacitor will discharge
into the output of the regulator. The discharge current de-
pends on the value of the capacitor, the output voltage of the
regulator, and rate of decrease of V
. In the LMS8117A
regulators, the internal diode between the output and input
pins can withstand microsecond surge currents of 10A to
20A. With an extremely large output capacitor (
≥
1000 μF),
and with input instantaneously shorted to ground, the regu-
lator could be damaged.
In this case, an external diode is recommended between the
output and input pins to protect the regulator, as shown in
Figure 4
.
5.0 HEATSINK REQUIREMENTS
When an integrated circuit operates with an appreciable
current, its junction temperature is elevated. It is important to
quantify its thermal limits in order to achieve acceptable
performance and reliability. This limit is determined by sum-
ming the individual parts consisting of a series of tempera-
ture rises from the semiconductor junction to the operating
environment. A one-dimensional steady-state model of con-
duction heat transfer is demonstrated in
Figure 5
. The heat
generated at the device junction flows through the die to the
die attach pad, through the lead frame to the surrounding
case material, to the printed circuit board, and eventually to
the ambient environment. Below is a list of variables that
may affect the thermal resistance and in turn the need for a
heatsink.
R
θ
JC
(Component
Variables)
Leadframe Size & Material
R
θ
CA
(Application
Variables)
Mounting Pad Size,
Material, & Location
Placement of Mounting
Pad
PCB Size & Material
Traces Length & Width
Adjacent Heat Sources
No. of Conduction Pins
Die Size
Die Attach Material
Molding Compound Size
and Material
Volume of Air
Ambient Temperatue
Shape of Mounting Pad
10119619
FIGURE 3. Best Load Regulation using Adjustable
Output Regulator
10119615
FIGURE 4. Regulator with Protection Diode
L
www.national.com
8
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