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| 型號(hào)： | LP2950CDT-3.3RK |
| 元件分類(lèi)： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | THREE-TERMINAL POSITIVE FIXED VOLTAGE REGULATORS |
| 中文描述： | 三端固定電壓調(diào)節(jié)器 |
| 文件頁(yè)數(shù)： | 10/21頁(yè) |
| 文件大?。?/td> | 640K |
| 代理商： | LP2950CDT-3.3RK |
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Application Hints
EXTERNAL CAPACITORS
A 1.0 μF (or greater) capacitor is required between the out-
put and ground for stability at output voltages of 5V or more.
At lower output voltages, more capacitance is required
(2.2 μF or more is recommended for 3V and 3.3V versions).
Without this capacitor the part will oscillate. Most types of
tantalum or aluminum electrolytics work fine here; even film
types work but are not recommended for reasons of cost.
Many aluminum electrolytics have electrolytes that freeze at
about 30C, so solid tantalums are recommended for op-
eration below 25C. The important parameters of the ca-
pacitor are an ESR of about 5
or less and a resonant fre-
quency above 500 kHz. The value of this capacitor may be
increased without limit.
At lower values of output current, less output capacitance is
required for stability. The capacitor can be reduced to
0.33 μF for currents below 10 mA or 0.1 μF for currents be-
low 1 mA. Using the adjustable versions at voltages below
5V runs the error amplifier at lower gains so that moreoutput
capacitance is needed. For the worst-case situation of a
100 mA load at 1.23V output (Output shorted to Feedback) a
3.3 μF (or greater) capacitor should be used.
Unlike many other regulators, the LP2950 will remain stable
and in regulation with no load in addition to the internal volt-
age divider. This is especially important in CMOS RAM
keep-alive applications. When setting the output voltage of
the LP2951 versions with external resistors, a minimum load
of 1 μA is recommended.
A 1 μF tantalum or aluminum electrolytic capacitor should be
placed from the LP2950/LP2951 input to ground if there is
more than 10 inches of wire between the input and theAC fil-
ter capacitor or if a battery is used as the input.
Stray capacitance to the LP2951 Feedback terminal can
cause instability. This may especially be a problem when us-
ing high value external resistors to set the output voltage.
Adding a 100 pF capacitor between Output and Feedback
and increasing the output capacitor to at least 3.3 μF will fix
this problem.
ERROR DETECTION COMPARATOR OUTPUT
The comparator produces a logic low output whenever the
LP2951 output falls out of regulation by more than approxi-
mately 5%. This figure is the comparator’s built-in offset of
about 60 mV divided by the 1.235 reference voltage. (Refer
to the block diagram in the front of the datasheet.) This trip
level remains “5% below normal” regardless of the pro-
grammed output voltage of the 2951. For example, the error
flag trip level is typically 4.75V for a 5V output or 11.4V for a
12V output. The out of regulation condition may be due ei-
ther to low input voltage, current limiting, or thermal limiting.
Figure 1 below gives a timing diagram depicting the ERROR
signal and the regulated output voltage as the LP2951 input
is ramped up and down. For 5V versions, the ERROR signal
becomes valid (low) at about 1.3V input. It goes high at
about 5V input (the input voltage at which V
= 4.75V).
Since the LP2951’s dropout voltage is load-dependent (see
curve in typical performance characteristics), the
input
volt-
age trip point (about 5V) will vary with the load current. The
output
voltage trip point (approx. 4.75V) does not vary with
load.
The error comparator has an open-collector output which re-
quires an external pullup resistor. This resistor may be re-
turned to the output or some other supply voltage depending
on system requirements. In determining a value for this re-
sistor, note that while the output is rated to sink 400 μA, this
sink current adds to battery drain in a low battery condition.
Suggested values range from 100k to 1 M
. The resistor is
not required if this output is unused.
PROGRAMMING THE OUTPUT VOLTAGE (LP2951)
The LP2951 may be pin-strapped for the nominal fixed out-
put voltage using its internal voltage divider by tying the out-
put and sense pins together, and also tying the feedback and
V
pins together. Alternatively, it may be programmed for
any output voltage between its 1.235V reference and its 30V
maximum rating. As seen in Figure 2 an external pair of re-
sistors is required.
The complete equation for the output voltage is
where V
is the nominal 1.235 reference voltage and I
is
the feedback pin bias current, nominally 20 nA. The mini-
mum recommended load current of 1 μA forces an upper
limit of 1.2 M
on the value of R
, if the regulator must work
with no load (a condition often found in CMOS in standby).
I
will produce a 2% typical error in V
which may be
eliminated at room temperature by trimming R
. For better
accuracy, choosing R
= 100k reduces this error to 0.17%
while increasing the resistor program current to 12 μA. Since
the LP2951 typically draws 60 μA at no load with Pin 2
open-circuited, this is a small price to pay.
DS008546-20
IN
≤
1.3V, the error flag pin becomes a high impas the pull-up
OUT
vWhen V
FIGURE 1. ERROR Output Timing
www.national.com
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LP2950CDT5.0 | 制造商:未知廠(chǎng)家 制造商全稱(chēng):未知廠(chǎng)家 功能描述:Positive Fixed Voltage Regulator |
| LP2950CDT-5.0 | 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(lèi)型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20 |
| LP2950CDT-5.0/NOPB | 功能描述:低壓差穩(wěn)壓器 - LDO 100MA LDO DPAK 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(lèi)型:Fixed 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-20 |
| LP2950CDT-5.0/NOPB | 制造商:Texas Instruments 功能描述:Linear Voltage Regulator IC |
| LP2950CDT-5.0G | 功能描述:低壓差穩(wěn)壓器 - LDO 5V 100mA Low Power 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(lèi)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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