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PDF LM2781 Data sheet ( Hoja de datos )
| Número de pieza | LM2781 | |
| Descripción | Ultra-Low Ripple Switched Capacitor Voltage Inverter | |
| Fabricantes | National Semiconductor | |
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March 2003
www.DataSheet4U.com
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 8Ω Typical Output
Resistance with Two Flying Capacitors
n Output Resistance of 18Ω 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
© 2003 National Semiconductor Corporation DS200447
www.national.com
1 page  
Block Diagram
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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 (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
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
20044710
flying capacitor transfers charge to the COUT, elevating both
the voltage across the COUT 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:
VRIPPLE = [(0.5xIOUT) / (fSWxCOUT) ] + (2xIOUTxESRCout)
5 www.national.com
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| Páginas | Total 7 Páginas | |
| PDF Descargar | [ Datasheet LM2781.PDF ] | |
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