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LP2951CP-X 参数 Datasheet PDF下载

LP2951CP-X图片预览
型号: LP2951CP-X
PDF下载: 下载PDF文件 查看货源
内容描述: 100mA时的低压差稳压器 [100mA Low Dropout Voltage Regulators]
分类和应用: 稳压器
文件页数/大小: 10 页 / 144 K
品牌: CALOGIC [ CALOGIC, LLC ]
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LP2950 / LP2951  
CORPORATION  
APPLICATION HINTS  
EXTERNAL CAPACITORS  
The error comparator has an open-collector output which  
requires an external pullup 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 400µA, 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.  
The stability of the LP2950/LP2951 requires a 1.0µF or  
greater capacitor between output and ground. Oscillation  
could occur without this capacitor. Most types of tantalum  
or aluminum electrolytic are acceptable. For operations  
below -25oC a solid tantalum is recommended since the  
many aluminum types have electrolytes that freeze at about  
-30oC. The ESR of about 5or less and resonant  
frequency about 500kHz are the most important parameters  
in the value of the capacitor. The capacitors value may be  
increased without limit.  
PROGRAMMING THE OUTPUT VOLTAGE OF LP2951  
The LP2951 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.  
At lower values of output current, less output capacitance is  
required for stability. For currents below 10mA the value of  
the capacitor can be reduced to 0.33µF and 0.1µF 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.  
Refer to the equation below for the programming of the output  
voltage:  
At worst case 3.3µF or greater must be used for the  
condition of 100mA load at 1.23V output.  
VOUT = VREF × (1 + R1\R2) + IFBR1  
The LP2950, unlike other low dropout regulators will remain  
stable and in regulation with no load in addition to the  
internal voltage divider. This feature is especially important  
in applications like CMOS RAM keep-alive circuits. When  
setting the output voltage of the LP2951 version with  
external resistors, a minimum load of 1µA is recommended.  
The VREF is 1.235 and IFB is the feedback bias current,  
nominally -20nA. The minimum recommended load current of  
1µA forces an upper limit of 1.2Mon value of R2. If no load  
is 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 12µA. Since the LP2951 typically draws  
60µA at no load with Pin 2 open-circuited this is a small price  
to pay.  
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 1µA tantalum or aluminum electrolytic capacitor  
should be placed from the input to the ground.  
REDUCING OUTPUT NOISE  
Instability can occur if there is a stray capacitance to the  
LP2951 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 eliminated by  
adding a 100pF capacitor between output and feedback  
and increasing the output capacitor to at least 3.3µF.  
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 lead 3 of LP2950, but is  
relatively inefficient, as increasing the capacitor from 1µF to  
220µF only decreases the noise from 430µV to 160µVrms for  
a 100kHz bandwidth at 5V output.  
ERROR DETECTION COMPARATOR OUTPUT  
The Comparator produces a logic low output whenever the  
LP2951 output falls out of regulation by more than around 5%.  
This occurs at approximately 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  
LP2951 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 LP2951’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.  
Noise could also be reduced fourfold by a bypass capacitor  
across R1, since it reduces the high frequency gain from 4 to  
unity.  
CBYPASS 1/2πR1 × 200Hz  
or choose 0.01µF. When doing this, the output capacitor must  
be increased to 3.3µF to maintain stability. These changes  
reduce the output noise from 430µV to 100µVrms 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 voltages.  
CALOGIC CORPORATION, 237 Whitney Place, Fremont, California 94539, Telephone: 510-656-2900, FAX: 510-651-3025