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參數(shù)資料
| 型號(hào)： | LP2957A |
| 廠商： | National Semiconductor Corporation |
| 文件頁(yè)數(shù)： | 8/14頁(yè) |
| 文件大?。?/td> | 378K |
| 代理商： | LP2957A |
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Application Hints
(Continued)
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T
R
(Max). This is calculated
by using the formula:
T
R
(Max) = T
J
(Max) T
A
(Max)
where: T
(Max) is the maximum allowable junction tem-
perature
T
A
(Max) is the maximum ambient temperature
Using the calculated values for T
(Max) and P(Max), the re-
quired value for junction-to-ambient thermal resistance,
θ
(JA)
, can now be found:
θ
(JA)
= T
R
(Max)/P(Max)
If the calculated value is 60C/W
or higher
, the regulator
may be operated without an external heatsink. If the calcu-
lated value is
below
60C/W, an external heatsink is re-
quired. The required thermal resistance for this heatsink,
θ
(HA)
, can be calculated using the formula:
θ
(HA)
=
θ
(JA)
θ
(JC)
θ
(CH)
where:
θ
is the junction-to-case thermal resistance, which is
specified as 3C/W for the LP2957.
θ
is the case-to-heatsink thermal resistance, which is de-
pendent on the interfacing material (see Table 1 and Table
2).
Typical TO-220 Case-To-Heatsink
Thermal Resistance in C/W
TABLE 1. (From AAVID)
Silicone Grease
Dry Interface
Mica with Grease
1.0
1.3
1.4
TABLE 2. (From Thermalloy)
Thermasil III
Thermasil II
Thermalfilm (0.002)
with Grease
1.3
1.5
2.2
θ
is the heatsink-to-ambient thermal resistance. It is this
specification (listed on the heatsink manufacturers data
sheet) which defines the effectiveness of the heatsink. The
heatsink selected must have a thermal resistance which is
equal to or lower
than the value of
θ
(HA)
calculated from the
above listed formula.
ERROR COMPARATOR
This comparator produces a logic “LOW” whenever the out-
put falls out of regulation by more than about 5%. This figure
results from the comparator’s built-in offset of 60 mV divided
by the 1.23V reference. An out-of-regulation condition can
result from low input voltage, current limiting, or thermal lim-
iting.
Figure 2 gives a timing diagram showing the relationship be-
tween the output voltage, the ERROR output, and input volt-
age as the input voltage is ramped up and down to the regu-
lator
without snap-on/snap-off output
. The ERROR signal
becomes low at about 1.3V input. It goes high at about 5V in-
put, where the output equals 4.75V. Since the dropout volt-
age is load dependent, the
input
voltage trip points will vary
with load current. The
output
voltage trip point does not
vary.
The comparator has an open-collector output which requires
an external pull-up resistor. This resistor may be connected
to the regulator output or some other supply voltage. Using
the regulator output prevents an invalid “HIGH” on the com-
parator output which occurs if it is pulled up to an external
voltage while the regulator input voltage is reduced below
1.3V. In selecting a value for the pull-up resistor, note that
while the output can sink 400 μA, this current adds to battery
drain. Suggested values range from 100k to 1 M
. The re-
sistor is not required if the output is unused.
DS011340-7
P
TOTAL
= (V
IN
5)I
L
+ (V
IN
)I
G
*See EXTERNAL CAPACITORS
FIGURE 1. Basic 5V Regulator Circuit
www.national.com
8
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LP2957A-12 WAF | 制造商:Texas Instruments 功能描述: |
| LP2957AIS/NOPB | 功能描述:IC REG LDO 5V .25A TO-263-5 RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - 線性 系列:- 標(biāo)準(zhǔn)包裝:1 系列:- 穩(wěn)壓器拓?fù)浣Y(jié)構(gòu):正，固定式 輸出電壓:4V 輸入電壓:最高 16V 電壓 - 壓降（標(biāo)準(zhǔn)）:0.28V @ 150mA 穩(wěn)壓器數(shù)量:1 電流 - 輸出:150mA 電流 - 限制（最?。?- 工作溫度:-40°C ~ 125°C 安裝類型:表面貼裝 封裝/外殼:5-VFBGA 供應(yīng)商設(shè)備封裝:5-uSMD 包裝:Digi-Reel® 其它名稱:LP2985AIBP-4.0DKR |
| LP2957AIS/NOPB | 制造商:Texas Instruments 功能描述:Voltage Regulator IC |
| LP2957AISX | 功能描述:低壓差穩(wěn)壓器 - LDO RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動(dòng)電壓（最大值）:307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20 |
| LP2957AISX/NOPB | 功能描述:低壓差穩(wěn)壓器 - LDO RoHS:否 制造商:Texas Instruments 最大輸入電壓:36 V 輸出電壓:1.4 V to 20.5 V 回動(dòng)電壓（最大值）:307 mV 輸出電流:1 A 負(fù)載調(diào)節(jié):0.3 % 輸出端數(shù)量: 輸出類型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20 |
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