LM2787BP [NSC]
Low Noise Regulated Switched Capacitor Voltage; 低噪声稳压开关电容电压
| 型号： | LM2787BP |
| 厂家： | 
National Semiconductor |
| 描述： | Low Noise Regulated Switched Capacitor Voltage |
| 文件： | 总8页 (文件大小：207K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
August 2002
LM2787
Low Noise Regulated Switched Capacitor Voltage
Inverter in micro SMD
General Description
Features
n Inverts and regulates the input supply voltage
n Small 8-Bump micro SMD package
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 a 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
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Ordering Information
*
Device Order Number
Package Number
BPA08CCB
Package Marking
Supplies As
LM2787BP
S8
S8
Tape and Reel (250 units/reel)
Tape and Reel (3000 units/reel)
LM2787BPX
BPA08CCB
*
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
Cap+
VIN
Function
A1
Positive terminal for C1.
B1
Positive power supply input.
C1
VOUT
VFB
Regulated negative output voltage.
C2
Feedback input. Connect VFB to an external resistor divider between VOUT and a positive
adjust voltage VADJ (0≤VADJ≤VIN). DO NOT leave unconnected.
Active low, logic-level shutdown input.
C3
B3
A3
A2
SD
VNEG
Cap−
GND
Negative unregulated output voltage.
Negative terminal for C1.
Ground.
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2
Absolute Maximum Ratings (Note 1)
TJMAX (Note 3)
150˚C
220˚C/W
θJA (Note 3)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Operating Input Voltage Range
Operating Output Current Range
Operating Ambient
2.7V to 5.5V
0mA to 10mA
−40˚C to 85˚C
Supply Voltage (VIN to GND or GND
to OUT)
SD
+ 5.8V
(GND − 0.3V) to
(VIN + 0.3V)
Temp. Range
Operating Junction Temp. Range
Storage Temperature
Lead Temp. (Soldering, 10 sec.)
ESD Rating (Note 4)
−40˚C to 110˚C
−65˚C to 150˚C
300˚C
VNEG and VOUT Continuous Output
Current
10mA
1 sec.
2kV
VOUT Short-Circuit Duration to GND
(Note 2)
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 VIN = 3.6V, C1 = C2 = C3 = 1µF.
Symbol
IQ
Parameter
Supply Current
Conditions
Min
Typ
400
0.05
260
Max
950
1
Units
µA
Open Circuit, No Load
ISD
Shutdown Supply Current
Switching Frequency
(Note 5)
µA
FSW
VIN = 3.6V
140
450
kHz
%
ηPOWER
Power Efficiency at VNEG
IL = 3.6mA
IL = 10mA
94
91
TSTART
RNEG
VR
Start Up time
120
30
600
µs
Output Resistance to VNEG (Note 6)
Ω
Output Voltage Ripple
(Note 7)
IL =2.5mA, VOUT = −2.7V
1
mV
V
IL = 10mA, VOUT = −3.8V
IL = 2.5mA (Note 8)
VFB
Feedback Pin Reference
Voltage
−1.25
−1.20
−1.15
VOUT
Adjustable Output Voltage
5.5V ≥ VIN ≥ 2.7V, 2.5mA ≥ IL
5.5V ≥ VIN ≥ 3.0V, 10mA ≥ IL
0mA
− (VIN −0.3V)
− (VIN −1.2V)
V
≥
Load Regulation
Line Regulation
0 to 10mA, VOUT = − 2.4V
5.5V ≥ VIN ≥ 2.7V, IL = 2.5mA
5
1
mV/mA
mV/V
V
VIH
VIL
Shutdown Pin Input Voltage 5.5V ≥ VIN ≥ 2.7V
2.4
High
Shutdown Pin Input Voltage 5.5V ≥ VIN ≥ 2.7V
0.8
V
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 may damage the device and must be avoided. Also, for
IN
temperatures above T = 85˚C, OUT must not be shorted to GND or V or device may be damaged.
A
IN
Note 3: The maximum power dissipation must be de-rated at elevated temperatures and is limited by T
(maximum junction temperature), T (ambient
A
JMAX
temperature) and θ (junction-to-ambient thermal resistance). The maximum power dissipation at any temperature is:
JA
PDissMAX = (TJMAX — TA)/θJA 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 kΩ resistor into each pin.
Note 5: The output switches operate at one half the oscillator frequency, f
= 2f
.
OSC
SW
Note 6: Current drawn from V
pin decreases power efficiency and will increase output voltage ripple.
NEG
Note 7: In the test circuit, capacitors C , C , and C are 1µF, 0.30Ω maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, increase
1
2
3
output voltage ripple, and reduce efficiency.
Note 8: The feedback resistors R1 and R2 are 200kΩ resistors.
3
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10131326
FIGURE 1. Standard Application Circuit
Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, VOUT = −2.5V.
Output Voltage vs. Output Current
Output Voltage vs. Input Voltage
10131305
10131306
Maximum VNEG Current vs. Input Voltage
No Load Supply Current vs. Input Voltage
10131308
10131309
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Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, VOUT
=
−2.5V. (Continued)
Switching Frequency vs. Input Voltage
VFB vs. Temperature
10131311
10131315
Start-up Time vs. Input Voltage
Start-up from Shutdown (no load)
10131312
10131310
Output Ripple
Output Noise Spectrum
10131313
10131324
5
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Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, VOUT
=
−2.5V. (Continued)
Line Transient Response
Load Transient Response
10131317
10131318
10131327
FIGURE 2. Functional Block Diagram
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10Ω. It is clear from this equation that low ESR capacitors
are desirable and that larger values of C1 will further reduce
the output resistance. The output resistance of the entire
circuit (in dropout) is:
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.
ROUT = RNEG + Rregulator
Rregulator (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
A switched capacitor charge-pump circuit is used to invert
the input voltage VIN to its corresponding negative value
which is seen at VNEG. This voltage is regulated by a low
dropout linear regulator at VOUT (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 VOUT. The regulator also attenuates
noise from the incoming supply due to its high PSRR.
*
IOUT ROUT
.
A larger value of capacitor and lower ESR for C2 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 VOUT
.
In summation, larger value capacitors with lower ESR will
give the lowest output noise and ripple. C1, C2, and C3
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.
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
VOUT and VNEG are switched to ground.
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 (C3)
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.
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 VADJ (Figure 2). VADJ can range from GND to
any positive voltage up to VIN. VADJ 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 VFB which
equals −1.2V. The output voltage can be selected using the
equation:
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
The current into the feedback pin IFB is in the range of 10nA
to 100nA. Therefore using a value of 500kΩ or smaller for R1
should make this current of little concern when setting the
output voltage. For best accuracy, use resistors with 1% or
better tolerance.
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.
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 (VIN-|VOUT|) of the circuit. The output resistance
of the charge-pump inverter is:
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.
The switching frequency is fixed at 260kHz and RSW (the
combined resistance of the internal switches) is typically
7
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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
X1 = 1.346 X2 = 1.346 X3 = 0.850
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