lm2787tpx National Semiconductor Corporation, lm2787tpx Datasheet - Page 7

• Manufacturer Part Number: lm2787tpx
• Description: Low Noise Regulated Switched Capacitor Voltage Inverter In Micro Smd
• Manufacturer: National Semiconductor Corporation

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
which is seen at V
dropout linear regulator at V
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
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
0.05µA (typical) when engaged. When shutdown is active
V
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
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
equals −1.2V. The output voltage can be selected using the
equation:
The current into the feedback pin I
to 100nA. Therefore using a value of 500kΩ or smaller for R
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
the charge-pump inverter is:
The switching frequency is fixed at 260kHz and R
combined resistance of the internal switches) is typically
OUT
and V
IN
NEG
-|V
ADJ
OUT
are switched to ground.
|) of the circuit. The output resistance of
(Figure 2). V
NEG
IN
to its corresponding negative value
. This voltage is regulated by a low
OUT
. The regulator also attenuates
IN
OUT
. V
ADJ
ADJ
(Figure 2). The output volt-
FB
can range from GND to
is usually chosen to be
is in the range of 10nA
FB
SW
which
(the
1
7
10Ω. It is clear from this equation that low ESR capacitors
are desirable and that larger values of C
the output resistance. The output resistance of the entire
circuit (in dropout) is:
R
approximately 10Ω. 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
A larger value of capacitor and lower ESR for C
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
In summation, larger value capacitors with lower ESR will
give the lowest output noise and ripple. C
should be 1.0µF minimum with less than 0.3Ω 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
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.
OUT
regulator
*
R
OUT
(the output impedance of the linear regulator) is
.
OUT
.
R
OUT
= R
NEG
+ R
regulator
1
will further reduce
1
, C
www.national.com
2
2
, and C
will lower
3
3
)