LM2781
Ultra-Low Ripple Switched Capacitor Voltage Inverter
General Description
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.
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.
See also: LM2780, a slow clock (12kHz typ) version of this
topology with extremely low supply current.
Features
n
Inverts Input Voltage
n
Ultra-Low Voltage Ripple and 8
Typical Output
Resistance with Two Flying Capacitors
n
Output Resistance of 18
Using One Flying Capacitor
n
210kHz (typ.) Switching Frequency
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
Typical Application Circuit
20044701
March 2003
LM2781
Ultra-Low
Ripple
Switched
Capacitor
V
oltage
Inverter
2003 National Semiconductor Corporation
DS200447
www.national.com
Connection Diagram
8-Bump Thin Micro SMD Package
NS Package Number TPA08
20044702
Pin Description
Pin
Name
Description
A1
C2+
Secondary flying-capacitor connection
B1
V
IN
Input voltage. Input range: 1.8V to 5.5V
C1
C1+
Primary flying-capacitor connection
A2
SD
Shutdown pin logic input. Low = on, High = shutdown
C2
GND
Ground
A3
C2-
Secondary flying-capacitor connection
B3
V
OUT
Output voltage. V
OUT
= -V
IN
C3
C1-
Primary flying capacitor connection
Ordering Information
Order Number
Package Description
Package
Marking
Supplied as
LM2781TP
TPA08: Thin Micro SMD
12
Tape and reel, 250 pcs.
LM2781TPX
TPA08: Thin Micro SMD
12
Tape and reel, 3000 pcs.
LM2781
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2
Absolute Maximum Ratings
(Notes 1,
2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
IN
pin: Voltage to GND
-0.3V to 5.8V
SD pin: Voltage to GND
-0.3V to
(V
IN
+ 0.3V), with
5.8V max
Output Short-to-GND Duration
(Note 3)
1 sec.
Maximum Junction Temperature
(T
J-MAX
)
150C
Storage Temperature Range
-65C to +150C
Pad Temperature
(Soldering, 10 sec.)
265C
ESD Rating (Note 4)
Human-body model
Machine model
2kV
200V
Operating Ratings
(Notes 1, 2)
Input Voltage Range
1.8V to 5.5V
Junction Temperature (T
J
) Range
-40C to +90C
Ambient Temperature (T
A
) Range
(Note 5)
-40C to +85C
Thermal Properties
Junction-to-Ambient Thermal
220C/W
Resistance (
JA
), TPA08 Package
(Note 6)
Electrical Characteristics
(Notes 2, 7)
Limits in standard typface are for T
J
= 25C, and limits in boldface type apply over the full operating junction temperature
range (-40C
T
J
+90C). Unless otherwise noted: specifications apply to the LM2781 Typical Application Circuit (pg. 1) with
V
IN
= 5.0V, V(SD) = 0V, and C
IN
= C
OUT
= C1 = C2 = 1F. (Note 8)
Symbol
Parameter
Conditions
Min
Typ
Max
Units
R
OUT
Output Resistance
(Note 9)
I
OUT
= 10mA, C2 = 1F
8
30
I
OUT
= 10mA, C2 removed
18
40
I
Q
Supply Current
No load
0.7
1.4
mA
I
SD
Shutdown Supply Current
V(SD) = 5.0V
0.1
0.5
A
F
SW
Switching Frequency
100
210
400
kHz
V
SD
Shutdown Pin Logic Levels
Normal Operation
1.8V
V
IN
5.5V
0
0.5
V
Shutdown Mode
1.8V
V
IN
5.5V
0.5xV
IN
V
IN
V
OUTp-p
Output Voltage Ripple
V
IN
= 5.0V, I
OUT
= 10mA
10
mV
P
EFF
Power Efficiency
V
IN
= 5.0V, I
OUT
= 10mA
92
%
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
OUT
may be shorted to GND for one second without damage when the ambient temperature is at or below 85C. Prolonged or repeated shorts of V
OUT
to GND may damage the device, and should be avoided.
Note 4: The human-body model is a 100pF capacitor discharged through a 1.5k
resistor into each pin. The machine model is a 200pF capacitor discharged
directly into each pin.
Note 5: Maximum ambient temperature (T
A-MAX
) is dependent on the maximum operating junction temperature (T
J-MAX-OP
= 90C), the maximum power dissipation
of the device in the application (P
D-MAX
), and the junction-to-ambient thermal resistance of the part in the application (
JA
), as given by the following equation: T
A-MAX
= T
J-MAX-OP
- (
JA
x P
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
IN
, C
OUT
, C1, and C2: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics.
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
OUT
= -[V
IN
- (I
OUT
x R
OUT
)]
LM2781
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Typical Performance Characteristics
Unless otherwise specified, T
A
= 25C, C
IN
= C
OUT
= C1 = C2 = 1F.
Output Voltage vs. Load Current
Power Efficiency vs. Load Current
20044703
20044704
Output Resistance vs. Temperature (I
OUT
= 10mA)
Supply Current vs. Temperature (I
OUT
= 0)
20044705
20044706
Switching Frequency vs. Temperature
Output Voltage Ripple vs. Output Current
20044707
20044708
LM2781
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Block Diagram
20044710
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.
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 (
1), 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 (
2), 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
to the input voltage, but opposite in sign (V
OUT
~= - V
IN
). 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 C
OUT
, the
voltage across the capacitor drops, and the magnitude of the
output voltage ( |V
OUT
| ) falls. During the pump phase, the
flying capacitor transfers charge to the C
OUT
, elevating both
the voltage across the C
OUT
and the magnitude of the output
voltage.
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.
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.
When only one charge pump is active (C2 removed), the
output ripple is more predictable, and can be estimated with
the following equation:
V
RIPPLE
= [(0.5xI
OUT
) / (f
SW
xC
OUT
) ] + (2xI
OUT
xESR
Cout
)
LM2781
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