LM2781 [NSC]
Ultra-Low Ripple Switched Capacitor Voltage Inverter; 超低纹波开关电容电压型逆变器
| 型号： | LM2781 |
| 厂家： | 
National Semiconductor |
| 描述： | Ultra-Low Ripple Switched Capacitor Voltage Inverter |
| 文件： | 总7页 (文件大小：162K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
March 2003
LM2781
Ultra-Low Ripple Switched Capacitor Voltage Inverter
General Description
Features
n Inverts Input Voltage
The LM2781 is a charge pump that inverts an input voltage
in the range of 1.8V to 5.5V to the corresponding output
voltage of -1.8V to -5.5V. With use of three or four low-cost
external ceramic capacitors, the LM2781 can provide up to
50mA of output current.
n Ultra-Low Voltage Ripple and 8Ω Typical Output
Resistance with Two Flying Capacitors
n Output Resistance of 18Ω Using One Flying Capacitor
n 210kHz (typ.) Switching Frequency
The LM2781 operates at a 210kHz typical switching fre-
quency to minimize supply current (0.7mA typ.). Two charge
pumps operate 180˚ out of phase to significantly reduce the
output voltage ripple compared to that of traditional single-
phase charge pump circuits. If larger output ripple can be
tolerated, optional capacitor C2 may be omitted to reduce
the cost and size of the solution. The LM2781 is available in
an 8-pin Micro SMD chip-scale package.
n 8-pin micro SMD Package (1.4mm x 1.4mm x 0.5mm)
Applications
n Cellular Phones
n Pagers
n PDAs
n Op-Amp Power Supplies
n Interface Power Supplies
n Handheld Instruments
See also: LM2780, a slow clock (12kHz typ) version of this
topology with extremely low supply current.
Typical Application Circuit
20044701
© 2003 National Semiconductor Corporation
DS200447
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Connection Diagram
8-Bump Thin Micro SMD Package
NS Package Number TPA08
20044702
Pin Description
Pin
A1
B1
C1
A2
C2
A3
B3
C3
Name
C2+
VIN
Description
Secondary flying-capacitor connection
Input voltage. Input range: 1.8V to 5.5V
Primary flying-capacitor connection
C1+
SD
Shutdown pin logic input. Low = on, High = shutdown
Ground
GND
C2-
Secondary flying-capacitor connection
Output voltage. VOUT = -VIN
VOUT
C1-
Primary flying capacitor connection
Ordering Information
Order Number
Package Description
Package
Marking
12
Supplied as
LM2781TP
TPA08: Thin Micro SMD
TPA08: Thin Micro SMD
Tape and reel, 250 pcs.
Tape and reel, 3000 pcs.
LM2781TPX
12
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2
Absolute Maximum Ratings (Notes 1,
ESD Rating (Note 4)
Human-body model
Machine model
2kV
2)
200V
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Operating Ratings (Notes 1, 2)
Input Voltage Range
VIN pin: Voltage to GND
SD pin: Voltage to GND
−0.3V to 5.8V
-0.3V to
1.8V to 5.5V
-40˚C to +90˚C
-40˚C to +85˚C
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range
(Note 5)
(VIN + 0.3V), with
5.8V max
Output Short-to-GND Duration
(Note 3)
1 sec.
Thermal Properties
Junction-to-Ambient Thermal
Resistance (θJA), TPA08 Package
(Note 6)
Maximum Junction Temperature
(TJ-MAX
)
150˚C
220˚C/W
Storage Temperature Range
Pad Temperature
-65˚C to +150˚C
(Soldering, 10 sec.)
265˚C
Electrical Characteristics (Notes 2, 7)
Limits in standard typface are for TJ = 25˚C, and limits in boldface type apply over the full operating junction temperature
range (-40˚C ≤ TJ ≤ +90˚C). Unless otherwise noted: specifications apply to the LM2781 Typical Application Circuit (pg. 1) with
VIN = 5.0V, V(SD) = 0V, and CIN = COUT = C1 = C2 = 1µF. (Note 8)
Symbol
Parameter
Output Resistance
(Note 9)
Conditions
IOUT = 10mA, C2 = 1µF
Min
Typ
8
Max
30
Units
Ω
ROUT
IOUT = 10mA, C2 removed
No load
18
40
Ω
IQ
Supply Current
0.7
0.1
210
1.4
0.5
400
mA
µA
ISD
FSW
VSD
Shutdown Supply Current
Switching Frequency
V(SD) = 5.0V
100
0
kHz
Shutdown Pin Logic Levels Normal Operation
1.8V ≤ VIN ≤ 5.5V
0.5
VIN
V
Shutdown Mode
0.5xVIN
1.8V ≤ VIN ≤ 5.5V
VOUTp-p
PEFF
Output Voltage Ripple
Power Efficiency
VIN = 5.0V, IOUT = 10mA
VIN = 5.0V, IOUT = 10mA
10
92
mV
%
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of
the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the
Electrical Characteristics table.
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: V
may be shorted to GND for one second without damage when the ambient temperature is at or below 85˚C. Prolonged or repeated shorts of V
OUT
OUT
to GND may damage the device, and should be avoided.
Note 4: The human-body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. The machine model is a 200pF capacitor discharged
directly into each pin.
Note 5: Maximum ambient temperature (T
) is dependent on the maximum operating junction temperature (T
= 90˚C), the maximum power dissipation
J-MAX-OP
A-MAX
of the device in the application (P
), and the junction-to-ambient thermal resistance of the part in the application (θ ), as given by the following equation: T
D-MAX
JA A-MAX
= T
- (θ x P
).
J-MAX-OP
JA
D-MAX
Note 6: Junction-to-ambient thermal resistance of the micro SMD package is highly application and board-layout dependent.
Note 7: Max and Min limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm.
Note 8: C , C , C1, and C2: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics.
IN
OUT
Note 9: Output resistance is a model for the voltage drop at the output, resulting from internal switch resistance, capacitor ESR, and charge pump charge transfer
characteristics. Output voltage can be predicted with the following equation: V
= -[V - (I
x R
)]
OUT
IN
OUT
OUT
3
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Typical Performance Characteristics
Unless otherwise specified, TA = 25˚C, CIN = COUT = C1 = C2 = 1µF.
Output Voltage vs. Load Current
Power Efficiency vs. Load Current
20044703
20044704
Output Resistance vs. Temperature (IOUT = 10mA)
Supply Current vs. Temperature (IOUT = 0)
20044705
20044706
Switching Frequency vs. Temperature
Output Voltage Ripple vs. Output Current
20044707
20044708
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4
Block Diagram
20044710
flying capacitor transfers charge to the COUT, elevating both
the voltage across the COUT and the magnitude of the output
voltage.
Operation Description
The LM2781 is a switched capacitor voltage inverter that
produces a negative output voltage from a positive input
voltage. To minimize output voltage ripple, the LM2781 con-
tains two charge pump inverters that operate 180˚ out of
phase. These two charge pumps can be seen in the block
diagram above.
The LM2781 contains two charge pump inverters running
180˚ out of phase. Notice in the block diagram that the clock
connections on Charge Pump 2 are opposite of the clock
connections to Charge Pump 1. Thus, when Charge Pump 1
is in the charge phase, Charge Pump 2 is in the pump phase,
and vice-versa. With this double-pump technology, there is
always a flying capacitor connected to the output. This re-
sults in significantly lower output voltage ripple than that of a
single-pump inverter.
A basic switched capacitor inverter is a two-phase charge
pump circuit that transfers charge from the input to the
output using a flying capacitor. Charge Pump 1 in the block
diagram will be used to illustrate the operation of a single
charge pump. Disregard Charge Pump 2 in the diagram for
now. In phase 1 (φ1), the CLK1A signal of Charge Pump 1 is
high, and MOSFET switches S1A and S1B are ON. For
Charge Pump 1, this is the charge phase, as the flying
capacitor C1 is charged to the input voltage. In the second
phase (φ2), the positive-charged plate of C1 is connected to
GND through switch S1C, and the negative-charged plate is
connected to the output through switch S1D. This results in
the output voltage being approximately equal in magnitude
OUTPUT VOLTAGE RIPPLE
Several factors influence the magnitude of voltage ripple on
the output of the LM2781. Primary influences are output
capacitance, flying capacitance, load current, input voltage,
and output capacitor ESR.
The magnitude of the output voltage ripple is difficult to
predict with equations when the LM2781 is double pumping.
Refer to the Typical Performance Characteristic curves for
this information. In general, ripple decreases with increased
output capacitance, decreased load current, a lower input
voltage, a smaller flying capacitance and/or a smaller output
capacitor ESR.
~
to the input voltage, but opposite in sign (VOUT = - VIN). In
this pump phase, charge stored on the flying capacitor sup-
plies the current to the load and transfers charge to the
output capacitor.
This basic inverter has ripple in the output voltage. During
the charge phase, the load current is supplied by the charge
on the output capacitor. As charge is pulled off COUT, the
voltage across the capacitor drops, and the magnitude of the
output voltage ( |VOUT| ) falls. During the pump phase, the
When only one charge pump is active (C2 removed), the
output ripple is more predictable, and can be estimated with
the following equation:
VRIPPLE = [(0.5xIOUT) / (fSWxCOUT) ] + (2xIOUTxESRCout
)
5
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CAPACITOR SELECTION
Operation Description (Continued)
OUTPUT RESISTANCE
Surface-mount multi-layer ceramic capacitors are recom-
mended for use with the LM2781. These capacitors are
small, inexpensive and have very low equivalent series re-
Output resistance (ROUT) models the internal non-idealities
of a charge pump that result in droop in the output voltage.
The effect of output resistance on output voltage is ex-
pressed in the following equation:
<
sistance (ESR, 15mΩ typ.). Tantalum capacitors, OS-CON
capacitors, and aluminum electrolytic capacitors generally
are not recommended for use with the LM2781 due to their
high ESR, as compared to ceramic capacitors.
VOUT = - [VIN - ( |IOUT| x ROUT )]
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM2781. These capacitors have tight capacitance tolerance
(as good as 10%), hold their value over temperature (X7R:
15% over -55˚C to 125˚C; X5R: 15% over -55˚C to 85˚C),
and typically have little voltage coefficient.
Output resistance is a function of the internal switch resis-
tance (RSW) , flying capacitance (C1), flying capacitor
equivalent series resistance (ESRC1), output capacitor ESR
(ESRCout), and internal switching frequency (fSW). When the
LM2781 operates as a single charge pump (C2 removed),
the output resistance can be approximated by the following
equation:
Capacitors with Y5V and/or Z5U temperature characteristic
are generally not recommended for use with the LM2781.
These types of capacitors typically have wide capacitance
tolerance (+80%, -20%), vary significantly over temperature
(Y5V: +22%, -82% over -30˚C to +85˚C range; Z5U: +22%,
-56% over +10˚C to +85˚C range), and have poor voltage
coefficients. Under some conditions, a nominal 1µF Y5V or
Z5U capacitor could have a capacitance of only 0.1µF. Such
deviation would severely affect performance of the LM2781
circuit.
~
1 Fly Cap: ROUT1 = 2RSW1+[1/(fSW*C1)]+4ESRC1+ESRCout
RSW1 is the sum of the ON resistance of the four internal
MOSFET switches of Charge Pump 1.
With both C1 and C2 in place, the double pumping topology
of the LM2781 cuts the output resistance in half:
~
2 Fly Caps: ROUT2 = 0.5xROUT1
Recommendations for minimizing output resistance include
increasing flying capacitance and minimizing output and fly-
ing capacitor ESR.
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6
Physical Dimensions inches (millimeters) unless otherwise noted
TPA08: 8-Bump Thin Micro SMD
X1 = X2 = 1.361mm 0.030mm
X3 = 0.500mm 0.075mm
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