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参数资料
| 型号： | LM2687LD/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 0.01 A SWITCHED CAPACITOR REGULATOR, 180 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | LLP-8 |
| 文件页数： | 9/10页 |
| 文件大小： | 297K |
| 代理商： | LM2687LD/NOPB |
Device Description
The LM2687 is an inverting, regulated charge-pump power
converter. It features low noise, small physical size, and is
simple to use. It is an ideal solution for biasing GaAsFET
devices such as power amplifier modules found in portable
devices and cellular phones.
A switched capacitor charge-pump circuit is used to invert
the input voltage V
IN to its corresponding negative value
which is seen at V
NEG. This voltage is regulated by a low
dropout linear regulator at V
OUT (Figure 3). The output volt-
age can be regulated anywhere from 1.5V to 5.2V and is
determined by a pair of feedback resistors (see Setting the
Output Voltage). The PSRR of the linear regulator reduces
the output voltage ripple produced by the charge-pump in-
verter to 1mV
P-P (typical) at the output VOUT. The regulator
also attenuates noise from the incoming supply due to its
high PSRR.
Shutdown
The LM2687 features a logic-level shutdown feature. The
function is active-low and will reduce the supply current to
0.05A (typical) when engaged. When shutdown is active
V
OUT and VNEG are switched to ground.
Application Information
SETTING THE OUTPUT VOLTAGE
The output voltage on the LM2687 is set by using a resistor
divider between the output, the feedback pin, and an arbi-
trary voltage V
any positive voltage up to V
IN.VADJ is usually chosen to be
GND and should not be connected to a different voltage
unless it is well regulated so the output will stay constant.
The feedback pin is held at a constant voltage V
FB which
equals 1.2V. The output voltage can be selected using the
equation:
The current into the feedback pin I
FB is in the range of 10nA
to 100nA. Therefore using a value of 500k
or smaller for R
1
should make this current of little concern when setting the
output voltage. For best accuracy, use resistors with 1% or
better tolerance.
CAPACITOR SELECTION
Selecting the right capacitors for your circuit is important.
The capacitors affect the output resistance of the charge-
pump, the output voltage ripple, and the overall dropout
voltage (V
IN-|VOUT|) of the circuit. The output resistance of
the charge-pump inverter is:
The switching frequency is fixed at 100kHz and R
SW (the
combined resistance of the internal switches) is typically
10
. It is clear from this equation that low ESR capacitors
are desirable and that larger values of C
1 will further reduce
the output resistance. The output resistance of the entire
circuit (in dropout) is:
R
OUT =RNEG +Rregulator
R
regulator (the output impedance of the linear regulator) is
approximately 10
. When the circuit is in regulation, the
overall output resistance is equal to the linear regulator load
regulation (5mV/mA). The dropout voltage is therefore af-
fected by the capacitors used since it is simply defined as
I
OUT*ROUT.
A larger value of capacitor and lower ESR for C
2 will lower
the output voltage ripple of the charge-pump. This ripple will
then be subject to the PSRR of the linear regulator and
reduced at V
OUT. A larger value and lower ESR for C3 will
further reduce this ripple.
The Low Dropout Linear Regulator uses an N-channel FET
device which behaves similarly to an NPN device. Because
of this and the internal compensation there are no strict ESR
requirements for the output capacitor to maintain stability.
Using the minimum recommended values will ensure stabil-
ity under all conditions.
In summation, larger value capacitors with lower ESR will
give the lowest output noise and ripple. C
1,C2, and C3
should be 1.0F minimum with less than 0.3
ESR. Larger
values may be used for any or all capacitors. All capacitors
should be either ceramic, surface-mount chip tantalum, or
polymer electrolytic.
OUTPUT NOISE AND RIPPLE
Low output noise and output voltage ripple are two of the
attractive features of the LM2687. Because they are small,
the noise and ripple (1mV typ.) can be hard to measure
accurately. Ground loop error between the circuit and the
oscilloscope caused by the switching of the charge-pump
produces ground currents in the probe wires. This causes
sharp voltage spikes on the oscilloscope waveform. To re-
duce this error measure, the output directly at the output
capacitor (C
3) and use the shortest wires possible. Also, do
not use the ground lead on the probe. Take the tip cover off
of the probe and touch the grounding ring of the probe
directly to the ground terminal of C
3. This should give the
most accurate reading of the actual output waveform.
LM2687
www.national.com
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相关代理商/技术参数 |
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| LM2687LDX | 功能描述:电荷泵 RoHS:否 制造商:Maxim Integrated 功能:Inverting, Step Up 输出电压:- 1.5 V to - 5.5 V, 3 V to 11 V 输出电流:100 mA 电源电流:1 mA 最大工作温度:+ 70 C 封装 / 箱体:SOIC-8 Narrow 封装:Tube |
| LM2687LDX/NOPB | 功能描述:电荷泵 RoHS:否 制造商:Maxim Integrated 功能:Inverting, Step Up 输出电压:- 1.5 V to - 5.5 V, 3 V to 11 V 输出电流:100 mA 电源电流:1 mA 最大工作温度:+ 70 C 封装 / 箱体:SOIC-8 Narrow 封装:Tube |
| LM2687MM | 功能描述:电荷泵 RoHS:否 制造商:Maxim Integrated 功能:Inverting, Step Up 输出电压:- 1.5 V to - 5.5 V, 3 V to 11 V 输出电流:100 mA 电源电流:1 mA 最大工作温度:+ 70 C 封装 / 箱体:SOIC-8 Narrow 封装:Tube |
| LM2687MM | 制造商:Texas Instruments 功能描述:VOLT CONVERTER SMD 2687 MSOP8 |
| LM2687MM/NOPB | 功能描述:电荷泵 RoHS:否 制造商:Maxim Integrated 功能:Inverting, Step Up 输出电压:- 1.5 V to - 5.5 V, 3 V to 11 V 输出电流:100 mA 电源电流:1 mA 最大工作温度:+ 70 C 封装 / 箱体:SOIC-8 Narrow 封装:Tube |
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