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参数资料
| 型号： | LM2787TP/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 0.01 A SWITCHED CAPACITOR REGULATOR, 450 kHz SWITCHING FREQ-MAX, PBGA8 |
| 封装： | SMD-8 |
| 文件页数： | 7/9页 |
| 文件大小： | 233K |
| 代理商： | LM2787TP/NOPB |
Device Description
The LM2787 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 2). 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 at the output V
OUT. The regulator also attenuates
noise from the incoming supply due to its high PSRR.
Shutdown
The LM2787 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 LM2787 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 260kHz 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.
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 LM2787. Because they are small,
the noise and ripple 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 volt-
age spikes on the oscilloscope waveform. To reduce 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
output ground. This should give the most accurate reading of
the actual output waveform.
MICRO SMD MOUNTING
The micro SMD package requires specific mounting tech-
niques which are detailed in National Semiconductor Appli-
cation Note # 1112. Referring to the section Surface Mount
Technology (SMT) Assembly Considerations, it should be
noted that the pad style which must be used with the 8-pin
package is the NSMD (non-solder mask defined) type.
For best results during assembly, alignment ordinals on the
PC board may be used to facilitate placement of the micro
SMD device.
MICRO SMD LIGHT SENSITIVITY
Exposing the micro SMD device to direct sunlight may cause
misoperation of the device. Light sources such as Halogen
lamps can also affect electrical performance if brought near
the device.
The wavelengths which have the most detrimental effect are
reds and infra-reds. The fluorescent lighting used inside of
most buildings has very little effect on performance.
LM2787
www.national.com
7
相关PDF资料 |
PDF描述 |
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| LM2787BP/NOPB | 0.01 A SWITCHED CAPACITOR REGULATOR, 450 kHz SWITCHING FREQ-MAX, PBGA8 |
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| LM185AYH-1.2 | 1-OUTPUT TWO TERM VOLTAGE REFERENCE, 1.235 V, MBCY2 |
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