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 500k

Ω 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 drop-

out voltage (V

of the charge-pump inverter is:

The switching frequency is fixed at 260kHz and R

combined resistance of the internal switches) is typically

10

Ω. 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

R

approximately 10

Ω. 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 0.3

Ω 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.

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