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 de-

vices such as power amplifier modules found in portable de-

vices 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 to 1mV

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

0.05µA (typical) when engaged. When shutdown is active

V

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

GND and should not be connected to a different voltage un-

less it is well regulated so the output will stay constant. The

feedback pin is held at a constant voltage V

−1.2V. The output voltage can be selected using the equa-

tion:

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 100kHz 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 cir-

cuit (in dropout) is:

R

R

proximately 10

Ω. When the circuit is in regulation, the overall

output resistance is equal to the linear regulator load regula-

tion (5mV/mA). The dropout voltage is therefore affected 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 re-

duced at V

ther 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

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 at-

tractive features of the LM2687. Because they are small, the

noise and ripple (1mV typ.) can be hard to measure accu-

rately. Ground loop error between the circuit and the oscillo-

scope 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 er-

ror 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

ground terminal of C

reading of the actual output waveform.

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