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參數(shù)資料
| 型號： | LP2957IT |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | 5V Low-Dropout Regulator for P Applications |
| 中文描述： | 5 V FIXED POSITIVE LDO REGULATOR, 0.8 V DROPOUT, PSFM5 |
| 封裝： | BENT STAGGERED, TO-220, 5 PIN |
| 文件頁數(shù)： | 9/14頁 |
| 文件大?。?/td> | 378K |
| 代理商： | LP2957IT |
Application Hints
(Continued)
If a single pull-up resistor is connected to the regulator out-
put, the error flag may briefly rise up to about 1.3V as the in-
put voltage ramps up or down through the 0V to 1.3V region.
In some cases, this 1.3V signal may be mis-interpreted as a
false high by a μP which is still “alive” with 1.3V applied to it.
To prevent this, the user may elect to use
two
resistors
which are equal in value on the error output (one connected
to ground and the other connected to the regulator output).
If this two-resistor divider is used, the error output will only be
pulled up to about 0.6V (not 1.3V) during power-up or
power-down, so it can not be interpreted as a high signal.
When the regulator output is in regulation (4.8V to 5V), the
error output voltage will be 2.4V to 2.5V, which is clearly a
high signal.
OUTPUT ISOLATION
The regulator output can be connected to an active voltage
source (such as a battery) with the regulator input turned off,
as long as the regulator ground pin is connected to
ground
. If the ground pin is left floating,
damage to the
regulator can occur
if the output is pulled up by an external
voltage source.
SNAP-ON/SNAP-OFF OPERATION
The LP2957 output can be wired for snap-on/snap-off opera-
tion using three external resistors:
When connected as shown, the shutdown input holds the
regulator off until the input voltage rises up to the turn-on
threshold (V
ON
), at which point the output “snaps on”.
When the input power is shut off (and the input voltage starts
to decay) the output voltage will snap off when the input volt-
age reaches the turn-off threshold, V
OFF
.
It is important to note that the voltage V
must always be
lower than V
(the difference in these voltage levels is
called the hysteresis).
Hysteresis is
required
when using snap-on/snap-off output,
with the minimum amount of hysteresis required for a spe-
cific application being dependent on the source impedance
of whatever is supplying V
IN
.
Caution:
A type of
low-frequency oscillation
can occur if
V
and V
OFF
are too close together (
insufficient
hysteresis
lator must draw sufficient current to power the
load and charge up the output capacitor (in most
cases, the regulator will briefly draw the maximum
current allowed by its internal limiter).
For this reason, it is best to assume the LP2957
may pull a peak current of about
600 mA
from the
source (which is the listed maximum short-circuit
load current of
530 mA
plus the ground pin current
of
70 mA
).
This high peak current causes V
to drop by an amount
equal to the source impedance multiplied by the current. If V
IN
drops below V
, the regulator will turn off and stop draw-
ing current from the source. This will allow V
to rise back up
above V
, and the cycle will start over. The regulator will
stay in this oscillating mode and never come into regulation.
HYSTERESIS IN TRANSFORMER-POWERED
APPLICATIONS:
If the unregulated DC input voltage to the regulator comes
from a transformer, the required hysteresis is easily mea-
sured by loading the source with a resistive load.
If the regulator is powered from a battery, the source imped-
ance will probably be low enough that other considerations
will determine the optimum values for hysteresis (see Design
Example
#
2).
For best results, the load resistance used to test the trans-
former should be selected to draw about
600 mA
for the
maximum load current test, since this is the maximum peak
current the LP2957 could be expected to draw from the
source.
DS011340-14
eIn shutdown mode, ERROR will go high if it has been pulled up to an
*
*
Exact value depends on dropout voltage, which varies with load current.
FIGURE 2. ERROR Output Timing
DS011340-8
*Minimum value (increase as required for smooth turn-on characteristic).
FIGURE 3. Snap-On/Snap-Off Output
DS011340-9
FIGURE 4. Snap-On/Snap-Off Input
and Output Voltage Diagram
DS011340-10
FIGURE 5. Transformer Powered Input Supply
www.national.com
9
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| LP2960AIM-3.3 | 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 |
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