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PDF LM2681M6X Data sheet ( Hoja de datos )
| Número de pieza | LM2681M6X | |
| Descripción | Switched Capacitor Voltage Converter | |
| Fabricantes | National Semiconductor | |
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March 1999
LM2681
Switched Capacitor Voltage Converter
General Description
The LM2681 CMOS charge-pump voltage converter oper-
ates as a voltage doubler for an input voltage in the range of
+2.5V to +5.5V. Two low cost capacitors and a diode
(needed during start-up) is used in this circuit to provide up
to 20 mA of output current. The LM2681 can also work as a
voltage divider to split a voltage in the range of +1.8V to
+11V in half.
The LM2681 operates at 160 kHz oscillator frequency to re-
duce output resistance and voltage ripple. With an operating
current of only 550 µA (operating efficiency greater than 90%
with most loads) the LM2681 provides ideal performance for
battery powered systems. The device is in SOT-23-6 pack-
age.
Features
n Doubles or Splits Input Supply Voltage
n SOT23-6 Package
n 15Ω Typical Output Impedance
n 90% Typical Conversion Efficiency at 20 mA
Applications
n Cellular Phones
n Pagers
n PDAs
n Operational Amplifier Power Suppliers
n Interface Power Suppliers
n Handheld Instruments
Basic Application Circuits
Voltage Doubler
Splitting Vin in Half
DS100965-1
DS100965-2
© 1999 National Semiconductor Corporation DS100965
www.national.com
1 page  
Pin Description
Pin Name
1 V+
2 GND
3 CAP−
4 GND
5 OUT
6 CAP+
Function
Voltage Doubler
Power supply positive voltage input
Power supply ground input
Connect this pin to the negative terminal of the
charge-pump capacitor
Power supply ground input
Positive voltage output
Connect this pin to the positive terminal of the
charge-pump capacitor
Voltage Split
Positive voltage output
Same as doubler
Same as doubler
Same as doubler
Power supply positive voltage
input
Same as doubler
Circuit Description
The LM2681 contains four large CMOS switches which are
switched in a sequence to double the input supply voltage.
Energy transfer and storage are provided by external capaci-
tors. Figure 2 illustrates the voltage conversion scheme.
When S2 and S4 are closed, C1 charges to the supply volt-
age V+. During this time interval, switches S1 and S3 are
open. In the next time interval, S2 and S4 are open; at the
same time, S1 and S3 are closed, the sum of the input volt-
age V+ and the voltage across C1 gives the 2V+ output volt-
age when there is no load. The output voltage drop when a
load is added is determined by the parasitic resistance (Rd-
s(on) of the MOSFET switches and the ESR of the capacitors)
and the charge transfer loss between capacitors. Details will
be discussed in the following application information section.
FIGURE 2. Voltage Doubling Principle
DS100965-14
Application Information
Positive Voltage Doubler
The main application of the LM2681 is to double the input
voltage. The range of the input supply voltage is 2.5V to
5.5V.
The output characteristics of this circuit can be approximated
by an ideal voltage source in series with a resistance. The
voltage source equals 2V+. The output resistance Rout is a
function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, the capacitance and ESR
of C1 and C2. Since the switching current charging and dis-
charging C1 is approximately twice as the output current, the
effect of the ESR of the pumping capacitor C1 will be multi-
plied by four in the output resistance. The output capacitor
C2 is charging and discharging at a current approximately
equal to the output current, therefore, its ESR only counts
once in the output resistance. A good approximation of Rout
is:
where RSW is the sum of the ON resistance of the internal
MOSFET switches shown in Figure 2.
The peak-to-peak output voltage ripple is determined by the
oscillator frequency, the capacitance and ESR of the output
capacitor C2:
High capacitance, low ESR capacitors can reduce both the
output reslistance and the voltage ripple.
The Schottky diode D1 is only needed for start-up. The inter-
nal oscillator circuit uses the OUT pin and the GND pin. Volt-
age across OUT and GND must be larger than 1.8V to insure
the operation of the oscillator. During start-up, D1 is used to
charge up the voltage at the OUT pin to start the oscillator;
also, it protects the device from turning-on its own parasitic
diode and potentially latching-up. Therefore, the Schottky di-
ode D1 should have enough current carrying capability to
charge the output capacitor at start-up, as well as a low for-
ward voltage to prevent the internal parasitic diode from
turning-on. A Schottky diode like 1N5817 can be used for
most applications. If the input voltage ramp is less than 10V/
ms, a smaller Schottky diode like MBR0520LT1 can be used
to reduce the circuit size.
Split V+ in Half
Another interesting application shown in the Basic Applica-
tion Circuits is using the LM2681 as a precision voltage di-
vider. . This circuit can be derived from the voltage doubler
by switching the input and output connections. In the voltage
divider, the input voltage applies across the OUT pin and the
GND pin (which are the power rails for the internal oscillator),
therefore no start-up diode is needed. Also, since the
off-voltage across each switch equals Vin/2, the input voltage
can be raised to +11V.
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| PDF Descargar | [ Datasheet LM2681M6X.PDF ] | |
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