Datasheet LM2787BP Datasheet (National) [ru]

LM2787
Low Noise Regulated Switched Capacitor Voltage
Inverter in micro SMD

LM2787 Low Noise Regulated Switched Capacitor Voltage Inverter in micro SMD

August 2002

General Description

The LM2787 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 LM2787 is
adjustable between −1.5V and −5.2V. The LM2787 operates
at 260 kHz (typical) switching frequency to reduce output
resistance and voltage ripple. With an operating current of
only 400 µA (charge pump power efficiency greater than
90% with most loads) and 0.05 µA typical shutdown current,
the LM2787 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-Bump
micro SMD package.

Features

n Inverts and regulates the input supply voltage
n Small 8-Bump micro SMD package
n 91% typical charge pump power efficiency at 10mA
n Low output ripple
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

Typical Application Circuit and Connection Diagram

10131325

8-Bump micro SMD (Top View)

10131302

© 2002 National Semiconductor Corporation DS101313 www.national.com

Ordering Information

LM2787

Device Order Number Package Number Package Marking

LM2787BP BPA08CCB S8 Tape and Reel (250 units/reel)

LM2787BPX BPA08CCB S8 Tape and Reel (3000 units/reel)

Note:*The small physical size of the micro SMD package does not allow for

the full part number marking. Devices will be marked with the desig­nation shown in the column Package Marking.

Pin Descriptions

Pin No. Name Function

A1 Cap+ Positive terminal for C

B1 V

C1 V

C2 V

IN

OUT

FB

Positive power supply input.

Regulated negative output voltage.

Feedback input. Connect VFBto an external resistor divider between V
adjust voltage V

C3 SD

B3 V

NEG

Active low, logic-level shutdown input.

Negative unregulated output voltage.

A3 Cap− Negative terminal for C

A2 GND Ground.

ADJ

1

(0≤V

.

ADJ≤VIN

.

1

). DO NOT leave unconnected.

*

Supplies As

and a positive

OUT

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LM2787

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
to OUT) + 5.8V

SD

V

and V

NEG

OUT

Current 10mA

Short-Circuit Duration to GND

V

OUT

to GND or GND

IN

Continuous Output

(GND − 0.3V) to

+ 0.3V)

(V

IN

T

(Note 3) 150˚C

JMAX

θ

(Note 3) 220˚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

(Note 2) 1 sec.

Continuous Power Dissipation (TA=
25˚C) (Note 3) 600mW

Electrical Characteristics

Limits with standard typeface apply for TJ= 25˚C, and limits in boldface type apply over the full temperature range. Unless
otherwise specified V

Symbol Parameter Conditions Min Typ Max Units

I

I

SD

F

SW

Supply Current Open Circuit, No Load 400 950 µA

Q

Shutdown Supply Current 0.05 1 µA

Switching Frequency
(Note 5)

η

POWER

T

R

START

NEG

V

Power Efficiency at V

Start Up time 120 600 µs

Output Resistance to V

Output Voltage Ripple

R

(Note 7)

V

Feedback Pin Reference

FB

Voltage

V

OUT

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

Load Regulation 0 to 10mA, V

Line Regulation 5.5V ≥ V

V

Shutdown Pin Input Voltage

IH

High

V

Shutdown Pin Input Voltage

IL

Low

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 V
temperatures above T

Note 3: The maximum power dissipation must be de-rated at elevated temperatures and is limited by T
temperature) 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: Current drawn from V

Note 7: In the test circuit, capacitors C

output voltage ripple, and reduce efficiency.

Note 8: The feedback resistors R1 and R2 are 200kΩ resistors.

JA

= 3.6V, C1=C2=C3= 1µF.

IN

VIN= 3.6V 140 260 450

NEG

IL= 3.6mA

= 10mA

I

L

(Note 6) 30 Ω

NEG

IL=2.5mA, V

= 10mA, V

I

L

OUT

OUT

= −2.7V
= −3.8V

IL= 2.5mA (Note 8) −1.25 −1.20 −1.15 V

L

5.5V ≥ VIN≥ 3.0V, 10mA ≥ IL≥

−(V

IN

IN

−1.2V)

−0.3V)

−(V

0mA

= − 2.4V 5 mV/mA

OUT

≥ 2.7V, IL= 2.5mA 1 mV/V

IN

5.5V ≥ VIN≥ 2.7V 2.4 V

5.5V ≥ VIN≥ 2.7V 0.8 V

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

= 85˚C, OUT must not be shorted to GND or VINor device may be damaged.

A

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

PDiss

=(T

MAX

pin decreases power efficiency and will increase output voltage ripple.

NEG

1,C2

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

JMAX

=2fSW.

OSC

, and C3are 1µF, 0.30Ω maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, increase

IN

(maximum junction temperature), TA(ambient

JMAX

kHz

94
91

%

1mV

V

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LM2787

10131326

FIGURE 1. Standard Application Circuit

Typical Performance Characteristics Unless otherwise specified, T

Output Voltage vs. Output Current Output Voltage vs. Input Voltage

10131305 10131306

Maximum V

Current vs. Input Voltage No Load Supply Current vs. Input Voltage

NEG

= 25˚C, V

A

OUT

= −2.5V.

10131308 10131309

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LM2787

Typical Performance Characteristics Unless otherwise specified, T

−2.5V. (Continued)

Switching Frequency vs. Input Voltage V

10131311 10131315

Start-up Time vs. Input Voltage Start-up from Shutdown (no load)

FB

= 25˚C, V

A

OUT

vs. Temperature

=

10131310

Output Ripple Output Noise Spectrum

10131313

10131312

10131324

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Typical Performance Characteristics Unless otherwise specified, T

−2.5V. (Continued)

LM2787

Line Transient Response Load Transient Response

= 25˚C, V

A

OUT

=

10131317

FIGURE 2. Functional Block Diagram

10131318

10131327

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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

to its corresponding negative value

IN

. This voltage is regulated by a low

NEG

(Figure 2). The output volt-

OUT

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 in­verter at the output V

. The regulator also attenuates

OUT

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
and V

V

OUT

are switched to ground.

NEG

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 arbi­trary voltage V

(Figure 2). V

ADJ

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:

can range from GND to

ADJ

IN.VADJ

is usually chosen to be

FB

which

10Ω. It is clear from this equation that low ESR capacitors
are desirable and that larger values of C

will further reduce

1

the output resistance. The output resistance of the entire
circuit (in dropout) is:

R

OUT=RNEG+Rregulator

R

(the output impedance of the linear regulator) is

regulator

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 af­fected by the capacitors used since it is simply defined as

*

R

OUT

.

will lower

2

I

OUT

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

OUT

.

In summation, larger value capacitors with lower ESR will
give the lowest output noise and ripple. C

1,C2

, and 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 volt­age 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.

3

LM2787

3

)

The current into the feedback pin IFBis in the range of 10nA
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 drop-

-|V

out voltage (V

IN

|) of the circuit. The output resistance

OUT

of the charge-pump inverter is:

The switching frequency is fixed at 260kHz and RSW(the
combined resistance of the internal switches) is typically

Micro SMD Mounting

The micro SMD package requires specific mounting tech­niques which are detailed in National Semiconductor Appli-

#

cation Note

1112. Referring to the section Surface Mount

Technology (SMT) Assembly Considerations, it should be

1

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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Physical Dimensions inches (millimeters)

unless otherwise noted

8-Bump micro SMD

NS Package Number BPA08CCB

For Ordering, Refer to Ordering Information Table

= 1.346 X2= 1.346 X3= 0.850

X

1

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COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

LM2787 Low Noise Regulated Switched Capacitor Voltage Inverter in micro SMD

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

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.

labeling, can be reasonably expected to result in a
significant injury to the user.

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