LM2687
Low Noise Regulated Switched Capacitor Voltage
Inverter
General Description
The LM2687 CMOS Negative Regulated Switched Capacitor
Voltage Inverter delivers a very low noise adjustable output
for an input voltage in the range of +2.7V to +5.5V. Four low
cost capacitors are used in this circuit to provide up to 10mA
of output current. The regulated output for the LM2687 is
adjustable between −1.5V and −5.2V. The LM2687 operates
at 100 kHz (typical) switching frequency to reduce output
resistance and voltage ripple. With an operating current of
only 500 µA (charge pump power efficiency greater than
90% with most loads) and 0.05 µA typical shutdown current,
the LM2687 provides ideal performance for cellular phone
power amplifier bias and other low current, low noise nega-
tive voltage needs. The device comes in a small 8-pin MSOP
or LLP package.
Features
nInverts and regulates the input supply voltage
nSmall MSOP-8 or LLP-8 package
n91% typical charge pump power efficiency at 10mA
nLow output ripple (1mV typical)
nShutdown lowers Quiescent current to 0.05 µA (typical)
Applications
nWireless Communication Systems
nCellular Phone Power Amplifier Biasing
nInterface Power Supplies
nHandheld Instrumentation
nLaptop Computers and PDA’s
Typical Application Circuit
10118001
Connection Diagrams
8-Pin MSOP 8-Pin LLP
10118002
10118025
April 2003
LM2687 Low Noise Regulated Switched Capacitor Voltage Inverter
© 2003 National Semiconductor Corporation DS101180 www.national.com
Ordering Information for MSOP Package
Device Order Number Package Number Package Marking*Supplies As
LM2687MM MUA08A S12A Tape and Reel (1000 units/reel)
LM2687MMX MUA08A S12A Tape and Reel (3500 units/reel)
Ordering Information for LLP Package
Device Order Number Package Number Package Marking*Supplies As
LM2687LD LDA08A L050B Tape and Reel (1000 units/reel)
LM2687LDX LDA08A L050B Tape and Reel (3500 units/reel)
Note: *The small physical sizes of the MSOP-8 and LLP-8 packages do not
allow for the full part number marking. Devices will be marked with the
designation shown in the column Package Marking.
Pin Description
Pin No. Name Function
1 Cap+ Positive terminal for C
1
.
2 GND Ground.
3 Cap− Negative terminal for C
1
.
4SD
Active low, logic-level shutdown input.
5V
NEG
Negative unregulated output voltage.
6V
FB
Feedback input. Connect V
FB
to an external resistor divider between V
OUT
and a positive
adjust voltage V
ADJ
(0≤V
ADJ
≤V
IN
). DO NOT leave unconnected.
7V
OUT
Regulated negative output voltage.
8V
IN
Positive power supply input.
DAP Die Attach Pad LLP package only. Connect this pad to V
NEG
.
LM2687
www.national.com 2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (V
IN
to GND or GND
to OUT) + 5.8V
SD (GND − 0.3V) to
(V
IN
+ 0.3V)
V
NEG
and V
OUT
Continuous Output
Current 10mA
V
OUT
Short-Circuit Duration to GND
(Note 2) 1 sec.
Continuous Power Dissipation (T
A
=
25˚C) (Note 3) 600mW
T
JMAX
(Note 3) 150˚C
θ
JA
(Note 3) 250˚C/W
Operating Input Voltage Range 2.7V to 5.5V
Operating Output Current Range 0mA to 10mA
Operating Ambient −40˚C to 85˚C
Temp. Range
Operating Junction Temp. Range −40˚C to 110˚C
Storage Temperature −65˚C to 150˚C
Lead Temp. (Soldering, 10 sec.) 300˚C
ESD Rating (Note 4) 2kV
Electrical Characteristics
Limits with standard typeface apply for T
J
= 25˚C, and limits in boldface type apply over the full temperature range. Unless
otherwise specified V
IN
= 3.6V, C
1
=C
2
= 1µF, C
3
= 10µF.
Symbol Parameter Conditions Min Typ Max Units
I
Q
Supply Current Open Circuit, No Load 500 950 µA
I
SD
Shutdown Supply Current 0.05 1 µA
F
SW
Switching Frequency
(Note 5)
2.7V ≤V
IN
≤.5.5V
V
IN
= 3.6V
50
60
110
105
180
170 kHz
η
POWER
Power Efficiency at V
NEG
I
L
= 3.6mA
I
L
= 10mA
94
91 %
T
START
Start Up time (Note 6) 120 600 µs
R
NEG
Output Resistance to V
NEG
(Note 7) 30 Ω
V
R
Output Voltage Ripple
(Note 8)
I
L
=2.5mA, V
OUT
= −2.7V
I
L
= 10mA, V
OUT
= −3.8V
1
2mV
V
FB
Feedback Pin Reference
Voltage
I
L
= 2.5mA (Note 9) −1.25 −1.20 −1.15 V
V
OUT
Adjustable Output Voltage 5.5V ≥V
IN
≥2.7V, 2.5mA ≥I
L
5.5V ≥V
IN
≥3.0V, 10mA ≥I
L
≥
0mA
−(V
IN
−0.3V)
−(V
IN
−1.2V)
V
Load Regulation 0 to 10mA, V
OUT
= − 2.4V 5 mV/mA
Line Regulation 5.5V ≥V
IN
≥2.7V, I
L
= 2.5mA 1 mV/V
V
IH
Shutdown Pin Input Voltage
High
5.5V ≥V
IN
≥2.7V 2.2 V
V
IL
Shutdown Pin Input Voltage
Low
5.5V ≥V
IN
≥2.7V 0.5 V
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device
beyond its rated operating conditions.
Note 2: OUT may be shorted to GND for one second without damage. However, shorting OUT to VIN may damage the device and must be avoided. Also, for
temperatures above TA= 85˚C, OUT must not be shorted to GND or VIN or device may be damaged.
Note 3: The maximum power dissipation must be de-rated at elevated temperatures and is limited by TJMAX (maximum junction temperature), TA(ambient
temperature) and θJA (junction-to-ambient thermal resistance). The maximum power dissipation at any temperature is:
PDiss
MAX
=(T
JMAX
—T
A
)/θ
JA
up to the value listed in the Absolute Maximum Ratings.
Note 4: Rating is for the human body model, a 100pF capacitor discharged through a 1.5 kΩresistor into each pin.
Note 5: The output switches operate at one half the oscillator frequency, fOSC =2f
SW.
Note 6: All capacitors are 1µF.
Note 7: Current drawn from VNEG pin decreases power efficiency and will increase output voltage ripple.
Note 8: In the test circuit, capacitors C1and C2are 1µF, C3= 10µF, 0.30Ωmaximum ESR capacitors. Capacitors with higher ESR will increase output resistance,
increase output voltage ripple, and reduce efficiency.
Note 9: The feedback resistors R1 and R2 are 200kΩresistors.
LM2687
www.national.com3
10118021
FIGURE 1. Standard Application Circuit for Minimum Capacitance Values
10118022
FIGURE 2. Standard Application Circuit for Low Output Noise
LM2687
www.national.com 4
Typical Performance Characteristics Unless otherwise specified, T
A
= 25˚C, V
OUT
= −2.5V.
Output Voltage vs. Output Current
Figure 2
Output Voltage vs. Output Current
Figure 1
10118004 10118005
Output Voltage vs. Input Voltage Maximum Output Current vs. Input Voltage
10118006 10118007
Maximum V
NEG
Current vs. Input Voltage No Load Supply Current vs. Input Voltage
10118008 10118009
LM2687
www.national.com5
Typical Performance Characteristics Unless otherwise specified, T
A
= 25˚C,
VOUT = −2.5V. (Continued)
Start-Up Time vs. Input Voltage
Figure 1 Switching Frequency vs. Input Voltage
10118010 10118011
Start-Up from Shutdown (no load)
Figure 2
Output Ripple
Figure 1
10118012 10118013
Output Ripple
Figure 2 Line Transient Response
10118014 10118017
LM2687
www.national.com 6
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 V
OUT
. 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
0.05µA (typical) when engaged. When shutdown is active
V
OUT
and V
NEG
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
ADJ
(Figure 3). V
ADJ
can range from GND to
any positive voltage up to V
IN
.V
ADJ
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 500kΩor 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
-|V
OUT
|) 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
10Ω. 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
=R
NEG
+R
regulator
R
regulator
(the output impedance of the linear regulator) is
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
OUT
*R
OUT
.
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 C
3
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
,C
2
, and C
3
should be 1.0µF minimum with less than 0.3ΩESR. 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 8
Physical Dimensions inches (millimeters) unless otherwise noted
MSOP-8 Package
8-Lead Mini SO-8 (MM)
For Ordering, Refer to Ordering Information Table
NS Package Number MUA08A
LM2687
www.national.com9
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
LLP-8 Package
8-Lead Leadless Leadframe Package (LD)
For Ordering, Refer to Ordering Information Table
NS Package Number LDA08A
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
National Semiconductor
Asia Pacific Customer
Support Center
Fax: +65-6250 4466
Email: ap.support@nsc.com
Tel: +65-6254 4466
National Semiconductor
Japan Customer Support Center
Fax: 81-3-5639-7507
Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560
www.national.com
LM2687 Low Noise Regulated Switched Capacitor Voltage Inverter
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
