參數(shù)資料
型號(hào)： LM2831ZMFX
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 穩(wěn)壓器
英文描述： High Frequency 1.5A Load - Step-Down DC-DC Regulator
中文描述： 2.5 A SWITCHING REGULATOR, 3750 kHz SWITCHING FREQ-MAX, PDSO5
封裝： SOT-23, 5 PIN
文件頁(yè)數(shù)： 15/27頁(yè)
文件大?。?/td> 893K
代理商： LM2831ZMFX
Thermal Definitions
(Continued)
Thermal impedance from the silicon junction to the ambient
air is defined as:
The PCB size, weight of copper used to route traces and
ground plane, and number of layers within the PCB can
greatly effect R
θ
. The type and number of thermal vias can
also make a large difference in the thermal impedance.
Thermal vias are necessary in most applications. They con-
duct heat from the surface of the PCB to the ground plane.
Four to six thermal vias should be placed under the exposed
pad to the ground plane if the LLP package is used.
Thermal impedance also depends on the thermal properties
of the application operating conditions (Vin, Vo, Io etc), and
the surrounding circuitry.
Silicon Junction Temperature Determination Method 1:
To accurately measure the silicon temperature for a given
application, two methods can be used. The first method
requires the user to know the thermal impedance of the
silicon junction to top case temperature.
Some clarification needs to be made before we go any
further.
R
θ
JC
is the thermal impedance from all six sides of an IC
package to silicon junction.
R
Φ
is the thermal impedance from top case to the silicon
junction.
In this data sheet we will use R
Φ
so that it allows the user
to measure top case temperature with a small thermocouple
attached to the top case.
R
Φ
is approximately 30C/Watt for the 6-pin LLP package
with the exposed pad. Knowing the internal dissipation from
the efficiency calculation given previously, and the case
temperature, which can be empirically measured on the
bench we have:
Therefore:
T
j
= (R
Φ
JC
x P
LOSS
) + T
C
From the previous example:
T
j
= (R
Φ
JC
x P
INTERNAL
) + T
C
T
j
= 30C/W x 0.189W + T
C
The second method can give a very accurate silicon junction
temperature.
The first step is to determine R
θ
of the application. The
LM2831 has over-temperature protection circuitry. When the
silicon temperature reaches 165C, the device stops switch-
ing. The protection circuitry has a hysteresis of about 15C.
Once the silicon temperature has decreased to approxi-
mately 150C, the device will start to switch again. Knowing
this, the R
θ
for any application can be characterized during
the early stages of the design one may calculate the R
θ
by
placing the PCB circuit into a thermal chamber. Raise the
ambient temperature in the given working application until
the circuit enters thermal shutdown. If the SW-pin is moni-
tored, it will be obvious when the internal PFET stops switch-
ing, indicating a junction temperature of 165C. Knowing the
internal power dissipation from the above methods, the junc-
tion temperature, and the ambient temperature R
θ
JA
can be
determined.
Once this is determined, the maximum ambient temperature
allowed for a desired junction temperature can be found.
An example of calculating R
θ
JA
for an application using the
National Semiconductor LM2831 LLP demonstration board
is shown below.
The four layer PCB is constructed using FR4 with
1
2
oz
copper traces. The copper ground plane is on the bottom
layer. The ground plane is accessed by two vias. The board
measures 3.0cm x 3.0cm. It was placed in an oven with no
forced airflow. The ambient temperature was raised to
144C, and at that temperature, the device went into thermal
shutdown.
From the previous example:
P
INTERNAL
= 189mW
If the junction temperature was to be kept below 125C, then
the ambient temperature could not go above 109C
T
j
- (R
θ
JA
x P
LOSS
) = T
A
125C - (111C/W x 189mW) = 104C
LLP Package
For certain high power applications, the PCB land may be
modified to a "dog bone" shape (see Figure 6). By increasing
the size of ground plane, and adding thermal vias, the R
θ
JA
for the application can be reduced.
20174868
FIGURE 4. Internal LLP Connection
L
www.national.com
15
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相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
LM2831ZMFX/NOPB 功能描述:直流/直流開關(guān)調(diào)節(jié)器 RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5
LM2831ZSD 功能描述:直流/直流開關(guān)調(diào)節(jié)器 RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5
LM2831ZSD/NOPB 功能描述:直流/直流開關(guān)調(diào)節(jié)器 RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5
LM2831ZSDNOPB 制造商:Texas Instruments 功能描述:
LM2831ZSDX 功能描述:直流/直流開關(guān)調(diào)節(jié)器 RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5
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