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参数资料
| 型号： | LM2735XMFEVAL/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 23/58页 |
| 文件大小： | 0K |
| 描述： | BOARD DEMO FOR LM2735 SOT23-5 |
| 标准包装： | 1 |
| 主要目的： | DC/DC，步升 |
| 输出及类型： | 1，非隔离 |
| 输出电压： | 12V |
| 电流 - 输出： | 500mA |
| 输入电压： | 3 ~ 5.5 V |
| 稳压器拓扑结构： | 升压 |
| 频率 - 开关： | 1.6MHz |
| 板类型： | 完全填充 |
| 已供物品： | 板，文档 |
| 已用 IC / 零件： | LM2735 |
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Total Power Losses are:
Table 3. Power Loss Tabulation
SNVS485F – JUNE 2007 – REVISED APRIL 2013
V IN
V OUT
5V
12V
I OUT
V D
F SW
T RISE
T FALL
I Q
R DSon
R DCR
D
η
P INTERNAL = P COND + P SW = 475 mW
500mA
0.4V
1.6MHz
6nS
5nS
4mA
250m ?
75m ?
0.623
86%
POUT
PDIODE
PSWR
PSWF
PQ
PCOND
PIND
PLOSS
6W
236mW
80mW
70mW
20mW
305mW
145mW
856mW
(46)
Calculating R T JA and R < JC
R T JA =
T J - T A
P Dissipatio n
and
R < JC =
T J - T CASE
P Dissipatio n
(47)
We now know the internal power dissipation, and we are trying to keep the junction temperature at or below
125°C. The next step is to calculate the value for R θ JA and/or R ψ JC . This is actually very simple to accomplish,
and necessary if you think you may be marginal with regards to thermals or determining what package option is
correct.
The LM2735 has a thermal shutdown comparator. When the silicon reaches a temperature of 160°C, the device
shuts down until the temperature reduces to 150°C. Knowing this, one can calculate the R θ JA or the R ψ JC of a
specific application. Because the junction to top case thermal impedance is much lower than the thermal
impedance of junction to ambient air, the error in calculating R ψ JC is lower than for R θ JA . However, you will need
to attach a small thermocouple onto the top case of the LM2735 to obtain the R ψ JC value.
Knowing the temperature of the silicon when the device shuts down allows us to know three of the four variables.
Once we calculate the thermal impedance, we then can work backwards with the junction temperature set to
125°C to see what maximum ambient air temperature keeps the silicon below the 125°C temperature.
Procedure:
Place your application into a thermal chamber. You will need to dissipate enough power in the device so you can
obtain a good thermal impedance value.
Raise the ambient air temperature until the device goes into thermal shutdown. Record the temperatures of the
ambient air and/or the top case temperature of the LM2735. Calculate the thermal impedances.
Copyright ? 2007–2013, Texas Instruments Incorporated
Product Folder Links: LM2735
23
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