- 您現(xiàn)在的位置：買賣IC網(wǎng) > PDF目錄385493 > LP2957 (National Semiconductor Corporation) 5V Low-Dropout Regulator for μP Applications(μP應(yīng)用的5V低壓差穩(wěn)壓器) PDF資料下載
參數(shù)資料
| 型號： | LP2957 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 5V Low-Dropout Regulator for μP Applications(μP應(yīng)用的5V低壓差穩(wěn)壓器) |
| 中文描述： | 5V的低壓差的微處理器應(yīng)用（微處理器應(yīng)用的5V的低壓差穩(wěn)壓器穩(wěn)壓器） |
| 文件頁數(shù)： | 10/14頁 |
| 文件大?。?/td> | 378K |
| 代理商： | LP2957 |
Application Hints
(Continued)
The difference in input voltage measured at no load and
full load defines the amount of hysteresis required for
proper snap-on/snap-off operation
(the programmed hys-
teresis must be
greater
than the difference in voltages).
CALCULATING RESISTOR VALUES:
The values of R1, R2 and R3 can be calculated assuming
the designer knows the hysteresis.
In most transformer-powered applications, it can be as-
sumed that
V
(the input voltage at turn-off)
should be set
for about 5.5V,
since this allows about 500 mV across the
LP2957 to keep the output in regulation until it snaps off.
V
(the input voltage at turn on) is found by
adding the hyster-
esis voltage to V
OFF
.
R1, R2 and R3 are found by solving the node equations for
the currents entering the node nearest the shutdown pin
(written at the turn-on and turn-off thresholds).
The shutdown pin bias current (10 nA typical) is not included
in the calculations:
Since these
two
equations contain
three
unknowns (R1, R2
and R3) one resistor value must be assumed and then the
remaining two values can be obtained by solving the equa-
tions.
The node equations will be simplified by solving both equa-
tions for R2, and then equating the two to generate an ex-
pression in terms of R1 and R3.
Setting these equal to each other and solving for R1 yields:
The same equation solved for R3 is:
A value for R1 or R3 can be derived using either one of the
above equations, if the designer assumes a value for one of
the resistors.
The simplest approach is to assume a value for R3. Best re-
sults will typically be obtained using values between about
20 k
and 100 k
(this keeps the current drain low, but also
generates realistic values for the other resistors).
There is no limit on the
minimum value
of R3, but current
should be minimized as it generates power that drains the
source and does not power the load.
SUMMARY: TO SOLVE FOR R1, R2 AND R3:
1.
Assume a value for either R1 or R3.
2.
Solve for the other variable using the equation for R1 or
R3.
3.
Take the values for R1 and R3 and plug them back into
either
equation for R2 and solve for this value.
DESIGN EXAMPLE
#
1:
A 5V regulated output is to be powered from a transformer
secondary which is rectified and filtered. The voltage V
is
measured at zero current and maximum current (600 mA) to
determine the minimum allowable hysteresis.
V
is measured using an oscilloscope (both traces are
shown on the same grid for clarity):
The full-load voltage waveform from a transformer-powered
supply will have ripple voltage as shown. The correct point to
measure is the
lowest
value of the waveform.
The 1.2V differential between no-load and full-load condi-
tions means that
at least 1.2V
of hysteresis is required for
proper snap-on/snap-off operation (for this example,
we will
use 1.5V
).
As a starting point, we will assume:
V
OFF
= 5.5V
V
ON
= V
OFF
+ HYST = 5.5 + 1.5 = 7V
R3 = 49.9k
Solving for R1:
Turn-ON Transition
DS011340-11
Turn-OFF Transition
DS011340-12
FIGURE 6. Equivalent Circuits
DS011340-13
FIGURE 7. V
IN
VOLTAGE WAVEFORMS
www.national.com
10
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| LP2957A | |
| LP2957AIS | 5V Low-Dropout Regulator for P Applications |
| LP2957AIT | 5V Low-Dropout Regulator for P Applications |
| LP2957IS | 5V Low-Dropout Regulator for P Applications |
| LP2957IT | 5V Low-Dropout Regulator for P Applications |
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（最小）:- 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 |
發(fā)布緊急采購，3分鐘左右您將得到回復(fù)。