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參數(shù)資料
| 型號： | SPX2951CS-L-3.0 |
| 元件分類： | 固定／可調(diào)正電壓單路輸出LDO穩(wěn)壓器 |
| 英文描述： | FIXED/ADJUSTABLE POSITIVE LDO REGULATOR, 0.5 V DROPOUT, PDSO8 |
| 封裝： | PLASTIC, SOIC-8 |
| 文件頁數(shù)： | 7/12頁 |
| 文件大小： | 258K |
| 代理商： | SPX2951CS-L-3.0 |
Rev. 10/30/00
SPX2950/SPX2951
APPLICATION HINTS
EXTERNAL CAPACITORS
The stability of the SPX2950/SPX2951 requires a 1.0
F or
greater capacitor between output and ground. Oscillation could
occur without this capacitor.
Most types of tantalum or
aluminum electrolytic works fine here. For operations below -
25
°C solid tantalum is recommended since many aluminum types
have electrolytes that freeze at about -30
°C. The ESR of about
5
or less and resonant frequency above 500kHz are the most
important parameters in the value of the capacitor. The capacitor
value can be increased without limit.
At lower values of output current, less output capacitance is
required for stability. For the currents below 10mA the value of
the capacitor can be reduced to 0.5
F and 0.15F for 1mA. More
output capacitance is needed for the 8-pin version at voltages
below 5V since it runs the error amplifier at lower gain. At worst
case 5
F or greater must be used for the condition of 150mA
load at 1.23V output.
The SPX2950, unlike other low dropout regulators will remain
stable in regulation with no load in addition to the internal
voltage divider.
This feature is especially important in
applications like CMOS RAM keep-alive. When setting the
output voltage of the SPX2951 version with external resistors, a
minimum load of 1uA is recommended.
If there is more than 10 inches of wire between the input and the
AC filter capacitor or if a battery is used as the input then a 1
A
tantalum or aluminum electrolytic capacitor should be placed
from the input to the ground.
Instability can occur if there is stray capacitance to the SPX2951
feedback terminal (pin 7). This could cause more problems when
using a higher value of external resistors to set the output voltage.
This problem can be fixed by adding a 100pF capacitor between
output and feedback and increasing the output capacitor to at
least 3.3
F.
ERROR DETECTION COMPARATOR OUTPUT
The Comparator produces a logic low output whenever the
SPX2951 output falls out of regulation by more than around 5%.
This is around 60mV offset divided by the 1.235 reference
voltage. This trip level remains 5% below normal regardless of
the programmed output voltage of the regulator. Figure 1 shows
the timing diagram depicting the ERROR signal and the regulator
output voltage as the SPX2951 input is ramped up and down.
The ERROR signal becomes low at around 1.3V input, and goes
high around 5V input (input voltage at which VOUT = 4.75 ).
Since the SPX2951’s dropout voltage is load dependent, the
input voltage trip point (around 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 requires an
external pull-up resistor. Depending on the system requirements the
resistor may be returned to 5V output or other supply voltage in
determining the value of this resistor, note that the output is rated to
sink 400
A, this value adds to battery drain in a low battery
condition. Suggested values range from 100K to 1M
. If the output
is unused this resistor is not required.
PROGRAMMING
THE
OUTPUT
VOLTAGE
OF
SPX2951
The SPX2951 may be pin-strapped for 5V using its internal voltage
divider by tying Pin 1 (output) to Pin 2 (sense) and Pin 7 (feedback)
to Pin 6 (5V Tap). Also 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 resistors is required.
Refer to the below equation for the programming of the output
voltage.
VOUT = VREF
× ( 1 + R1/R2 )+ IFBR1
The VREF is 1.235 and IFB is the feedback bias current, nominally
-20 nA. The minimum recommended load current of 1
A forces an
upper limit of 1.2 M
on value of R2. If no load presented the IFB
produces an error of typically 2% in VOUT which may be eliminated at
room temperature by trimming R1. To improve the accuracy choose
the value of R2 = 100k this reduces the error by 0.17% and increases
the resistor program current by 12
A. Since the SPX2951 typically
draws 60
A at no load with Pin 2 open-circuited this is a small price
to pay.
REDUCING OUTPUT NOISE
It may be an advantage to reduce the AC noise present at the output.
One way is to reduce the regulator bandwidth by increasing the size of
the output capacitor. This is the only way that noise can be reduced
on the SPX2950 but is relatively inefficient, as increasing the
capacitor from 1
F to 220 F only decreases the noise from 430V to
160
VRMS. for a 100kHz bandwidth at 5V output.
Noise could also be reduced fourfold by a bypass capacitor across R1,
since it reduces the high frequency gain from 4 to unity. Pick
CBYPASS
1 / 2πR1 × 200 Hz
or choose 0.01
F. When doing this, the output capacitor must be
increased to 3.3
F to maintain stability. These changes reduce the
output noise from 430
V to 100VRMS. for a 100kHz bandwidth at
5V output. With the bypass capacitor added, noise no longer scales
with output voltage so that improvements are more dramatic at higher
output voltages.
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| SPX2951CS-L-3-3 | 制造商:Rochester Electronics LLC 功能描述: 制造商:Exar Corporation 功能描述: |
| SPX2951CS-L-5-0 | 制造商:Rochester Electronics LLC 功能描述: 制造商:Exar Corporation 功能描述: |
| SPX2951CS-L-5-0/TR | 制造商:Rochester Electronics LLC 功能描述: 制造商:Exar Corporation 功能描述: |
| SPX2954 | 制造商:SIPEX 制造商全稱:Sipex Corporation 功能描述:250 mA Low Drop Out Voltage Regulator |
| SPX2954_05 | 制造商:SIPEX 制造商全稱:Sipex Corporation 功能描述:250mA Low Dropout Voltage Regulator |
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