LM2787TP/NOPB - NATIONAL SEMICONDUCTOR

LM2787

LM2787 Low Noise Regulated Switched Capacitor Voltage Inverter in micro SMD

Literature Number: SNVS080E

LM2787

Low Noise Regulated Switched Capacitor Voltage

Inverter in micro SMD

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 nega-

tive voltage needs. The device comes in small 8-Bump micro

SMD and thin micro SMD packages.

Features

nInverts and regulates the input supply voltage

nSmall 8-Bump micro SMD and thin micro SMD

packages

n91% typical charge pump power efficiency at 10mA

nLow output ripple

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 and Connection Diagram

10131325

8-Bump micro SMD (Top View)

10131302

April 2003

LM2787 Low Noise Regulated Switched Capacitor Voltage Inverter in micro SMD

Ordering Information

micro SMD

Device Order Number Package Number Package Marking*Supplies As

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

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

Thin micro SMD

Device Order Number Package Number Package Marking*Supplies As

LM2787TP TPA08CCA S8 Tape and Reel (250 units/reel)

LM2787TPX TPA08CCA 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 desig-

nation shown in the column Package Marking.

Pin Descriptions

Pin No. Name Function

A1 Cap+ Positive terminal for C

B1 V

Positive power supply input.

C1 V

OUT

Regulated negative output voltage.

C2 V

Feedback input. Connect V

to an external resistor divider between V

OUT

and a positive

adjust voltage V

ADJ

(0≤V

ADJ

≤V

). DO NOT leave unconnected.

C3 SD Active low, logic-level shutdown input.

B3 V

NEG

Negative unregulated output voltage.

A3 Cap− Negative terminal for C

A2 GND Ground.

LM2787

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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

to GND or GND

to OUT) + 5.8V

SD (GND − 0.3V) to

+ 0.3V)

NEG

and V

OUT

Continuous Output

Current 10mA

OUT

Short-Circuit Duration to GND

(Note 2) 1 sec.

Continuous Power Dissipation (T

25˚C) (Note 3) 600mW

JMAX

(Note 3) 150˚C

(Note 3) 220˚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

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

otherwise specified V

= 3.6V, C

= 1µF.

Symbol Parameter Conditions Min Typ Max Units

Supply Current Open Circuit, No Load 400 950 µA

Shutdown Supply Current 0.05 1 µA

Switching Frequency

(Note 5)

= 3.6V 140 260 450 kHz

POWER

Power Efficiency at V

NEG

= 3.6mA

= 10mA

91 %

START

Start Up time 120 600 µs

NEG

Output Resistance to V

NEG

(Note 6) 30 Ω

Output Voltage Ripple

(Note 7)

=2.5mA, V

OUT

= −2.7V

= 10mA, V

OUT

= −3.8V 1mV

Feedback Pin Reference

Voltage

= 2.5mA (Note 8) −1.25 −1.20 −1.15 V

OUT

Adjustable Output Voltage 5.5V ≥V

≥2.7V, 2.5mA ≥I

5.5V ≥V

≥3.0V, 10mA ≥I

≥

0mA

−(V

−0.3V)

−(V

−1.2V)

Load Regulation 0 to 10mA, V

OUT

= − 2.4V 5 mV/mA

Line Regulation 5.5V ≥V

≥2.7V, I

= 2.5mA 1 mV/V

Shutdown Pin Input Voltage

High

5.5V ≥V

≥2.7V 2.4 V

Shutdown Pin Input Voltage

Low

5.5V ≥V

≥2.7V 0.8 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

)/θ

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: Current drawn from VNEG pin decreases power efficiency and will increase output voltage ripple.

Note 7: In the test circuit, capacitors C1,C

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

output voltage ripple, and reduce efficiency.

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

LM2787

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Typical Performance Characteristics Unless otherwise specified, T

= 25˚C, V

OUT

= −2.5V.

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

10131305 10131306

Maximum V

NEG

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

10131308 10131309

10131326

FIGURE 1. Standard Application Circuit

LM2787

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Typical Performance Characteristics Unless otherwise specified, T

= 25˚C, V

OUT

−2.5V. (Continued)

Switching Frequency vs. Input Voltage V

vs. Temperature

10131311 10131315

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

10131310

10131312

Output Ripple Output Noise Spectrum

10131313

10131324

LM2787

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Typical Performance Characteristics Unless otherwise specified, T

= 25˚C, V

OUT

−2.5V. (Continued)

Line Transient Response Load Transient Response

10131317 10131318

Functional Block Diagram

10131327

FIGURE 2. Functional Block Diagram

LM2787

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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

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 2). 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 at the output V

OUT

. The regulator also attenuates

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

OUT

and V

NEG

are switched to ground.

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

ADJ

(Figure 2). V

ADJ

can range from GND to

any positive voltage up to 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

which

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

equation:

The current into the feedback pin I

is in the range of 10nA

to 100nA. Therefore using a value of 500kΩor 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 dropout

voltage (V

-|V

OUT

|) of the circuit. The output resistance of

the charge-pump inverter is:

The switching frequency is fixed at 260kHz and R

(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

will further reduce

the output resistance. The output resistance of the entire

circuit (in dropout) is:

OUT

NEG

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

OUT

A larger value of capacitor and lower ESR for C

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

In summation, larger value capacitors with lower ESR will

give the lowest output noise and ripple. C

, and C

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 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.

LM2787

www.national.com7

Physical Dimensions inches (millimeters)

unless otherwise noted

8-Bump micro SMD

NS Package Number BPA08CCB

For Ordering, Refer to Ordering Information Table

= 1.346 X

= 0.850

8-Bump thin micro SMD

NS Package Number TPA08CCA

For Ordering, Refer to Ordering Information Table

= 1.346 X

= 0.500

LM2787

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Notes

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COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

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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.

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www.national.com

LM2787 Low Noise Regulated Switched Capacitor Voltage Inverter in micro SMD

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.

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