Datasheet LM2687MMX Datasheet (NSC)

LM2687
Low Noise Regulated Switched Capacitor Voltage
Inverter

LM2687 Low Noise Regulated Switched Capacitor Voltage Inverter

August 1999

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 outputfor the LM2687 is ad­justable between −1.5V and −5.2V. The LM2687operates at
100 kHz (typical) switching frequencyto 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 negative volt­age needs. The device comes in a small 8-pin MSOP pack­age.

Typical Application Circuit

Features

n Inverts and regulates the input supply voltage
n Small MSOP-8 package
n 91%typical charge pump power efficiency at 10mA
n Low output ripple (1mV typical)
n Shutdown lowers Quiescent current to 0.05 µA (typical)

Applications

n Wireless Communication Systems

%

n Cellular Phone Power Amplifier Biasing
n Interface Power Supplies
n Handheld Instrumentation
n Laptop Computers and PDA’s

DS101180-1

Connection Diagram

8-Pin MSOP

DS101180-2

© 1999 National Semiconductor Corporation DS101180 www.national.com

Ordering Information

Device Order Number Package Number Package Marking

LM2687MM MUA08A S12A Tape and Reel (1000 units/reel)

LM2687MMX MUA08A S12A Tape and Reel (3500 units/reel)

Note:*The small physical size of the MSOP-8 package does not allow for the full part number marking. Devices will be marked with the designation shown in the

column Package Marking.

*

Supplies As

Pin Description

Pin No. Name Function

1 Cap+ Positive terminal for C
2 GND Ground.
3 Cap− Negative terminal for C
4SD
5V
6

7V
8V

NEG

V

FB

OUT

IN

Active low, logic-level shutdown input.
Negative unregulated output voltage.
Feedback input. Connect VFBto an external resistor divider between V

adjust voltage V
Regulated negative output voltage.
Positive power supply input.

ADJ

1

(0≤V

.

.

1

). DO NOT leave unconnected.

ADJ≤VIN

and a positive

OUT

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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
GND to OUT) + 5.8V

SD

V

and V

NEG

Current 10mA

Short-Circuit Duration to GND

V

OUT

(Note 2) 1 sec.
Continuous Power Dissipation (T

=

25˚C) (Note 3) 600mW

to GND or

IN

Continuous Output

OUT

A

(GND − 0.3V) to

+ 0.3V)

(V

IN

T

(Note 3) 150˚C

JMAX

θ

(Note 3) 250˚C/W

JA

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
otherwise specified V

IN

=

3.6V, C

Symbol Parameter Conditions Min Typ Max Units

Supply Current Open Circuit, No Load 500 950 µA

Q

Shutdown Supply Current 0.05 1 µA

SD

Switching Frequency

SW

(Note 5)
Power Efficiency at V

Start Up time (Note 6) 120 600 µs
Output Resistance to V
Output Voltage Ripple

R

(Note 8)
Feedback Pin Reference

FB

Voltage
Adjustable Output Voltage 5.5V ≥ VIN≥ 2.7V, 2.5mA ≥ I

η

POWER

T

R

V

I

I

F

START

NEG

V

V

OUT

Load Regulation 0 to 10mA, V
Line Regulation 5.5V ≥ V

V

V

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Electrical specifications donotapplywhenoperatingthedevice
beyond its rated operating conditions.

Note 2: OUT may be shorted to GND for one second without damage. However, shorting OUT to V
peratures above T

Note 3: The maximum powerdissipationmust be de-rated at elevated temperatures and is limited by T
ture) and θ

Note 4: Rating is for the human body model, a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Note 5: The output switches operate at one half the oscillator frequency, f
Note 6: All capacitors are 1µF.
Note 7: Current drawn from V
Note 8: In the test circuit, capacitors C

increase output voltage ripple, and reduce efficiency.
Note 9: The feedback resistors R1 and R2 are 200kΩ resistors.

Shutdown Pin Input

IH

Voltage High
Shutdown Pin Input

IL

Voltage Low

=

85˚C, OUT must not be shorted to GND or V

A

(junction-to-ambient thermal resistance). The maximum power dissipation at any temperature is:

JA

PDiss

=

(T

MAX

pin decreases power efficiency and will increase output voltage ripple.

NEG

and C2are 1µF,C

1

=

25˚C, and limits in boldface type apply over the full temperature range. Unless

J

=

C

1

2

NEGIL

NEG

=

=

1µF, C

2.7V ≤ VIN≤.5.5V
V

I

L

IN

=

=
=

3

3.6V

3.6mA
10mA

10µF.

50
60

110
105

94
91

(Note 7) 30 Ω

=

I

2.5mA, V

L

=

10mA, V

I

L

=

I

2.5mA (Note 9) −1.25 −1.20 −1.15 V

L

5.5V ≥ VIN≥ 3.0V, 10mA ≥ IL≥
0mA

≥ 2.7V, I

IN

=

−2.7V

OUT

=

−3.8V

OUT

−0.3V)

−(V

L

=

− 2.4V 5 mV/mA

OUT

=

2.5mA 1 mV/V

L

−(V

IN
IN

−1.2V)

1
2

5.5V ≥ VIN≥ 2.7V 2.2 V

5.5V ≥ VIN≥ 2.7V 0.5 V

may damage the device and must be avoided. Also, for tem-

or device may be damaged.

IN

—TA)/θJAup to the value listed in the Absolute Maximum Ratings.

JMAX

=

.

2f

OSC

SW

=

10µF,0.30Ω maximum ESR capacitors. Capacitors with higher ESR will increase output resistance,

3

IN

(maximum junction temperature), TA(ambient tempera-

JMAX

180
170

kHz

%

mV

V

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Electrical Characteristics (Continued)

FIGURE 1. Standard Application Circuit for Minimum Capacitance Values

DS101180-21

FIGURE 2. Standard Application Circuit for Low Output Noise

Typical Performance Characteristics Unless otherwise specified, T

Output Voltage vs. Output Current

Figure 2

DS101180-4

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Output Voltage vs. Output Current

Figure 1

DS101180-5

DS101180-22

A

=

25˚C, V

OUT

=

−2.5V.

Output Voltage vs. Input Voltage

DS101180-6

Typical Performance Characteristics Unless otherwise specified, T

−2.5V. (Continued)

A

=

25˚C, V

OUT

=

Maximum Output Current vs. Input
Voltage

DS101180-7

Start-Up Time vs. Input Voltage

Figure 1

DS101180-10

Output Ripple

Figure 1

Maximum V
Voltage

Current vs. Input

NEG

DS101180-8

Switching Frequency vs. Input
Voltage

DS101180-11

Output Ripple

Figure 2

No Load Supply Current vs. Input
Voltage

DS101180-9

Start-Up from Shutdown (no load)

Figure 2

DS101180-12

Line Transient Response

DS101180-13

DS101180-14

DS101180-17

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Typical Performance Characteristics Unless otherwise specified, T

−2.5V. (Continued)

A

=

25˚C, V

OUT

=

Load Transient Response

Output Noise Spectrum

Figure 1

DS101180-18

DS101180-24

VFBvs. Temperature

DS101180-15

Output Noise Spectrum

Figure 2

DS101180-23

FIGURE 3. Functional Block Diagram

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DS101180-3

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 isan idealsolution 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

to its corresponding negative value

IN

. This voltage is regulated by a low

NEG

OUT

(

Figure 3

). The output volt-

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 in­verter to 1mV
also attenuates noise from the incoming supply due to its

(typical) at the output V

P-P

. The regulator

OUT

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

are switched to ground.

NEG

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 arbi­trary 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

−1.2V. The output voltage can be selected using the equa­tion:

The current into the feedback pin IFBis in the range of 10nA
to 100nA. Therefore using a value of 500kΩ or smaller forR
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
of the charge-pump inverter is:

(

Figure 3

). V

ADJ

-|V

|) of the circuit. The output resistance

IN

OUT

can range from GND to

ADJ

is usually chosen to be

IN.VADJ

which equals

FB

The switching frequency is fixed at 100kHz and RSW(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
the output resistance. The output resistance of the entire cir­cuit (in dropout) is:

=

R

R

OUT

R

(the output impedance of the linear regulator) isap-

regulator

proximately 10Ω. When the circuit is in regulation, the overall

NEG+Rregulator

output resistance is equal to the linear regulator load regula­tion (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 re­duced at V
ther reduce this ripple.

. A larger value and lower ESR for C3will fur-

OUT

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 thereare nostrict ESR
requirements for the output capacitor to maintain stability.
Using the minimum recommended values will ensure stabil­ity 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 0.3Ω 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 theLM2687. Becausethey aresmall, 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 volt-

1

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
ground terminal of C
reading of the actual output waveform.

. This should give the most accurate

3

will further reduce

1

*

R

OUT

will lower

2

, and C

1,C2

OUT

.

3

)

3

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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 Low Noise Regulated Switched Capacitor Voltage Inverter

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